mm: reformat the Unevictable-LRU documentation

Do a bit of reformatting on the Unevictable-LRU documentation.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 572 insertions, 469 deletions

diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt
index 0706a72..2d70d0d 100644
--- a/Documentation/vm/unevictable-lru.txt
+++ b/Documentation/vm/unevictable-lru.txt
@@ -1,588 +1,691 @@
-
-This document describes the Linux memory management "Unevictable LRU"
-infrastructure and the use of this infrastructure to manage several types
-of "unevictable" pages.  The document attempts to provide the overall
-rationale behind this mechanism and the rationale for some of the design
-decisions that drove the implementation.  The latter design rationale is
-discussed in the context of an implementation description.  Admittedly, one
-can obtain the implementation details--the "what does it do?"--by reading the
-code.  One hopes that the descriptions below add value by provide the answer
-to "why does it do that?".
-
-Unevictable LRU Infrastructure:
-
-The Unevictable LRU adds an additional LRU list to track unevictable pages
-and to hide these pages from vmscan.  This mechanism is based on a patch by
-Larry Woodman of Red Hat to address several scalability problems with page
+			==============================
+			UNEVICTABLE LRU INFRASTRUCTURE
+			==============================
+
+========
+CONTENTS
+========
+
+ (*) The Unevictable LRU
+
+     - The unevictable page list.
+     - Memory control group interaction.
+     - Marking address spaces unevictable.
+     - Detecting Unevictable Pages.
+     - vmscan's handling of unevictable pages.
+
+ (*) mlock()'d pages.
+
+     - History.
+     - Basic management.
+     - mlock()/mlockall() system call handling.
+     - Filtering special vmas.
+     - munlock()/munlockall() system call handling.
+     - Migrating mlocked pages.
+     - mmap(MAP_LOCKED) system call handling.
+     - munmap()/exit()/exec() system call handling.
+     - try_to_unmap().
+     - try_to_munlock() reverse map scan.
+     - Page reclaim in shrink_*_list().
+
+
+============
+INTRODUCTION
+============
+
+This document describes the Linux memory manager's "Unevictable LRU"
+infrastructure and the use of this to manage several types of "unevictable"
+pages.
+
+The document attempts to provide the overall rationale behind this mechanism
+and the rationale for some of the design decisions that drove the
+implementation.  The latter design rationale is discussed in the context of an
+implementation description.  Admittedly, one can obtain the implementation
+details - the "what does it do?" - by reading the code.  One hopes that the
+descriptions below add value by provide the answer to "why does it do that?".
+
+
+===================
+THE UNEVICTABLE LRU
+===================
+
+The Unevictable LRU facility adds an additional LRU list to track unevictable
+pages and to hide these pages from vmscan.  This mechanism is based on a patch
+by Larry Woodman of Red Hat to address several scalability problems with page
 reclaim in Linux.  The problems have been observed at customer sites on large
-memory x86_64 systems.  For example, a non-numal x86_64 platform with 128GB
-of main memory will have over 32 million 4k pages in a single zone.  When a
-large fraction of these pages are not evictable for any reason [see below],
-vmscan will spend a lot of time scanning the LRU lists looking for the small
-fraction of pages that are evictable.  This can result in a situation where
-all cpus are spending 100% of their time in vmscan for hours or days on end,
-with the system completely unresponsive.
-
-The Unevictable LRU infrastructure addresses the following classes of
-unevictable pages:
-
-+ page owned by ramfs
-+ page mapped into SHM_LOCKed shared memory regions
-+ page mapped into VM_LOCKED [mlock()ed] vmas
-
-The infrastructure might be able to handle other conditions that make pages
+memory x86_64 systems.
+
+To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of
+main memory will have over 32 million 4k pages in a single zone.  When a large
+fraction of these pages are not evictable for any reason [see below], vmscan
+will spend a lot of time scanning the LRU lists looking for the small fraction
+of pages that are evictable.  This can result in a situation where all CPUs are
+spending 100% of their time in vmscan for hours or days on end, with the system
+completely unresponsive.
+
+The unevictable list addresses the following classes of unevictable pages:
+
+ (*) Those owned by ramfs.
+
+ (*) Those mapped into SHM_LOCK'd shared memory regions.
+
+ (*) Those mapped into VM_LOCKED [mlock()ed] VMAs.
+
+The infrastructure may also be able to handle other conditions that make pages
 unevictable, either by definition or by circumstance, in the future.
 
 
-The Unevictable LRU List
+THE UNEVICTABLE PAGE LIST
+-------------------------
 
 The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list
 called the "unevictable" list and an associated page flag, PG_unevictable, to
-indicate that the page is being managed on the unevictable list.  The
-PG_unevictable flag is analogous to, and mutually exclusive with, the PG_active
-flag in that it indicates on which LRU list a page resides when PG_lru is set.
-The unevictable LRU list is source configurable based on the UNEVICTABLE_LRU
-Kconfig option.
+indicate that the page is being managed on the unevictable list.
+
+The PG_unevictable flag is analogous to, and mutually exclusive with, the
+PG_active flag in that it indicates on which LRU list a page resides when
+PG_lru is set.  The unevictable list is compile-time configurable based on the
+UNEVICTABLE_LRU Kconfig option.
 
 The Unevictable LRU infrastructure maintains unevictable pages on an additional
 LRU list for a few reasons:
 
-1) We get to "treat unevictable pages just like we treat other pages in the
-   system, which means we get to use the same code to manipulate them, the
-   same code to isolate them (for migrate, etc.), the same code to keep track
-   of the statistics, etc..." [Rik van Riel]
+ (1) We get to "treat unevictable pages just like we treat other pages in the
+     system - which means we get to use the same code to manipulate them, the
+     same code to isolate them (for migrate, etc.), the same code to keep track
+     of the statistics, etc..." [Rik van Riel]
+
+ (2) We want to be able to migrate unevictable pages between nodes for memory
+     defragmentation, workload management and memory hotplug.  The linux kernel
+     can only migrate pages that it can successfully isolate from the LRU
+     lists.  If we were to maintain pages elsewhere than on an LRU-like list,
+     where they can be found by isolate_lru_page(), we would prevent their
+     migration, unless we reworked migration code to find the unevictable pages
+     itself.
 
-2) We want to be able to migrate unevictable pages between nodes--for memory
-   defragmentation, workload management and memory hotplug.  The linux kernel
-   can only migrate pages that it can successfully isolate from the lru lists.
-   If we were to maintain pages elsewise than on an lru-like list, where they
-   can be found by isolate_lru_page(), we would prevent their migration, unless
-   we reworked migration code to find the unevictable pages.
 
+The unevictable list does not differentiate between file-backed and anonymous,
+swap-backed pages.  This differentiation is only important while the pages are,
+in fact, evictable.
 
-The unevictable LRU list does not differentiate between file backed and swap
-backed [anon] pages.  This differentiation is only important while the pages
-are, in fact, evictable.
+The unevictable list benefits from the "arrayification" of the per-zone LRU
+lists and statistics originally proposed and posted by Christoph Lameter.
 
-The unevictable LRU list benefits from the "arrayification" of the per-zone
-LRU lists and statistics originally proposed and posted by Christoph Lameter.
+The unevictable list does not use the LRU pagevec mechanism. Rather,
+unevictable pages are placed directly on the page's zone's unevictable list
+under the zone lru_lock.  This allows us to prevent the stranding of pages on
+the unevictable list when one task has the page isolated from the LRU and other
+tasks are changing the "evictability" state of the page.
 
