Documentation: vm, add hugetlbfs reservation overview

Adding a brief overview of hugetlbfs reservation design and
implementation as an aid to those making code modifications in this
area.

Link: http://lkml.kernel.org/r/1491586995-13085-1-git-send-email-mike.kravetz@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 531 insertions, 0 deletions

diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX
index 6a5e2a1..11d3d8d 100644
--- a/Documentation/vm/00-INDEX
+++ b/Documentation/vm/00-INDEX
@@ -12,6 +12,8 @@ highmem.txt
 	- Outline of highmem and common issues.
 hugetlbpage.txt
 	- a brief summary of hugetlbpage support in the Linux kernel.
+hugetlbfs_reserv.txt
+	- A brief overview of hugetlbfs reservation design/implementation.
 hwpoison.txt
 	- explains what hwpoison is
 idle_page_tracking.txt
diff --git a/Documentation/vm/hugetlbfs_reserv.txt b/Documentation/vm/hugetlbfs_reserv.txt
new file mode 100644
index 0000000..9aca09a
--- /dev/null
+++ b/Documentation/vm/hugetlbfs_reserv.txt
@@ -0,0 +1,529 @@
+Hugetlbfs Reservation Overview
+------------------------------
+Huge pages as described at 'Documentation/vm/hugetlbpage.txt' are typically
+preallocated for application use.  These huge pages are instantiated in a
+task's address space at page fault time if the VMA indicates huge pages are
+to be used.  If no huge page exists at page fault time, the task is sent
+a SIGBUS and often dies an unhappy death.  Shortly after huge page support
+was added, it was determined that it would be better to detect a shortage
+of huge pages at mmap() time.  The idea is that if there were not enough
+huge pages to cover the mapping, the mmap() would fail.  This was first
+done with a simple check in the code at mmap() time to determine if there
+were enough free huge pages to cover the mapping.  Like most things in the
+kernel, the code has evolved over time.  However, the basic idea was to
+'reserve' huge pages at mmap() time to ensure that huge pages would be
+available for page faults in that mapping.  The description below attempts to
+describe how huge page reserve processing is done in the v4.10 kernel.
+
+
+Audience
+--------
+This description is primarily targeted at kernel developers who are modifying
+hugetlbfs code.
+
+
+The Data Structures
+-------------------
+resv_huge_pages
+	This is a global (per-hstate) count of reserved huge pages.  Reserved
+	huge pages are only available to the task which reserved them.
+	Therefore, the number of huge pages generally available is computed
+	as (free_huge_pages - resv_huge_pages).
+Reserve Map
+	A reserve map is described by the structure:
+	struct resv_map {
+		struct kref refs;
+		spinlock_t lock;
+		struct list_head regions;
+		long adds_in_progress;
+		struct list_head region_cache;
+		long region_cache_count;
+	};
+	There is one reserve map for each huge page mapping in the system.
+	The regions list within the resv_map describes the regions within
+	the mapping.  A region is described as:
+	struct file_region {
+		struct list_head link;
+		long from;
+		long to;
+	};
+	The 'from' and 'to' fields of the file region structure are huge page
+	indices into the mapping.  Depending on the type of mapping, a
+	region in the reserv_map may indicate reservations exist for the
+	range, or reservations do not exist.
+Flags for MAP_PRIVATE Reservations
+	These are stored in the bottom bits of the reservation map pointer.
+	#define HPAGE_RESV_OWNER    (1UL << 0) Indicates this task is the
+		owner of the reservations associated with the mapping.
+	#define HPAGE_RESV_UNMAPPED (1UL << 1) Indicates task originally
+		mapping this range (and creating reserves) has unmapped a
+		page from this task (the child) due to a failed COW.
+Page Flags
+	The PagePrivate page flag is used to indicate that a huge page
+	reservation must be restored when the huge page is freed.  More
+	details will be discussed in the "Freeing huge pages" section.
+
+
+Reservation Map Location (Private or Shared)
+--------------------------------------------
+A huge page mapping or segment is either private or shared.  If private,
+it is typically only available to a single address space (task).  If shared,
+it can be mapped into multiple address spaces (tasks).  The location and
+semantics of the reservation map is significantly different for two types
+of mappings.  Location differences are:
+- For private mappings, the reservation map hangs off the the VMA structure.
+  Specifically, vma->vm_private_data.  This reserve map is created at the
+  time the mapping (mmap(MAP_PRIVATE)) is created.
