ksm: let shared pages be swappable

Initial implementation for swapping out KSM's shared pages: add
page_referenced_ksm() and try_to_unmap_ksm(), which rmap.c calls when
faced with a PageKsm page.

Most of what's needed can be got from the rmap_items listed from the
stable_node of the ksm page, without discovering the actual vma: so in
this patch just fake up a struct vma for page_referenced_one() or
try_to_unmap_one(), then refine that in the next patch.

Add VM_NONLINEAR to ksm_madvise()'s list of exclusions: it has always been
implicit there (being only set with VM_SHARED, already excluded), but
let's make it explicit, to help justify the lack of nonlinear unmap.

Rely on the page lock to protect against concurrent modifications to that
page's node of the stable tree.

The awkward part is not swapout but swapin: do_swap_page() and
page_add_anon_rmap() now have to allow for new possibilities - perhaps a
ksm page still in swapcache, perhaps a swapcache page associated with one
location in one anon_vma now needed for another location or anon_vma.
(And the vma might even be no longer VM_MERGEABLE when that happens.)

ksm_might_need_to_copy() checks for that case, and supplies a duplicate
page when necessary, simply leaving it to a subsequent pass of ksmd to
rediscover the identity and merge them back into one ksm page.
Disappointingly primitive: but the alternative would have to accumulate
unswappable info about the swapped out ksm pages, limiting swappability.

Remove page_add_ksm_rmap(): page_add_anon_rmap() now has to allow for the
particular case it was handling, so just use it instead.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 27 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index ebee816..869aaa3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -49,6 +49,7 @@
 #include <linux/swapops.h>
 #include <linux/slab.h>
 #include <linux/init.h>
+#include <linux/ksm.h>
 #include <linux/rmap.h>
 #include <linux/rcupdate.h>
 #include <linux/module.h>
@@ -336,9 +337,9 @@ int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma)
  * Subfunctions of page_referenced: page_referenced_one called
  * repeatedly from either page_referenced_anon or page_referenced_file.
  */
-static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
-			       unsigned long address, unsigned int *mapcount,
-			       unsigned long *vm_flags)
+int page_referenced_one(struct page *page, struct vm_area_struct *vma,
+			unsigned long address, unsigned int *mapcount,
+			unsigned long *vm_flags)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	pte_t *pte;
@@ -507,28 +508,33 @@ int page_referenced(struct page *page,
 		    unsigned long *vm_flags)
 {
 	int referenced = 0;
+	int we_locked = 0;
 
 	if (TestClearPageReferenced(page))
 		referenced++;
 
 	*vm_flags = 0;
 	if (page_mapped(page) && page_rmapping(page)) {
-		if (PageAnon(page))
+		if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
+			we_locked = trylock_page(page);
+			if (!we_locked) {
+				referenced++;
+				goto out;
+			}
+		}
+		if (unlikely(PageKsm(page)))
+			referenced += page_referenced_ksm(page, mem_cont,
+								vm_flags);
+		else if (PageAnon(page))
 			referenced += page_referenced_anon(page, mem_cont,
 								vm_flags);
-		else if (is_locked)
+		else if (page->mapping)
 			referenced += page_referenced_file(page, mem_cont,
 								vm_flags);
-		else if (!trylock_page(page))
-			referenced++;
-		else {
-			if (page->mapping)
-				referenced += page_referenced_file(page,
-							mem_cont, vm_flags);
+		if (we_locked)
 			unlock_page(page);
-		}
 	}
-
+out:
 	if (page_test_and_clear_young(page))
 		referenced++;
 
@@ -620,14 +626,7 @@ static void __page_set_anon_rmap(struct page *page,
 	BUG_ON(!anon_vma);
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;
-
 	page->index = linear_page_index(vma, address);
-
-	/*
-	 * nr_mapped state can be updated without turning off
-	 * interrupts because it is not modified via interrupt.
-	 */
-	__inc_zone_page_state(page, NR_ANON_PAGES);
 }
 
 /**
@@ -665,14 +664,21 @@ static void __page_check_anon_rmap(struct page *page,
  * @vma:	the vm area in which the mapping is added
  * @address:	the user virtual address mapped
  *
- * The caller needs to hold the pte lock and the page must be locked.
+ * The caller needs to hold the pte lock, and the page must be locked in
+ * the anon_vma case: to serialize mapping,index checking after setting.
  */
 void page_add_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address)
 {
+	int first = atomic_inc_and_test(&page->_mapcount);
+	if (first)
+		__inc_zone_page_state(page, NR_ANON_PAGES);
+	if (unlikely(PageKsm(page)))
+		return;
+
 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
-	if (atomic_inc_and_test(&page->_mapcount))
+	if (first)
 		__page_set_anon_rmap(page, vma, address);
 	else
 		__page_check_anon_rmap(page, vma, address);
@@ -694,6 +700,7 @@ void page_add_new_anon_rmap(struct page *page,
 	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	SetPageSwapBacked(page);
 	atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
+	__inc_zone_page_state(page, NR_ANON_PAGES);
 	__page_set_anon_rmap(page, vma, address);
 	if (page_evictable(page, vma))
 		lru_cache_add_lru(page, LRU_ACTIVE_ANON);
@@ -760,8 +767,8 @@ void page_remove_rmap(struct page *page)
  * Subfunctions of try_to_unmap: try_to_unmap_one called
  * repeatedly from either try_to_unmap_anon or try_to_unmap_file.
  */
-static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
-			    unsigned long address, enum ttu_flags flags)
+int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
+		     unsigned long address, enum ttu_flags flags)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	pte_t *pte;
@@ -1156,7 +1163,9 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
 
 	BUG_ON(!PageLocked(page));
 
-	if (PageAnon(page))
+	if (unlikely(PageKsm(page)))
+		ret = try_to_unmap_ksm(page, flags);
+	else if (PageAnon(page))
 		ret = try_to_unmap_anon(page, flags);
 	else
 		ret = try_to_unmap_file(page, flags);
@@ -1177,15 +1186,17 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
  *
  * SWAP_AGAIN	- no vma is holding page mlocked, or,
  * SWAP_AGAIN	- page mapped in mlocked vma -- couldn't acquire mmap sem
+ * SWAP_FAIL	- page cannot be located at present
  * SWAP_MLOCK	- page is now mlocked.
  */
 int try_to_munlock(struct page *page)
 {
 	VM_BUG_ON(!PageLocked(page) || PageLRU(page));
 
-	if (PageAnon(page))
+	if (unlikely(PageKsm(page)))
+		return try_to_unmap_ksm(page, TTU_MUNLOCK);
+	else if (PageAnon(page))
 		return try_to_unmap_anon(page, TTU_MUNLOCK);
 	else
 		return try_to_unmap_file(page, TTU_MUNLOCK);
 }
-