-The unevictable list does not use the lru pagevec mechanism. Rather,
-unevictable pages are placed directly on the page's zone's unevictable
-list under the zone lru_lock.  The reason for this is to prevent stranding
-of pages on the unevictable list when one task has the page isolated from the
-lru and other tasks are changing the "evictability" state of the page.
 
+MEMORY CONTROL GROUP INTERACTION
+--------------------------------
 
-Unevictable LRU and Memory Controller Interaction
+The unevictable LRU facility interacts with the memory control group [aka
+memory controller; see Documentation/cgroups/memory.txt] by extending the
+lru_list enum.
+
+The memory controller data structure automatically gets a per-zone unevictable
+list as a result of the "arrayification" of the per-zone LRU lists (one per
+lru_list enum element).  The memory controller tracks the movement of pages to
+and from the unevictable list.
 
-The memory controller data structure automatically gets a per zone unevictable
-lru list as a result of the "arrayification" of the per-zone LRU lists.  The
-memory controller tracks the movement of pages to and from the unevictable list.
 When a memory control group comes under memory pressure, the controller will
 not attempt to reclaim pages on the unevictable list.  This has a couple of
-effects.  Because the pages are "hidden" from reclaim on the unevictable list,
-the reclaim process can be more efficient, dealing only with pages that have
-a chance of being reclaimed.  On the other hand, if too many of the pages
-charged to the control group are unevictable, the evictable portion of the
-working set of the tasks in the control group may not fit into the available
-memory.  This can cause the control group to thrash or to oom-kill tasks.
-
-
-Unevictable LRU:  Detecting Unevictable Pages
-
-The function page_evictable(page, vma) in vmscan.c determines whether a
-page is evictable or not.  For ramfs pages and pages in SHM_LOCKed regions,
-page_evictable() tests a new address space flag, AS_UNEVICTABLE, in the page's
-address space using a wrapper function.  Wrapper functions are used to set,
-clear and test the flag to reduce the requirement for #ifdef's throughout the
-source code.  AS_UNEVICTABLE is set on ramfs inode/mapping when it is created.
-This flag remains for the life of the inode.
-
-For shared memory regions, AS_UNEVICTABLE is set when an application
-successfully SHM_LOCKs the region and is removed when the region is
-SHM_UNLOCKed.  Note that shmctl(SHM_LOCK, ...) does not populate the page
-tables for the region as does, for example, mlock().   So, we make no special
-effort to push any pages in the SHM_LOCKed region to the unevictable list.
-Vmscan will do this when/if it encounters the pages during reclaim.  On
-SHM_UNLOCK, shmctl() scans the pages in the region and "rescues" them from the
-unevictable list if no other condition keeps them unevictable.  If a SHM_LOCKed
-region is destroyed, the pages are also "rescued" from the unevictable list in
-the process of freeing them.
-
-page_evictable() detects mlock()ed pages by testing an additional page flag,
-PG_mlocked via the PageMlocked() wrapper.  If the page is NOT mlocked, and a
-non-NULL vma is supplied, page_evictable() will check whether the vma is
+effects:
+
+ (1) Because the pages are "hidden" from reclaim on the unevictable list, the
+     reclaim process can be more efficient, dealing only with pages that have a
+     chance of being reclaimed.
+
+ (2) On the other hand, if too many of the pages charged to the control group
+     are unevictable, the evictable portion of the working set of the tasks in
+     the control group may not fit into the available memory.  This can cause
+     the control group to thrash or to OOM-kill tasks.
+
+
+MARKING ADDRESS SPACES UNEVICTABLE
+----------------------------------
+
+For facilities such as ramfs none of the pages attached to the address space
+may be evicted.  To prevent eviction of any such pages, the AS_UNEVICTABLE
+address space flag is provided, and this can be manipulated by a filesystem
+using a number of wrapper functions:
+
+ (*) void mapping_set_unevictable(struct address_space *mapping);
+
+	Mark the address space as being completely unevictable.
+
+ (*) void mapping_clear_unevictable(struct address_space *mapping);
+
+	Mark the address space as being evictable.
+
+ (*) int mapping_unevictable(struct address_space *mapping);
+
+	Query the address space, and return true if it is completely
+	unevictable.
+
+These are currently used in two places in the kernel:
+
+ (1) By ramfs to mark the address spaces of its inodes when they are created,
+     and this mark remains for the life of the inode.
+
+ (2) By SYSV SHM to mark SHM_LOCK'd address spaces until SHM_UNLOCK is called.
+
+     Note that SHM_LOCK is not required to page in the locked pages if they're
+     swapped out; the application must touch the pages manually if it wants to
+     ensure they're in memory.
+
+
+DETECTING UNEVICTABLE PAGES
+---------------------------
+
+The function page_evictable() in vmscan.c determines whether a page is
+evictable or not using the query function outlined above [see section "Marking
+address spaces unevictable"] to check the AS_UNEVICTABLE flag.
+
+For address spaces that are so marked after being populated (as SHM regions
+might be), the lock action (eg: SHM_LOCK) can be lazy, and need not populate
+the page tables for the region as does, for example, mlock(), nor need it make
+any special effort to push any pages in the SHM_LOCK'd area to the unevictable
+list.  Instead, vmscan will do this if and when it encounters the pages during
+a reclamation scan.
+
+On an unlock action (such as SHM_UNLOCK), the unlocker (eg: shmctl()) must scan
+the pages in the region and "rescue" them from the unevictable list if no other
+condition is keeping them unevictable.  If an unevictable region is destroyed,
+the pages are also "rescued" from the unevictable list in the process of
+freeing them.
+
+page_evictable() also checks for mlocked pages by testing an additional page
+flag, PG_mlocked (as wrapped by PageMlocked()).  If the page is NOT mlocked,
+and a non-NULL VMA is supplied, page_evictable() will check whether the VMA is
 VM_LOCKED via is_mlocked_vma().  is_mlocked_vma() will SetPageMlocked() and
 update the appropriate statistics if the vma is VM_LOCKED.  This method allows
 efficient "culling" of pages in the fault path that are being faulted in to
-VM_LOCKED vmas.
+VM_LOCKED VMAs.
 
 
-Unevictable Pages and Vmscan [shrink_*_list()]
+VMSCAN'S HANDLING OF UNEVICTABLE PAGES
+--------------------------------------
 
 If unevictable pages are culled in the fault path, or moved to the unevictable
-list at mlock() or mmap() time, vmscan will never encounter the pages until
-they have become evictable again, for example, via munlock() and have been
-"rescued" from the unevictable list.  However, there may be situations where we
-decide, for the sake of expediency, to leave a unevictable page on one of the
-regular active/inactive LRU lists for vmscan to deal with.  Vmscan checks for
-such pages in all of the shrink_{active|inactive|page}_list() functions and
-will "cull" such pages that it encounters--that is, it diverts those pages to
-the unevictable list for the zone being scanned.
-
-There may be situations where a page is mapped into a VM_LOCKED vma, but the
-page is not marked as PageMlocked.  Such pages will make it all the way to
+list at mlock() or mmap() time, vmscan will not encounter the pages until they
+have become evictable again (via munlock() for example) and have been "rescued"
+from the unevictable list.  However, there may be situations where we decide,
+for the sake of expediency, to leave a unevictable page on one of the regular
+active/inactive LRU lists for vmscan to deal with.  vmscan checks for such
+pages in all of the shrink_{active|inactive|page}_list() functions and will
+"cull" such pages that it encounters: that is, it diverts those pages to the
+unevictable list for the zone being scanned.
+
+There may be situations where a page is mapped into a VM_LOCKED VMA, but the
+page is not marked as PG_mlocked.  Such pages will make it all the way to
 shrink_page_list() where they will be detected when vmscan walks the reverse
-map in try_to_unmap().  If try_to_unmap() returns SWAP_MLOCK, shrink_page_list()
-will cull the page at that point.
+map in try_to_unmap().  If try_to_unmap() returns SWAP_MLOCK,
+shrink_page_list() will cull the page at that point.
 