+- For shared mappings, the reservation map hangs off the inode.  Specifically,
+  inode->i_mapping->private_data.  Since shared mappings are always backed
+  by files in the hugetlbfs filesystem, the hugetlbfs code ensures each inode
+  contains a reservation map.  As a result, the reservation map is allocated
+  when the inode is created.
+
+
+Creating Reservations
+---------------------
+Reservations are created when a huge page backed shared memory segment is
+created (shmget(SHM_HUGETLB)) or a mapping is created via mmap(MAP_HUGETLB).
+These operations result in a call to the routine hugetlb_reserve_pages()
+
+int hugetlb_reserve_pages(struct inode *inode,
+					long from, long to,
+					struct vm_area_struct *vma,
+					vm_flags_t vm_flags)
+
+The first thing hugetlb_reserve_pages() does is check for the NORESERVE
+flag was specified in either the shmget() or mmap() call.  If NORESERVE
+was specified, then this routine returns immediately as no reservation
+are desired.
+
+The arguments 'from' and 'to' are huge page indices into the mapping or
+underlying file.  For shmget(), 'from' is always 0 and 'to' corresponds to
+the length of the segment/mapping.  For mmap(), the offset argument could
+be used to specify the offset into the underlying file.  In such a case
+the 'from' and 'to' arguments have been adjusted by this offset.
+
+One of the big differences between PRIVATE and SHARED mappings is the way
+in which reservations are represented in the reservation map.
+- For shared mappings, an entry in the reservation map indicates a reservation
+  exists or did exist for the corresponding page.  As reservations are
+  consumed, the reservation map is not modified.
+- For private mappings, the lack of an entry in the reservation map indicates
+  a reservation exists for the corresponding page.  As reservations are
+  consumed, entries are added to the reservation map.  Therefore, the
+  reservation map can also be used to determine which reservations have
+  been consumed.
+
+For private mappings, hugetlb_reserve_pages() creates the reservation map and
+hangs it off the VMA structure.  In addition, the HPAGE_RESV_OWNER flag is set
+to indicate this VMA owns the reservations.
+
+The reservation map is consulted to determine how many huge page reservations
+are needed for the current mapping/segment.  For private mappings, this is
+always the value (to - from).  However, for shared mappings it is possible that some reservations may already exist within the range (to - from).  See the
+section "Reservation Map Modifications" for details on how this is accomplished.
+
+The mapping may be associated with a subpool.  If so, the subpool is consulted
+to ensure there is sufficient space for the mapping.  It is possible that the
+subpool has set aside reservations that can be used for the mapping.  See the
+section "Subpool Reservations" for more details.
+
+After consulting the reservation map and subpool, the number of needed new
+reservations is known.  The routine hugetlb_acct_memory() is called to check
+for and take the requested number of reservations.  hugetlb_acct_memory()
+calls into routines that potentially allocate and adjust surplus page counts.
+However, within those routines the code is simply checking to ensure there
+are enough free huge pages to accommodate the reservation.  If there are,
+the global reservation count resv_huge_pages is adjusted something like the
+following.
+	if (resv_needed <= (resv_huge_pages - free_huge_pages))
+		resv_huge_pages += resv_needed;
+Note that the global lock hugetlb_lock is held when checking and adjusting
+these counters.
+
+If there were enough free huge pages and the global count resv_huge_pages
+was adjusted, then the reservation map associated with the mapping is
+modified to reflect the reservations.  In the case of a shared mapping, a
+file_region will exist that includes the range 'from' 'to'.  For private
+mappings, no modifications are made to the reservation map as lack of an
+entry indicates a reservation exists.
+
+If hugetlb_reserve_pages() was successful, the global reservation count and
+reservation map associated with the mapping will be modified as required to
+ensure reservations exist for the range 'from' - 'to'.
+
+
+Consuming Reservations/Allocating a Huge Page
+---------------------------------------------
+Reservations are consumed when huge pages associated with the reservations
+are allocated and instantiated in the corresponding mapping.  The allocation
+is performed within the routine alloc_huge_page().
+struct page *alloc_huge_page(struct vm_area_struct *vma,
+                                    unsigned long addr, int avoid_reserve)
+alloc_huge_page is passed a VMA pointer and a virtual address, so it can
+consult the reservation map to determine if a reservation exists.  In addition,
+alloc_huge_page takes the argument avoid_reserve which indicates reserves
+should not be used even if it appears they have been set aside for the
+specified address.  The avoid_reserve argument is most often used in the case
+of Copy on Write and Page Migration where additional copies of an existing
+page are being allocated.
