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authorHugh Dickins <hugh.dickins@tiscali.co.uk>2009-12-14 17:59:29 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2009-12-15 08:53:19 -0800
commit80e148226028257ec0a1909d99b2c40d0ffe17f2 (patch)
tree1e3cae42144f5c80e215ba254e01bd6847ba1b36 /mm/rmap.c
parent4035c07a895974d0ac06a56fe870ad293fc451a7 (diff)
ksm: share anon page without allocating
When ksm pages were unswappable, it made no sense to include them in mem cgroup accounting; but now that they are swappable (although I see no strict logical connection) the principle of least surprise implies that they should be accounted (with the usual dissatisfaction, that a shared page is accounted to only one of the cgroups using it). This patch was intended to add mem cgroup accounting where necessary; but turned inside out, it now avoids allocating a ksm page, instead upgrading an anon page to ksm - which brings its existing mem cgroup accounting with it. Thus mem cgroups don't appear in the patch at all. This upgrade from PageAnon to PageKsm takes place under page lock (via a somewhat hacky NULL kpage interface), and audit showed only one place which needed to cope with the race - page_referenced() is sometimes used without page lock, so page_lock_anon_vma() needs an ACCESS_ONCE() to be sure of getting anon_vma and flags together (no problem if the page goes ksm an instant after, the integrity of that anon_vma list is unaffected). 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>
Diffstat (limited to 'mm/rmap.c')
-rw-r--r--mm/rmap.c6
1 files changed, 4 insertions, 2 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index ebdf582ef185..2e38e9048327 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -204,7 +204,7 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
unsigned long anon_mapping;
rcu_read_lock();
- anon_mapping = (unsigned long) page->mapping;
+ anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
goto out;
if (!page_mapped(page))
@@ -666,7 +666,9 @@ static void __page_check_anon_rmap(struct page *page,
* @address: the user virtual address mapped
*
* 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.
+ * the anon_vma case: to serialize mapping,index checking after setting,
+ * and to ensure that PageAnon is not being upgraded racily to PageKsm
+ * (but PageKsm is never downgraded to PageAnon).
*/
void page_add_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)

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