-To "cull" an unevictable page, vmscan simply puts the page back on the lru
-list using putback_lru_page()--the inverse operation to isolate_lru_page()--
-after dropping the page lock.  Because the condition which makes the page
-unevictable may change once the page is unlocked, putback_lru_page() will
-recheck the unevictable state of a page that it places on the unevictable lru
-list.  If the page has become unevictable, putback_lru_page() removes it from
-the list and retries, including the page_unevictable() test.  Because such a
-race is a rare event and movement of pages onto the unevictable list should be
-rare, these extra evictabilty checks should not occur in the majority of calls
-to putback_lru_page().
+To "cull" an unevictable page, vmscan simply puts the page back on the LRU list
+using putback_lru_page() - the inverse operation to isolate_lru_page() - after
+dropping the page lock.  Because the condition which makes the page unevictable
+may change once the page is unlocked, putback_lru_page() will recheck the
+unevictable state of a page that it places on the unevictable list.  If the
+page has become unevictable, putback_lru_page() removes it from the list and
+retries, including the page_unevictable() test.  Because such a race is a rare
+event and movement of pages onto the unevictable list should be rare, these
+extra evictabilty checks should not occur in the majority of calls to
+putback_lru_page().
 
 
-Mlocked Page:  Prior Work
+=============
+MLOCKED PAGES
+=============
 
-The "Unevictable Mlocked Pages" infrastructure is based on work originally
+The unevictable page list is also useful for mlock(), in addition to ramfs and
+SYSV SHM.  Note that mlock() is only available in CONFIG_MMU=y situations; in
+NOMMU situations, all mappings are effectively mlocked.
+
+
+HISTORY
+-------
+
+The "Unevictable mlocked Pages" infrastructure is based on work originally
 posted by Nick Piggin in an RFC patch entitled "mm: mlocked pages off LRU".
-Nick posted his patch as an alternative to a patch posted by Christoph
-Lameter to achieve the same objective--hiding mlocked pages from vmscan.
-In Nick's patch, he used one of the struct page lru list link fields as a count
-of VM_LOCKED vmas that map the page.  This use of the link field for a count
-prevented the management of the pages on an LRU list.  Thus, mlocked pages were
-not migratable as isolate_lru_page() could not find them and the lru list link
-field was not available to the migration subsystem.  Nick resolved this by
-putting mlocked pages back on the lru list before attempting to isolate them,
-thus abandoning the count of VM_LOCKED vmas.  When Nick's patch was integrated
-with the Unevictable LRU work, the count was replaced by walking the reverse
-map to determine whether any VM_LOCKED vmas mapped the page.  More on this
-below.
-
-
-Mlocked Pages:  Basic Management
-
-Mlocked pages--pages mapped into a VM_LOCKED vma--represent one class of
-unevictable pages.  When such a page has been "noticed" by the memory
-management subsystem, the page is marked with the PG_mlocked [PageMlocked()]
-flag.  A PageMlocked() page will be placed on the unevictable LRU list when
-it is added to the LRU.   Pages can be "noticed" by memory management in
-several places:
-
-1) in the mlock()/mlockall() system call handlers.
-2) in the mmap() system call handler when mmap()ing a region with the
-   MAP_LOCKED flag, or mmap()ing a region in a task that has called
-   mlockall() with the MCL_FUTURE flag.  Both of these conditions result
-   in the VM_LOCKED flag being set for the vma.
-3) in the fault path, if mlocked pages are "culled" in the fault path,
-   and when a VM_LOCKED stack segment is expanded.
-4) as mentioned above, in vmscan:shrink_page_list() when attempting to
-   reclaim a page in a VM_LOCKED vma via try_to_unmap().
-
-Mlocked pages become unlocked and rescued from the unevictable list when:
-
-1) mapped in a range unlocked via the munlock()/munlockall() system calls.
-2) munmapped() out of the last VM_LOCKED vma that maps the page, including
-   unmapping at task exit.
-3) when the page is truncated from the last VM_LOCKED vma of an mmap()ed file.
-4) before a page is COWed in a VM_LOCKED vma.
-
-
-Mlocked Pages:  mlock()/mlockall() System Call Handling
+Nick posted his patch as an alternative to a patch posted by Christoph Lameter
+to achieve the same objective: hiding mlocked pages from vmscan.
+
+In Nick's patch, he used one of the struct page LRU list link fields as a count
+of VM_LOCKED VMAs that map the page.  This use of the link field for a count
+prevented the management of the pages on an LRU list, and thus mlocked pages
+were not migratable as isolate_lru_page() could not find them, and the LRU list
+link field was not available to the migration subsystem.
+
+Nick resolved this by putting mlocked pages back on the lru list before
+attempting to isolate them, thus abandoning the count of VM_LOCKED VMAs.  When
+Nick's patch was integrated with the Unevictable LRU work, the count was
+replaced by walking the reverse map to determine whether any VM_LOCKED VMAs
+mapped the page.  More on this below.
+
+
+BASIC MANAGEMENT
+----------------
+
+mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable
+pages.  When such a page has been "noticed" by the memory management subsystem,
+the page is marked with the PG_mlocked flag.  This can be manipulated using the
+PageMlocked() functions.
+
+A PG_mlocked page will be placed on the unevictable list when it is added to
+the LRU.  Such pages can be "noticed" by memory management in several places:
+
+ (1) in the mlock()/mlockall() system call handlers;
+
+ (2) in the mmap() system call handler when mmapping a region with the
+     MAP_LOCKED flag;
+
+ (3) mmapping a region in a task that has called mlockall() with the MCL_FUTURE
+     flag
+
+ (4) in the fault path, if mlocked pages are "culled" in the fault path,
+     and when a VM_LOCKED stack segment is expanded; or
+
+ (5) as mentioned above, in vmscan:shrink_page_list() when attempting to
+     reclaim a page in a VM_LOCKED VMA via try_to_unmap()
+
+all of which result in the VM_LOCKED flag being set for the VMA if it doesn't
+already have it set.
+
+mlocked pages become unlocked and rescued from the unevictable list when:
+
+ (1) mapped in a range unlocked via the munlock()/munlockall() system calls;
+
+ (2) munmap()'d out of the last VM_LOCKED VMA that maps the page, including
+     unmapping at task exit;
+
+ (3) when the page is truncated from the last VM_LOCKED VMA of an mmapped file;
+     or
+
+ (4) before a page is COW'd in a VM_LOCKED VMA.
+
+
+mlock()/mlockall() SYSTEM CALL HANDLING
+---------------------------------------
 
 Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup()
-for each vma in the range specified by the call.  In the case of mlockall(),
+for each VMA in the range specified by the call.  In the case of mlockall(),
 this is the entire active address space of the task.  Note that mlock_fixup()
-is used for both mlock()ing and munlock()ing a range of memory.  A call to
-mlock() an already VM_LOCKED vma, or to munlock() a vma that is not VM_LOCKED
-is treated as a no-op--mlock_fixup() simply returns.
-
-If the vma passes some filtering described in "Mlocked Pages:  Filtering Vmas"
-below, mlock_fixup() will attempt to merge the vma with its neighbors or split
-off a subset of the vma if the range does not cover the entire vma.  Once the
-vma has been merged or split or neither, mlock_fixup() will call
-__mlock_vma_pages_range() to fault in the pages via get_user_pages() and
-to mark the pages as mlocked via mlock_vma_page().
-
-Note that the vma being mlocked might be mapped with PROT_NONE.  In this case,
-get_user_pages() will be unable to fault in the pages.  That's OK.  If pages
-do end up getting faulted into this VM_LOCKED vma, we'll handle them in the
+is used for both mlocking and munlocking a range of memory.  A call to mlock()
+an already VM_LOCKED VMA, or to munlock() a VMA that is not VM_LOCKED is
+treated as a no-op, and mlock_fixup() simply returns.
+
+If the VMA passes some filtering as described in "Filtering Special Vmas"
+below, mlock_fixup() will attempt to merge the VMA with its neighbors or split
+off a subset of the VMA if the range does not cover the entire VMA.  Once the
+VMA has been merged or split or neither, mlock_fixup() will call
+__mlock_vma_pages_range() to fault in the pages via get_user_pages() and to
+mark the pages as mlocked via mlock_vma_page().
+
+Note that the VMA being mlocked might be mapped with PROT_NONE.  In this case,
+get_user_pages() will be unable to fault in the pages.  That's okay.  If pages
+do end up getting faulted into this VM_LOCKED VMA, we'll handle them in the
 fault path or in vmscan.
 
 Also note that a page returned by get_user_pages() could be truncated or
-migrated out from under us, while we're trying to mlock it.  To detect
-this, __mlock_vma_pages_range() tests the page_mapping after acquiring
-the page lock.  If the page is still associated with its mapping, we'll
-go ahead and call mlock_vma_page().  If the mapping is gone, we just
-unlock the page and move on.  Worse case, this results in page mapped
-in a VM_LOCKED vma remaining on a normal LRU list without being
-PageMlocked().  Again, vmscan will detect and cull such pages.
-
-mlock_vma_page(), called with the page locked [N.B., not "mlocked"], will
-TestSetPageMlocked() for each page returned by get_user_pages().  We use
-TestSetPageMlocked() because the page might already be mlocked by another
-task/vma and we don't want to do extra work.  We especially do not want to
-count an mlocked page more than once in the statistics.  If the page was
-already mlocked, mlock_vma_page() is done.
+migrated out from under us, while we're trying to mlock it.  To detect this,
+__mlock_vma_pages_range() checks page_mapping() after acquiring the page lock.
+If the page is still associated with its mapping, we'll go ahead and call
+mlock_vma_page().  If the mapping is gone, we just unlock the page and move on.
+In the worst case, this will result in a page mapped in a VM_LOCKED VMA
+remaining on a normal LRU list without being PageMlocked().  Again, vmscan will
+detect and cull such pages.
+
+mlock_vma_page() will call TestSetPageMlocked() for each page returned by
+get_user_pages().  We use TestSetPageMlocked() because the page might already
+be mlocked by another task/VMA and we don't want to do extra work.  We
+especially do not want to count an mlocked page more than once in the
+statistics.  If the page was already mlocked, mlock_vma_page() need do nothing
+more.
 
 If the page was NOT already mlocked, mlock_vma_page() attempts to isolate the
 page from the LRU, as it is likely on the appropriate active or inactive list
-at that time.  If the isolate_lru_page() succeeds, mlock_vma_page() will
-putback the page--putback_lru_page()--which will notice that the page is now
-mlocked and divert the page to the zone's unevictable LRU list.  If
+at that time.  If the isolate_lru_page() succeeds, mlock_vma_page() will put
+back the page - by calling putback_lru_page() - which will notice that the page
+is now mlocked and divert the page to the zone's unevictable list.  If
 mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
-it later if/when it attempts to reclaim the page.
+it later if and when it attempts to reclaim the page.
 
 
-Mlocked Pages:  Filtering Special Vmas
+FILTERING SPECIAL VMAS
+----------------------
 
-mlock_fixup() filters several classes of "special" vmas:
+mlock_fixup() filters several classes of "special" VMAs:
 
-1) vmas with VM_IO|VM_PFNMAP set are skipped entirely.  The pages behind
+1) VMAs with VM_IO or VM_PFNMAP set are skipped entirely.  The pages behind
    these mappings are inherently pinned, so we don't need to mark them as
-   mlocked.  In any case, most of the pages have no struct page in which to
-   so mark the page.  Because of this, get_user_pages() will fail for these
-   vmas, so there is no sense in attempting to visit them.
-
-2) vmas mapping hugetlbfs page are already effectively pinned into memory.
-   We don't need nor want to mlock() these pages.  However, to preserve the
-   prior behavior of mlock()--before the unevictable/mlock changes--
-   mlock_fixup() will call make_pages_present() in the hugetlbfs vma range
-   to allocate the huge pages and populate the ptes.
-
-3) vmas with VM_DONTEXPAND|VM_RESERVED are generally user space mappings of
-   kernel pages, such as the vdso page, relay channel pages, etc.  These pages
+   mlocked.  In any case, most of the pages have no struct page in which to so
+   mark the page.  Because of this, get_user_pages() will fail for these VMAs,
+   so there is no sense in attempting to visit them.
+
+2) VMAs mapping hugetlbfs page are already effectively pinned into memory.  We
+   neither need nor want to mlock() these pages.  However, to preserve the
+   prior behavior of mlock() - before the unevictable/mlock changes -
+   mlock_fixup() will call make_pages_present() in the hugetlbfs VMA range to
+   allocate the huge pages and populate the ptes.
+
+3) VMAs with VM_DONTEXPAND or VM_RESERVED are generally userspace mappings of
+   kernel pages, such as the VDSO page, relay channel pages, etc.  These pages
    are inherently unevictable and are not managed on the LRU lists.
-   mlock_fixup() treats these vmas the same as hugetlbfs vmas.  It calls
+   mlock_fixup() treats these VMAs the same as hugetlbfs VMAs.  It calls
    make_pages_present() to populate the ptes.
 