+
+The helper routine vma_needs_reservation() is called to determine if a
+reservation exists for the address within the mapping(vma).  See the section
+"Reservation Map Helper Routines" for detailed information on what this
+routine does.  The value returned from vma_needs_reservation() is generally
+0 or 1.  0 if a reservation exists for the address, 1 if no reservation exists.
+If a reservation does not exist, and there is a subpool associated with the
+mapping the subpool is consulted to determine if it contains reservations.
+If the subpool contains reservations, one can be used for this allocation.
+However, in every case the avoid_reserve argument overrides the use of
+a reservation for the allocation.  After determining whether a reservation
+exists and can be used for the allocation, the routine dequeue_huge_page_vma()
+is called.  This routine takes two arguments related to reservations:
+- avoid_reserve, this is the same value/argument passed to alloc_huge_page()
+- chg, even though this argument is of type long only the values 0 or 1 are
+  passed to dequeue_huge_page_vma.  If the value is 0, it indicates a
+  reservation exists (see the section "Memory Policy and Reservations" for
+  possible issues).  If the value is 1, it indicates a reservation does not
+  exist and the page must be taken from the global free pool if possible.
+The free lists associated with the memory policy of the VMA are searched for
+a free page.  If a page is found, the value free_huge_pages is decremented
+when the page is removed from the free list.  If there was a reservation
+associated with the page, the following adjustments are made:
+	SetPagePrivate(page);	/* Indicates allocating this page consumed
+				 * a reservation, and if an error is
+				 * encountered such that the page must be
+				 * freed, the reservation will be restored. */
+	resv_huge_pages--;	/* Decrement the global reservation count */
+Note, if no huge page can be found that satisfies the VMA's memory policy
+an attempt will be made to allocate one using the buddy allocator.  This
+brings up the issue of surplus huge pages and overcommit which is beyond
+the scope reservations.  Even if a surplus page is allocated, the same
+reservation based adjustments as above will be made: SetPagePrivate(page) and
+resv_huge_pages--.
+
+After obtaining a new huge page, (page)->private is set to the value of
+the subpool associated with the page if it exists.  This will be used for
+subpool accounting when the page is freed.
+
+The routine vma_commit_reservation() is then called to adjust the reserve
+map based on the consumption of the reservation.  In general, this involves
+ensuring the page is represented within a file_region structure of the region
+map.  For shared mappings where the the reservation was present, an entry
+in the reserve map already existed so no change is made.  However, if there
+was no reservation in a shared mapping or this was a private mapping a new
+entry must be created.
+
+It is possible that the reserve map could have been changed between the call
+to vma_needs_reservation() at the beginning of alloc_huge_page() and the
+call to vma_commit_reservation() after the page was allocated.  This would
+be possible if hugetlb_reserve_pages was called for the same page in a shared
+mapping.  In such cases, the reservation count and subpool free page count
+will be off by one.  This rare condition can be identified by comparing the
+return value from vma_needs_reservation and vma_commit_reservation.  If such
+a race is detected, the subpool and global reserve counts are adjusted to
+compensate.  See the section "Reservation Map Helper Routines" for more
+information on these routines.
+
+
+Instantiate Huge Pages
+----------------------
+After huge page allocation, the page is typically added to the page tables
+of the allocating task.  Before this, pages in a shared mapping are added
+to the page cache and pages in private mappings are added to an anonymous
+reverse mapping.  In both cases, the PagePrivate flag is cleared.  Therefore,
+when a huge page that has been instantiated is freed no adjustment is made
+to the global reservation count (resv_huge_pages).
+
+
+Freeing Huge Pages
+------------------
+Huge page freeing is performed by the routine free_huge_page().  This routine
+is the destructor for hugetlbfs compound pages.  As a result, it is only
+passed a pointer to the page struct.  When a huge page is freed, reservation
+accounting may need to be performed.  This would be the case if the page was
+associated with a subpool that contained reserves, or the page is being freed
+on an error path where a global reserve count must be restored.
+
+The page->private field points to any subpool associated with the page.
+If the PagePrivate flag is set, it indicates the global reserve count should
+be adjusted (see the section "Consuming Reservations/Allocating a Huge Page"
+for information on how these are set).
+
+The routine first calls hugepage_subpool_put_pages() for the page.  If this
+routine returns a value of 0 (which does not equal the value passed 1) it
+indicates reserves are associated with the subpool, and this newly free page
+must be used to keep the number of subpool reserves above the minimum size.
+Therefore, the global resv_huge_pages counter is incremented in this case.
+
+If the PagePrivate flag was set in the page, the global resv_huge_pages counter
+will always be incremented.
+
+
+Subpool Reservations
+--------------------
+There is a struct hstate associated with each huge page size.  The hstate
+tracks all huge pages of the specified size.  A subpool represents a subset
+of pages within a hstate that is associated with a mounted hugetlbfs
+filesystem.
+
+When a hugetlbfs filesystem is mounted a min_size option can be specified
+which indicates the minimum number of huge pages required by the filesystem.
+If this option is specified, the number of huge pages corresponding to
+min_size are reserved for use by the filesystem.  This number is tracked in
+the min_hpages field of a struct hugepage_subpool.  At mount time,
+hugetlb_acct_memory(min_hpages) is called to reserve the specified number of
+huge pages.  If they can not be reserved, the mount fails.
+
+The routines hugepage_subpool_get/put_pages() are called when pages are
+obtained from or released back to a subpool.  They perform all subpool
+accounting, and track any reservations associated with the subpool.
+hugepage_subpool_get/put_pages are passed the number of huge pages by which
+to adjust the subpool 'used page' count (down for get, up for put).  Normally,
+they return the same value that was passed or an error if not enough pages
+exist in the subpool.
+
+However, if reserves are associated with the subpool a return value less
+than the passed value may be returned.  This return value indicates the
+number of additional global pool adjustments which must be made.  For example,
+suppose a subpool contains 3 reserved huge pages and someone asks for 5.
+The 3 reserved pages associated with the subpool can be used to satisfy part
+of the request.  But, 2 pages must be obtained from the global pools.  To
+relay this information to the caller, the value 2 is returned.  The caller
+is then responsible for attempting to obtain the additional two pages from
+the global pools.
+
+
+COW and Reservations
+--------------------
+Since shared mappings all point to and use the same underlying pages, the
+biggest reservation concern for COW is private mappings.  In this case,
+two tasks can be pointing at the same previously allocated page.  One task
+attempts to write to the page, so a new page must be allocated so that each
+task points to its own page.
+
+When the page was originally allocated, the reservation for that page was
+consumed.  When an attempt to allocate a new page is made as a result of
+COW, it is possible that no free huge pages are free and the allocation
+will fail.
+
+When the private mapping was originally created, the owner of the mapping
+was noted by setting the HPAGE_RESV_OWNER bit in the pointer to the reservation
+map of the owner.  Since the owner created the mapping, the owner owns all
+the reservations associated with the mapping.  Therefore, when a write fault
+occurs and there is no page available, different action is taken for the owner
+and non-owner of the reservation.
+
+In the case where the faulting task is not the owner, the fault will fail and
+the task will typically receive a SIGBUS.
+
+If the owner is the faulting task, we want it to succeed since it owned the
+original reservation.  To accomplish this, the page is unmapped from the
+non-owning task.  In this way, the only reference is from the owning task.
+In addition, the HPAGE_RESV_UNMAPPED bit is set in the reservation map pointer
+of the non-owning task.  The non-owning task may receive a SIGBUS if it later
+faults on a non-present page.  But, the original owner of the
+mapping/reservation will behave as expected.
+
+
+Reservation Map Modifications
+-----------------------------
+The following low level routines are used to make modifications to a
+reservation map.  Typically, these routines are not called directly.  Rather,
+a reservation map helper routine is called which calls one of these low level
+routines.  These low level routines are fairly well documented in the source
+code (mm/hugetlb.c).  These routines are:
+long region_chg(struct resv_map *resv, long f, long t);
+long region_add(struct resv_map *resv, long f, long t);
+void region_abort(struct resv_map *resv, long f, long t);
+long region_count(struct resv_map *resv, long f, long t);
+
+Operations on the reservation map typically involve two operations:
+1) region_chg() is called to examine the reserve map and determine how
+   many pages in the specified range [f, t) are NOT currently represented.
+
+   The calling code performs global checks and allocations to determine if
+   there are enough huge pages for the operation to succeed.
+
+2a) If the operation can succeed, region_add() is called to actually modify
+    the reservation map for the same range [f, t) previously passed to
+    region_chg().