-Note that for all of these special vmas, mlock_fixup() does not set the
+Note that for all of these special VMAs, mlock_fixup() does not set the
 VM_LOCKED flag.  Therefore, we won't have to deal with them later during
-munlock() or munmap()--for example, at task exit.  Neither does mlock_fixup()
-account these vmas against the task's "locked_vm".
-
-Mlocked Pages:  Downgrading the Mmap Semaphore.
-
-mlock_fixup() must be called with the mmap semaphore held for write, because
-it may have to merge or split vmas.  However, mlocking a large region of
-memory can take a long time--especially if vmscan must reclaim pages to
-satisfy the regions requirements.  Faulting in a large region with the mmap
-semaphore held for write can hold off other faults on the address space, in
-the case of a multi-threaded task.  It can also hold off scans of the task's
-address space via /proc.  While testing under heavy load, it was observed that
-the ps(1) command could be held off for many minutes while a large segment was
-mlock()ed down.
-
-To address this issue, and to make the system more responsive during mlock()ing
-of large segments, mlock_fixup() downgrades the mmap semaphore to read mode
-during the call to __mlock_vma_pages_range().  This works fine.  However, the
-callers of mlock_fixup() expect the semaphore to be returned in write mode.
-So, mlock_fixup() "upgrades" the semphore to write mode.  Linux does not
-support an atomic upgrade_sem() call, so mlock_fixup() must drop the semaphore
-and reacquire it in write mode.  In a multi-threaded task, it is possible for
-the task memory map to change while the semaphore is dropped.  Therefore,
-mlock_fixup() looks up the vma at the range start address after reacquiring
-the semaphore in write mode and verifies that it still covers the original
-range.  If not, mlock_fixup() returns an error [-EAGAIN].  All callers of
-mlock_fixup() have been changed to deal with this new error condition.
-
-Note:  when munlocking a region, all of the pages should already be resident--
-unless we have racing threads mlocking() and munlocking() regions.  So,
-unlocking should not have to wait for page allocations nor faults  of any kind.
-Therefore mlock_fixup() does not downgrade the semaphore for munlock().
-
-
-Mlocked Pages:  munlock()/munlockall() System Call Handling
-
-The munlock() and munlockall() system calls are handled by the same functions--
-do_mlock[all]()--as the mlock() and mlockall() system calls with the unlock
-vs lock operation indicated by an argument.  So, these system calls are also
-handled by mlock_fixup().  Again, if called for an already munlock()ed vma,
-mlock_fixup() simply returns.  Because of the vma filtering discussed above,
-VM_LOCKED will not be set in any "special" vmas.  So, these vmas will be
+munlock(), munmap() or task exit.  Neither does mlock_fixup() account these
+VMAs against the task's "locked_vm".
+
+
+munlock()/munlockall() SYSTEM CALL HANDLING
+-------------------------------------------
+
+The munlock() and munlockall() system calls are handled by the same functions -
+do_mlock[all]() - as the mlock() and mlockall() system calls with the unlock vs
+lock operation indicated by an argument.  So, these system calls are also
+handled by mlock_fixup().  Again, if called for an already munlocked VMA,
+mlock_fixup() simply returns.  Because of the VMA filtering discussed above,
+VM_LOCKED will not be set in any "special" VMAs.  So, these VMAs will be
 ignored for munlock.
 
-If the vma is VM_LOCKED, mlock_fixup() again attempts to merge or split off
-the specified range.  The range is then munlocked via the function
-__mlock_vma_pages_range()--the same function used to mlock a vma range--
+If the VMA is VM_LOCKED, mlock_fixup() again attempts to merge or split off the
+specified range.  The range is then munlocked via the function
+__mlock_vma_pages_range() - the same function used to mlock a VMA range -
 passing a flag to indicate that munlock() is being performed.
 
-Because the vma access protections could have been changed to PROT_NONE after
+Because the VMA access protections could have been changed to PROT_NONE after
 faulting in and mlocking pages, get_user_pages() was unreliable for visiting
-these pages for munlocking.  Because we don't want to leave pages mlocked(),
+these pages for munlocking.  Because we don't want to leave pages mlocked,
 get_user_pages() was enhanced to accept a flag to ignore the permissions when
-fetching the pages--all of which should be resident as a result of previous
-mlock()ing.
+fetching the pages - all of which should be resident as a result of previous
+mlocking.
 
 For munlock(), __mlock_vma_pages_range() unlocks individual pages by calling
 munlock_vma_page().  munlock_vma_page() unconditionally clears the PG_mlocked
-flag using TestClearPageMlocked().  As with mlock_vma_page(), munlock_vma_page()
-use the Test*PageMlocked() function to handle the case where the page might
-have already been unlocked by another task.  If the page was mlocked,
-munlock_vma_page() updates that zone statistics for the number of mlocked
-pages.  Note, however, that at this point we haven't checked whether the page
-is mapped by other VM_LOCKED vmas.
-
-We can't call try_to_munlock(), the function that walks the reverse map to check
-for other VM_LOCKED vmas, without first isolating the page from the LRU.
+flag using TestClearPageMlocked().  As with mlock_vma_page(),
+munlock_vma_page() use the Test*PageMlocked() function to handle the case where
+the page might have already been unlocked by another task.  If the page was
+mlocked, munlock_vma_page() updates that zone statistics for the number of
+mlocked pages.  Note, however, that at this point we haven't checked whether
+the page is mapped by other VM_LOCKED VMAs.
+
+We can't call try_to_munlock(), the function that walks the reverse map to
+check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
 try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
-not be on an lru list.  [More on these below.]  However, the call to
-isolate_lru_page() could fail, in which case we couldn't try_to_munlock().
-So, we go ahead and clear PG_mlocked up front, as this might be the only chance
-we have.  If we can successfully isolate the page, we go ahead and
+not be on an LRU list [more on these below].  However, the call to
+isolate_lru_page() could fail, in which case we couldn't try_to_munlock().  So,
+we go ahead and clear PG_mlocked up front, as this might be the only chance we
+have.  If we can successfully isolate the page, we go ahead and
 try_to_munlock(), which will restore the PG_mlocked flag and update the zone
-page statistics if it finds another vma holding the page mlocked.  If we fail
+page statistics if it finds another VMA holding the page mlocked.  If we fail
 to isolate the page, we'll have left a potentially mlocked page on the LRU.
-This is fine, because we'll catch it later when/if vmscan tries to reclaim the
-page.  This should be relatively rare.
-
-Mlocked Pages:  Migrating Them...
-
-A page that is being migrated has been isolated from the lru lists and is
-held locked across unmapping of the page, updating the page's mapping
-[address_space] entry and copying the contents and state, until the
-page table entry has been replaced with an entry that refers to the new
-page.  Linux supports migration of mlocked pages and other unevictable
-pages.  This involves simply moving the PageMlocked and PageUnevictable states
-from the old page to the new page.
-
-Note that page migration can race with mlocking or munlocking of the same
-page.  This has been discussed from the mlock/munlock perspective in the
-respective sections above.  Both processes [migration, m[un]locking], hold
-the page locked.  This provides the first level of synchronization.  Page
-migration zeros out the page_mapping of the old page before unlocking it,
-so m[un]lock can skip these pages by testing the page mapping under page
-lock.
-
-When completing page migration, we place the new and old pages back onto the
-lru after dropping the page lock.  The "unneeded" page--old page on success,
-new page on failure--will be freed when the reference count held by the
-migration process is released.  To ensure that we don't strand pages on the
-unevictable list because of a race between munlock and migration, page
-migration uses the putback_lru_page() function to add migrated pages back to
-the lru.
-
-
-Mlocked Pages:  mmap(MAP_LOCKED) System Call Handling
+This is fine, because we'll catch it later if and if vmscan tries to reclaim
+the page.  This should be relatively rare.
+
+
+MIGRATING MLOCKED PAGES
+-----------------------
+
+A page that is being migrated has been isolated from the LRU lists and is held
+locked across unmapping of the page, updating the page's address space entry
+and copying the contents and state, until the page table entry has been
+replaced with an entry that refers to the new page.  Linux supports migration
+of mlocked pages and other unevictable pages.  This involves simply moving the
+PG_mlocked and PG_unevictable states from the old page to the new page.
+
+Note that page migration can race with mlocking or munlocking of the same page.
+This has been discussed from the mlock/munlock perspective in the respective
+sections above.  Both processes (migration and m[un]locking) hold the page
+locked.  This provides the first level of synchronization.  Page migration
+zeros out the page_mapping of the old page before unlocking it, so m[un]lock
+can skip these pages by testing the page mapping under page lock.
+
+To complete page migration, we place the new and old pages back onto the LRU
+after dropping the page lock.  The "unneeded" page - old page on success, new
+page on failure - will be freed when the reference count held by the migration
+process is released.  To ensure that we don't strand pages on the unevictable
+list because of a race between munlock and migration, page migration uses the
+putback_lru_page() function to add migrated pages back to the LRU.
+
+
+mmap(MAP_LOCKED) SYSTEM CALL HANDLING
+-------------------------------------
 