+2b) If the operation can not succeed, region_abort is called for the same range
+    [f, t) to abort the operation.
+
+Note that this is a two step process where region_add() and region_abort()
+are guaranteed to succeed after a prior call to region_chg() for the same
+range.  region_chg() is responsible for pre-allocating any data structures
+necessary to ensure the subsequent operations (specifically region_add()))
+will succeed.
+
+As mentioned above, region_chg() determines the number of pages in the range
+which are NOT currently represented in the map.  This number is returned to
+the caller.  region_add() returns the number of pages in the range added to
+the map.  In most cases, the return value of region_add() is the same as the
+return value of region_chg().  However, in the case of shared mappings it is
+possible for changes to the reservation map to be made between the calls to
+region_chg() and region_add().  In this case, the return value of region_add()
+will not match the return value of region_chg().  It is likely that in such
+cases global counts and subpool accounting will be incorrect and in need of
+adjustment.  It is the responsibility of the caller to check for this condition
+and make the appropriate adjustments.
+
+The routine region_del() is called to remove regions from a reservation map.
+It is typically called in the following situations:
+- When a file in the hugetlbfs filesystem is being removed, the inode will
+  be released and the reservation map freed.  Before freeing the reservation
+  map, all the individual file_region structures must be freed.  In this case
+  region_del is passed the range [0, LONG_MAX).
+- When a hugetlbfs file is being truncated.  In this case, all allocated pages
+  after the new file size must be freed.  In addition, any file_region entries
+  in the reservation map past the new end of file must be deleted.  In this
+  case, region_del is passed the range [new_end_of_file, LONG_MAX).
+- When a hole is being punched in a hugetlbfs file.  In this case, huge pages
+  are removed from the middle of the file one at a time.  As the pages are
+  removed, region_del() is called to remove the corresponding entry from the
+  reservation map.  In this case, region_del is passed the range
+  [page_idx, page_idx + 1).
+In every case, region_del() will return the number of pages removed from the
+reservation map.  In VERY rare cases, region_del() can fail.  This can only
+happen in the hole punch case where it has to split an existing file_region
+entry and can not allocate a new structure.  In this error case, region_del()
+will return -ENOMEM.  The problem here is that the reservation map will
+indicate that there is a reservation for the page.  However, the subpool and
+global reservation counts will not reflect the reservation.  To handle this
+situation, the routine hugetlb_fix_reserve_counts() is called to adjust the
+counters so that they correspond with the reservation map entry that could
+not be deleted.
+
+region_count() is called when unmapping a private huge page mapping.  In
+private mappings, the lack of a entry in the reservation map indicates that
+a reservation exists.  Therefore, by counting the number of entries in the
+reservation map we know how many reservations were consumed and how many are
+outstanding (outstanding = (end - start) - region_count(resv, start, end)).
+Since the mapping is going away, the subpool and global reservation counts
+are decremented by the number of outstanding reservations.
+
+
+Reservation Map Helper Routines
+-------------------------------
+Several helper routines exist to query and modify the reservation maps.
+These routines are only interested with reservations for a specific huge
+page, so they just pass in an address instead of a range.  In addition,
+they pass in the associated VMA.  From the VMA, the type of mapping (private
+or shared) and the location of the reservation map (inode or VMA) can be
+determined.  These routines simply call the underlying routines described
+in the section "Reservation Map Modifications".  However, they do take into
+account the 'opposite' meaning of reservation map entries for private and
+shared mappings and hide this detail from the caller.
+
+long vma_needs_reservation(struct hstate *h,
+				struct vm_area_struct *vma, unsigned long addr)
+This routine calls region_chg() for the specified page.  If no reservation
+exists, 1 is returned.  If a reservation exists, 0 is returned.
+
+long vma_commit_reservation(struct hstate *h,
+				struct vm_area_struct *vma, unsigned long addr)
+This calls region_add() for the specified page.  As in the case of region_chg
+and region_add, this routine is to be called after a previous call to
+vma_needs_reservation.  It will add a reservation entry for the page.  It
+returns 1 if the reservation was added and 0 if not.  The return value should
+be compared with the return value of the previous call to
+vma_needs_reservation.  An unexpected difference indicates the reservation
+map was modified between calls.