 In addition the the mlock()/mlockall() system calls, an application can request
-that a region of memory be mlocked using the MAP_LOCKED flag with the mmap()
+that a region of memory be mlocked supplying the MAP_LOCKED flag to the mmap()
 call.  Furthermore, any mmap() call or brk() call that expands the heap by a
 task that has previously called mlockall() with the MCL_FUTURE flag will result
-in the newly mapped memory being mlocked.  Before the unevictable/mlock changes,
-the kernel simply called make_pages_present() to allocate pages and populate
-the page table.
+in the newly mapped memory being mlocked.  Before the unevictable/mlock
+changes, the kernel simply called make_pages_present() to allocate pages and
+populate the page table.
 
 To mlock a range of memory under the unevictable/mlock infrastructure, the
 mmap() handler and task address space expansion functions call
 mlock_vma_pages_range() specifying the vma and the address range to mlock.
-mlock_vma_pages_range() filters vmas like mlock_fixup(), as described above in
-"Mlocked Pages:  Filtering Vmas".  It will clear the VM_LOCKED flag, which will
-have already been set by the caller, in filtered vmas.  Thus these vma's need
-not be visited for munlock when the region is unmapped.
+mlock_vma_pages_range() filters VMAs like mlock_fixup(), as described above in
+"Filtering Special VMAs".  It will clear the VM_LOCKED flag, which will have
+already been set by the caller, in filtered VMAs.  Thus these VMA's need not be
+visited for munlock when the region is unmapped.
 
-For "normal" vmas, mlock_vma_pages_range() calls __mlock_vma_pages_range() to
+For "normal" VMAs, mlock_vma_pages_range() calls __mlock_vma_pages_range() to
 fault/allocate the pages and mlock them.  Again, like mlock_fixup(),
 mlock_vma_pages_range() downgrades the mmap semaphore to read mode before
-attempting to fault/allocate and mlock the pages; and "upgrades" the semaphore
+attempting to fault/allocate and mlock the pages and "upgrades" the semaphore
 back to write mode before returning.
 
-The callers of mlock_vma_pages_range() will have already added the memory
-range to be mlocked to the task's "locked_vm".  To account for filtered vmas,
+The callers of mlock_vma_pages_range() will have already added the memory range
+to be mlocked to the task's "locked_vm".  To account for filtered VMAs,
 mlock_vma_pages_range() returns the number of pages NOT mlocked.  All of the
-callers then subtract a non-negative return value from the task's locked_vm.
-A negative return value represent an error--for example, from get_user_pages()
-attempting to fault in a vma with PROT_NONE access.  In this case, we leave
-the memory range accounted as locked_vm, as the protections could be changed
-later and pages allocated into that region.
+callers then subtract a non-negative return value from the task's locked_vm.  A
+negative return value represent an error - for example, from get_user_pages()
+attempting to fault in a VMA with PROT_NONE access.  In this case, we leave the
+memory range accounted as locked_vm, as the protections could be changed later
+and pages allocated into that region.
 
 
-Mlocked Pages:  munmap()/exit()/exec() System Call Handling
+munmap()/exit()/exec() SYSTEM CALL HANDLING
+-------------------------------------------
 
 When unmapping an mlocked region of memory, whether by an explicit call to
 munmap() or via an internal unmap from exit() or exec() processing, we must
-munlock the pages if we're removing the last VM_LOCKED vma that maps the pages.
+munlock the pages if we're removing the last VM_LOCKED VMA that maps the pages.
 Before the unevictable/mlock changes, mlocking did not mark the pages in any
 way, so unmapping them required no processing.
 
 To munlock a range of memory under the unevictable/mlock infrastructure, the
-munmap() hander and task address space tear down function call
+munmap() handler and task address space call tear down function
 munlock_vma_pages_all().  The name reflects the observation that one always
-specifies the entire vma range when munlock()ing during unmap of a region.
-Because of the vma filtering when mlocking() regions, only "normal" vmas that
+specifies the entire VMA range when munlock()ing during unmap of a region.
+Because of the VMA filtering when mlocking() regions, only "normal" VMAs that
 actually contain mlocked pages will be passed to munlock_vma_pages_all().
 
-munlock_vma_pages_all() clears the VM_LOCKED vma flag and, like mlock_fixup()
+munlock_vma_pages_all() clears the VM_LOCKED VMA flag and, like mlock_fixup()
 for the munlock case, calls __munlock_vma_pages_range() to walk the page table
-for the vma's memory range and munlock_vma_page() each resident page mapped by
-the vma.  This effectively munlocks the page, only if this is the last
-VM_LOCKED vma that maps the page.
-
+for the VMA's memory range and munlock_vma_page() each resident page mapped by
+the VMA.  This effectively munlocks the page, only if this is the last
+VM_LOCKED VMA that maps the page.
 
-Mlocked Page:  try_to_unmap()
 
-[Note:  the code changes represented by this section are really quite small
-compared to the text to describe what happening and why, and to discuss the
-implications.]
+try_to_unmap()
+--------------
 
-Pages can, of course, be mapped into multiple vmas.  Some of these vmas may
+Pages can, of course, be mapped into multiple VMAs.  Some of these VMAs may
 have VM_LOCKED flag set.  It is possible for a page mapped into one or more
-VM_LOCKED vmas not to have the PG_mlocked flag set and therefore reside on one
-of the active or inactive LRU lists.  This could happen if, for example, a
-task in the process of munlock()ing the page could not isolate the page from
-the LRU.  As a result, vmscan/shrink_page_list() might encounter such a page
-as described in "Unevictable Pages and Vmscan [shrink_*_list()]".  To
-handle this situation, try_to_unmap() has been enhanced to check for VM_LOCKED
-vmas while it is walking a page's reverse map.
+VM_LOCKED VMAs not to have the PG_mlocked flag set and therefore reside on one
+of the active or inactive LRU lists.  This could happen if, for example, a task
+in the process of munlocking the page could not isolate the page from the LRU.
+As a result, vmscan/shrink_page_list() might encounter such a page as described
+in section "vmscan's handling of unevictable pages".  To handle this situation,
+try_to_unmap() checks for VM_LOCKED VMAs while it is walking a page's reverse
+map.
 