+
+void vma_end_reservation(struct hstate *h,
+				struct vm_area_struct *vma, unsigned long addr)
+This calls region_abort() for the specified page.  As in the case of region_chg
+and region_abort, this routine is to be called after a previous call to
+vma_needs_reservation.  It will abort/end the in progress reservation add
+operation.
+
+long vma_add_reservation(struct hstate *h,
+				struct vm_area_struct *vma, unsigned long addr)
+This is a special wrapper routine to help facilitate reservation cleanup
+on error paths.  It is only called from the routine restore_reserve_on_error().
+This routine is used in conjunction with vma_needs_reservation in an attempt
+to add a reservation to the reservation map.  It takes into account the
+different reservation map semantics for private and shared mappings.  Hence,
+region_add is called for shared mappings (as an entry present in the map
+indicates a reservation), and region_del is called for private mappings (as
+the absence of an entry in the map indicates a reservation).  See the section
+"Reservation cleanup in error paths" for more information on what needs to
+be done on error paths.
+
+
+Reservation Cleanup in Error Paths
+----------------------------------
+As mentioned in the section "Reservation Map Helper Routines", reservation
+map modifications are performed in two steps.  First vma_needs_reservation
+is called before a page is allocated.  If the allocation is successful,
+then vma_commit_reservation is called.  If not, vma_end_reservation is called.
+Global and subpool reservation counts are adjusted based on success or failure
+of the operation and all is well.
+
+Additionally, after a huge page is instantiated the PagePrivate flag is
+cleared so that accounting when the page is ultimately freed is correct.
+
+However, there are several instances where errors are encountered after a huge
+page is allocated but before it is instantiated.  In this case, the page
+allocation has consumed the reservation and made the appropriate subpool,
+reservation map and global count adjustments.  If the page is freed at this
+time (before instantiation and clearing of PagePrivate), then free_huge_page
+will increment the global reservation count.  However, the reservation map
+indicates the reservation was consumed.  This resulting inconsistent state
+will cause the 'leak' of a reserved huge page.  The global reserve count will
+be  higher than it should and prevent allocation of a pre-allocated page.
+
+The routine restore_reserve_on_error() attempts to handle this situation.  It
+is fairly well documented.  The intention of this routine is to restore
+the reservation map to the way it was before the page allocation.   In this
+way, the state of the reservation map will correspond to the global reservation
+count after the page is freed.
+
+The routine restore_reserve_on_error itself may encounter errors while
+attempting to restore the reservation map entry.  In this case, it will
+simply clear the PagePrivate flag of the page.  In this way, the global
+reserve count will not be incremented when the page is freed.  However, the
+reservation map will continue to look as though the reservation was consumed.
+A page can still be allocated for the address, but it will not use a reserved
+page as originally intended.
+
+There is some code (most notably userfaultfd) which can not call
+restore_reserve_on_error.  In this case, it simply modifies the PagePrivate
+so that a reservation will not be leaked when the huge page is freed.
+
+
+Reservations and Memory Policy
+------------------------------
+Per-node huge page lists existed in struct hstate when git was first used
+to manage Linux code.  The concept of reservations was added some time later.
+When reservations were added, no attempt was made to take memory policy
+into account.  While cpusets are not exactly the same as memory policy, this
+comment in hugetlb_acct_memory sums up the interaction between reservations
+and cpusets/memory policy.
+	/*
+	 * When cpuset is configured, it breaks the strict hugetlb page
+	 * reservation as the accounting is done on a global variable. Such
+	 * reservation is completely rubbish in the presence of cpuset because
+	 * the reservation is not checked against page availability for the
+	 * current cpuset. Application can still potentially OOM'ed by kernel
+	 * with lack of free htlb page in cpuset that the task is in.
+	 * Attempt to enforce strict accounting with cpuset is almost
+	 * impossible (or too ugly) because cpuset is too fluid that
+	 * task or memory node can be dynamically moved between cpusets.
+	 *
+	 * The change of semantics for shared hugetlb mapping with cpuset is
+	 * undesirable. However, in order to preserve some of the semantics,
+	 * we fall back to check against current free page availability as
+	 * a best attempt and hopefully to minimize the impact of changing
+	 * semantics that cpuset has.
+	 */
+
+Huge page reservations were added to prevent unexpected page allocation
+failures (OOM) at page fault time.  However, if an application makes use
+of cpusets or memory policy there is no guarantee that huge pages will be
+available on the required nodes.  This is true even if there are a sufficient
+number of global reservations.
+
+
+Mike Kravetz, 7 April 2017