 try_to_unmap() is always called, by either vmscan for reclaim or for page
-migration, with the argument page locked and isolated from the LRU.  BUG_ON()
-assertions enforce this requirement.  Separate functions handle anonymous and
-mapped file pages, as these types of pages have different reverse map
-mechanisms.
-
-	try_to_unmap_anon()
-
-To unmap anonymous pages, each vma in the list anchored in the anon_vma must be
-visited--at least until a VM_LOCKED vma is encountered.  If the page is being
-unmapped for migration, VM_LOCKED vmas do not stop the process because mlocked
-pages are migratable.  However, for reclaim, if the page is mapped into a
-VM_LOCKED vma, the scan stops.  try_to_unmap() attempts to acquire the mmap
-semphore of the mm_struct to which the vma belongs in read mode.  If this is
-successful, try_to_unmap() will mlock the page via mlock_vma_page()--we
-wouldn't have gotten to try_to_unmap() if the page were already mlocked--and
-will return SWAP_MLOCK, indicating that the page is unevictable.  If the
-mmap semaphore cannot be acquired, we are not sure whether the page is really
-unevictable or not.  In this case, try_to_unmap() will return SWAP_AGAIN.
-
-	try_to_unmap_file() -- linear mappings
-
-Unmapping of a mapped file page works the same, except that the scan visits
-all vmas that maps the page's index/page offset in the page's mapping's
-reverse map priority search tree.  It must also visit each vma in the page's
-mapping's non-linear list, if the list is non-empty.  As for anonymous pages,
-on encountering a VM_LOCKED vma for a mapped file page, try_to_unmap() will
-attempt to acquire the associated mm_struct's mmap semaphore to mlock the page,
-returning SWAP_MLOCK if this is successful, and SWAP_AGAIN, if not.
-
-	try_to_unmap_file() -- non-linear mappings
-
-If a page's mapping contains a non-empty non-linear mapping vma list, then
-try_to_un{map|lock}() must also visit each vma in that list to determine
-whether the page is mapped in a VM_LOCKED vma.  Again, the scan must visit
-all vmas in the non-linear list to ensure that the pages is not/should not be
-mlocked.  If a VM_LOCKED vma is found in the list, the scan could terminate.
-However, there is no easy way to determine whether the page is actually mapped
-in a given vma--either for unmapping or testing whether the VM_LOCKED vma
-actually pins the page.
-
-So, try_to_unmap_file() handles non-linear mappings by scanning a certain
-number of pages--a "cluster"--in each non-linear vma associated with the page's
-mapping, for each file mapped page that vmscan tries to unmap.  If this happens
-to unmap the page we're trying to unmap, try_to_unmap() will notice this on
-return--(page_mapcount(page) == 0)--and return SWAP_SUCCESS.  Otherwise, it
-will return SWAP_AGAIN, causing vmscan to recirculate this page.  We take
-advantage of the cluster scan in try_to_unmap_cluster() as follows:
-
-For each non-linear vma, try_to_unmap_cluster() attempts to acquire the mmap
-semaphore of the associated mm_struct for read without blocking.  If this
-attempt is successful and the vma is VM_LOCKED, try_to_unmap_cluster() will
-retain the mmap semaphore for the scan; otherwise it drops it here.  Then,
-for each page in the cluster, if we're holding the mmap semaphore for a locked
-vma, try_to_unmap_cluster() calls mlock_vma_page() to mlock the page.  This
-call is a no-op if the page is already locked, but will mlock any pages in
-the non-linear mapping that happen to be unlocked.  If one of the pages so
-mlocked is the page passed in to try_to_unmap(), try_to_unmap_cluster() will
-return SWAP_MLOCK, rather than the default SWAP_AGAIN.  This will allow vmscan
-to cull the page, rather than recirculating it on the inactive list.  Again,
-if try_to_unmap_cluster() cannot acquire the vma's mmap sem, it returns
-SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED vma, but
-couldn't be mlocked.
-
-
-Mlocked pages:  try_to_munlock() Reverse Map Scan
-
-TODO/FIXME:  a better name might be page_mlocked()--analogous to the
-page_referenced() reverse map walker.
-
-When munlock_vma_page()--see "Mlocked Pages:  munlock()/munlockall()
-System Call Handling" above--tries to munlock a page, it needs to
-determine whether or not the page is mapped by any VM_LOCKED vma, without
-actually attempting to unmap all ptes from the page.  For this purpose, the
-unevictable/mlock infrastructure introduced a variant of try_to_unmap() called
-try_to_munlock().
+migration, with the argument page locked and isolated from the LRU.  Separate
+functions handle anonymous and mapped file pages, as these types of pages have
+different reverse map mechanisms.
+
+ (*) try_to_unmap_anon()
+
+     To unmap anonymous pages, each VMA in the list anchored in the anon_vma
+     must be visited - at least until a VM_LOCKED VMA is encountered.  If the
+     page is being unmapped for migration, VM_LOCKED VMAs do not stop the
+     process because mlocked pages are migratable.  However, for reclaim, if
+     the page is mapped into a VM_LOCKED VMA, the scan stops.
+
+     try_to_unmap_anon() attempts to acquire in read mode the mmap semphore of
+     the mm_struct to which the VMA belongs.  If this is successful, it will
+     mlock the page via mlock_vma_page() - we wouldn't have gotten to
+     try_to_unmap_anon() if the page were already mlocked - and will return
+     SWAP_MLOCK, indicating that the page is unevictable.
+
+     If the mmap semaphore cannot be acquired, we are not sure whether the page
+     is really unevictable or not.  In this case, try_to_unmap_anon() will
+     return SWAP_AGAIN.
+
+ (*) try_to_unmap_file() - linear mappings
+
+     Unmapping of a mapped file page works the same as for anonymous mappings,
+     except that the scan visits all VMAs that map the page's index/page offset
+     in the page's mapping's reverse map priority search tree.  It also visits
+     each VMA in the page's mapping's non-linear list, if the list is
+     non-empty.
+
+     As for anonymous pages, on encountering a VM_LOCKED VMA for a mapped file
+     page, try_to_unmap_file() will attempt to acquire the associated
+     mm_struct's mmap semaphore to mlock the page, returning SWAP_MLOCK if this
+     is successful, and SWAP_AGAIN, if not.
+
+ (*) try_to_unmap_file() - non-linear mappings
+
+     If a page's mapping contains a non-empty non-linear mapping VMA list, then
+     try_to_un{map|lock}() must also visit each VMA in that list to determine
+     whether the page is mapped in a VM_LOCKED VMA.  Again, the scan must visit
+     all VMAs in the non-linear list to ensure that the pages is not/should not
+     be mlocked.
+
+     If a VM_LOCKED VMA is found in the list, the scan could terminate.
+     However, there is no easy way to determine whether the page is actually
+     mapped in a given VMA - either for unmapping or testing whether the
+     VM_LOCKED VMA actually pins the page.
+
+     try_to_unmap_file() handles non-linear mappings by scanning a certain
+     number of pages - a "cluster" - in each non-linear VMA associated with the
+     page's mapping, for each file mapped page that vmscan tries to unmap.  If
+     this happens to unmap the page we're trying to unmap, try_to_unmap() will
+     notice this on return (page_mapcount(page) will be 0) and return
+     SWAP_SUCCESS.  Otherwise, it will return SWAP_AGAIN, causing vmscan to
+     recirculate this page.  We take advantage of the cluster scan in
+     try_to_unmap_cluster() as follows:
+
+	For each non-linear VMA, try_to_unmap_cluster() attempts to acquire the
+	mmap semaphore of the associated mm_struct for read without blocking.
+
+	If this attempt is successful and the VMA is VM_LOCKED,
+	try_to_unmap_cluster() will retain the mmap semaphore for the scan;
+	otherwise it drops it here.
+
+	Then, for each page in the cluster, if we're holding the mmap semaphore
+	for a locked VMA, try_to_unmap_cluster() calls mlock_vma_page() to
+	mlock the page.  This call is a no-op if the page is already locked,
+	but will mlock any pages in the non-linear mapping that happen to be
+	unlocked.
+
+	If one of the pages so mlocked is the page passed in to try_to_unmap(),
+	try_to_unmap_cluster() will return SWAP_MLOCK, rather than the default
+	SWAP_AGAIN.  This will allow vmscan to cull the page, rather than
+	recirculating it on the inactive list.
+
+	Again, if try_to_unmap_cluster() cannot acquire the VMA's mmap sem, it
+	returns SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED
+	VMA, but couldn't be mlocked.
+
+
+try_to_munlock() REVERSE MAP SCAN
+---------------------------------
+
+ [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
+     page_referenced() reverse map walker.
+
+When munlock_vma_page() [see section "munlock()/munlockall() System Call
+Handling" above] tries to munlock a page, it needs to determine whether or not
+the page is mapped by any VM_LOCKED VMA without actually attempting to unmap
+all PTEs from the page.  For this purpose, the unevictable/mlock infrastructure
+introduced a variant of try_to_unmap() called try_to_munlock().
 
 try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
 mapped file pages with an additional argument specifing unlock versus unmap
 processing.  Again, these functions walk the respective reverse maps looking
-for VM_LOCKED vmas.  When such a vma is found for anonymous pages and file
+for VM_LOCKED VMAs.  When such a VMA is found for anonymous pages and file
 pages mapped in linear VMAs, as in the try_to_unmap() case, the functions
 attempt to acquire the associated mmap semphore, mlock the page via
 mlock_vma_page() and return SWAP_MLOCK.  This effectively undoes the
 pre-clearing of the page's PG_mlocked done by munlock_vma_page.
 
-If try_to_unmap() is unable to acquire a VM_LOCKED vma's associated mmap
-semaphore, it will return SWAP_AGAIN.  This will allow shrink_page_list()
-to recycle the page on the inactive list and hope that it has better luck
-with the page next time.
-
-For file pages mapped into non-linear vmas, the try_to_munlock() logic works
-slightly differently.  On encountering a VM_LOCKED non-linear vma that might
-map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking
-the page.  munlock_vma_page() will just leave the page unlocked and let
-vmscan deal with it--the usual fallback position.
-
-Note that try_to_munlock()'s reverse map walk must visit every vma in a pages'
-reverse map to determine that a page is NOT mapped into any VM_LOCKED vma.
-However, the scan can terminate when it encounters a VM_LOCKED vma and can
-successfully acquire the vma's mmap semphore for read and mlock the page.
-Although try_to_munlock() can be called many [very many!] times when
-munlock()ing a large region or tearing down a large address space that has been
-mlocked via mlockall(), overall this is a fairly rare event.
-
-Mlocked Page:  Page Reclaim in shrink_*_list()
-
-shrink_active_list() culls any obviously unevictable pages--i.e.,
-!page_evictable(page, NULL)--diverting these to the unevictable lru
-list.  However, shrink_active_list() only sees unevictable pages that
-made it onto the active/inactive lru lists.  Note that these pages do not
-have PageUnevictable set--otherwise, they would be on the unevictable list and
-shrink_active_list would never see them.
+If try_to_unmap() is unable to acquire a VM_LOCKED VMA's associated mmap
+semaphore, it will return SWAP_AGAIN.  This will allow shrink_page_list() to
+recycle the page on the inactive list and hope that it has better luck with the
+page next time.
+
+For file pages mapped into non-linear VMAs, the try_to_munlock() logic works
+slightly differently.  On encountering a VM_LOCKED non-linear VMA that might
+map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking the
+page.  munlock_vma_page() will just leave the page unlocked and let vmscan deal
+with it - the usual fallback position.
+
+Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
+reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
+However, the scan can terminate when it encounters a VM_LOCKED VMA and can
+successfully acquire the VMA's mmap semphore for read and mlock the page.
+Although try_to_munlock() might be called a great many times when munlocking a
+large region or tearing down a large address space that has been mlocked via
+mlockall(), overall this is a fairly rare event.
+
+
+PAGE RECLAIM IN shrink_*_list()
+-------------------------------
+
+shrink_active_list() culls any obviously unevictable pages - i.e.
+!page_evictable(page, NULL) - diverting these to the unevictable list.
+However, shrink_active_list() only sees unevictable pages that made it onto the
+active/inactive lru lists.  Note that these pages do not have PageUnevictable
+set - otherwise they would be on the unevictable list and shrink_active_list
+would never see them.
 
 Some examples of these unevictable pages on the LRU lists are:
 
-1) ramfs pages that have been placed on the lru lists when first allocated.
+ (1) ramfs pages that have been placed on the LRU lists when first allocated.
+
+ (2) SHM_LOCK'd shared memory pages.  shmctl(SHM_LOCK) does not attempt to
+     allocate or fault in the pages in the shared memory region.  This happens
+     when an application accesses the page the first time after SHM_LOCK'ing
+     the segment.
 
-2) SHM_LOCKed shared memory pages.  shmctl(SHM_LOCK) does not attempt to
-   allocate or fault in the pages in the shared memory region.  This happens
-   when an application accesses the page the first time after SHM_LOCKing
-   the segment.
+ (3) mlocked pages that could not be isolated from the LRU and moved to the
+     unevictable list in mlock_vma_page().
 
-3) Mlocked pages that could not be isolated from the lru and moved to the
-   unevictable list in mlock_vma_page().
+ (4) Pages mapped into multiple VM_LOCKED VMAs, but try_to_munlock() couldn't
+     acquire the VMA's mmap semaphore to test the flags and set PageMlocked.
+     munlock_vma_page() was forced to let the page back on to the normal LRU
+     list for vmscan to handle.
 
-3) Pages mapped into multiple VM_LOCKED vmas, but try_to_munlock() couldn't
-   acquire the vma's mmap semaphore to test the flags and set PageMlocked.
-   munlock_vma_page() was forced to let the page back on to the normal
-   LRU list for vmscan to handle.
+shrink_inactive_list() also diverts any unevictable pages that it finds on the
+inactive lists to the appropriate zone's unevictable list.
 
-shrink_inactive_list() also culls any unevictable pages that it finds on
-the inactive lists, again diverting them to the appropriate zone's unevictable
-lru list.  shrink_inactive_list() should only see SHM_LOCKed pages that became
-SHM_LOCKed after shrink_active_list() had moved them to the inactive list, or
-pages mapped into VM_LOCKED vmas that munlock_vma_page() couldn't isolate from
-the lru to recheck via try_to_munlock().  shrink_inactive_list() won't notice
-the latter, but will pass on to shrink_page_list().
+shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
+after shrink_active_list() had moved them to the inactive list, or pages mapped
+into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
+recheck via try_to_munlock().  shrink_inactive_list() won't notice the latter,
+but will pass on to shrink_page_list().
 
 shrink_page_list() again culls obviously unevictable pages that it could
 encounter for similar reason to shrink_inactive_list().  Pages mapped into
-VM_LOCKED vmas but without PG_mlocked set will make it all the way to
+VM_LOCKED VMAs but without PG_mlocked set will make it all the way to
 try_to_unmap().  shrink_page_list() will divert them to the unevictable list
 when try_to_unmap() returns SWAP_MLOCK, as discussed above.