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authorKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>2012-03-21 16:34:25 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-03-21 17:55:01 -0700
commit89c06bd52fb9ffceddf84f7309d2e8c9f1666216 (patch)
tree43ec3d97a89988bc143bed5796bcd7bef64212dc /mm/rmap.c
parent312734c04e2fecc58429aec98194e4ff12d8f7d6 (diff)
downloadlwn-89c06bd52fb9ffceddf84f7309d2e8c9f1666216.tar.gz
lwn-89c06bd52fb9ffceddf84f7309d2e8c9f1666216.zip
memcg: use new logic for page stat accounting
Now, page-stat-per-memcg is recorded into per page_cgroup flag by duplicating page's status into the flag. The reason is that memcg has a feature to move a page from a group to another group and we have race between "move" and "page stat accounting", Under current logic, assume CPU-A and CPU-B. CPU-A does "move" and CPU-B does "page stat accounting". When CPU-A goes 1st, CPU-A CPU-B update "struct page" info. move_lock_mem_cgroup(memcg) see pc->flags copy page stat to new group overwrite pc->mem_cgroup. move_unlock_mem_cgroup(memcg) move_lock_mem_cgroup(mem) set pc->flags update page stat accounting move_unlock_mem_cgroup(mem) stat accounting is guarded by move_lock_mem_cgroup() and "move" logic (CPU-A) doesn't see changes in "struct page" information. But it's costly to have the same information both in 'struct page' and 'struct page_cgroup'. And, there is a potential problem. For example, assume we have PG_dirty accounting in memcg. PG_..is a flag for struct page. PCG_ is a flag for struct page_cgroup. (This is just an example. The same problem can be found in any kind of page stat accounting.) CPU-A CPU-B TestSet PG_dirty (delay) TestClear PG_dirty if (TestClear(PCG_dirty)) memcg->nr_dirty-- if (TestSet(PCG_dirty)) memcg->nr_dirty++ Here, memcg->nr_dirty = +1, this is wrong. This race was reported by Greg Thelen <gthelen@google.com>. Now, only FILE_MAPPED is supported but fortunately, it's serialized by page table lock and this is not real bug, _now_, If this potential problem is caused by having duplicated information in struct page and struct page_cgroup, we may be able to fix this by using original 'struct page' information. But we'll have a problem in "move account" Assume we use only PG_dirty. CPU-A CPU-B TestSet PG_dirty (delay) move_lock_mem_cgroup() if (PageDirty(page)) new_memcg->nr_dirty++ pc->mem_cgroup = new_memcg; move_unlock_mem_cgroup() move_lock_mem_cgroup() memcg = pc->mem_cgroup new_memcg->nr_dirty++ accounting information may be double-counted. This was original reason to have PCG_xxx flags but it seems PCG_xxx has another problem. I think we need a bigger lock as move_lock_mem_cgroup(page) TestSetPageDirty(page) update page stats (without any checks) move_unlock_mem_cgroup(page) This fixes both of problems and we don't have to duplicate page flag into page_cgroup. Please note: move_lock_mem_cgroup() is held only when there are possibility of "account move" under the system. So, in most path, status update will go without atomic locks. This patch introduces mem_cgroup_begin_update_page_stat() and mem_cgroup_end_update_page_stat() both should be called at modifying 'struct page' information if memcg takes care of it. as mem_cgroup_begin_update_page_stat() modify page information mem_cgroup_update_page_stat() => never check any 'struct page' info, just update counters. mem_cgroup_end_update_page_stat(). This patch is slow because we need to call begin_update_page_stat()/ end_update_page_stat() regardless of accounted will be changed or not. A following patch adds an easy optimization and reduces the cost. [akpm@linux-foundation.org: s/lock/locked/] [hughd@google.com: fix deadlock by avoiding stat lock when anon] Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Greg Thelen <gthelen@google.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@suse.cz> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Cc: Ying Han <yinghan@google.com> Signed-off-by: Hugh Dickins <hughd@google.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.c28
1 files changed, 24 insertions, 4 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index ebeb95e9150a..5b5ad584ffb7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1148,10 +1148,15 @@ void page_add_new_anon_rmap(struct page *page,
*/
void page_add_file_rmap(struct page *page)
{
+ bool locked;
+ unsigned long flags;
+
+ mem_cgroup_begin_update_page_stat(page, &locked, &flags);
if (atomic_inc_and_test(&page->_mapcount)) {
__inc_zone_page_state(page, NR_FILE_MAPPED);
mem_cgroup_inc_page_stat(page, MEMCG_NR_FILE_MAPPED);
}
+ mem_cgroup_end_update_page_stat(page, &locked, &flags);
}
/**
@@ -1162,9 +1167,21 @@ void page_add_file_rmap(struct page *page)
*/
void page_remove_rmap(struct page *page)
{
+ bool anon = PageAnon(page);
+ bool locked;
+ unsigned long flags;
+
+ /*
+ * The anon case has no mem_cgroup page_stat to update; but may
+ * uncharge_page() below, where the lock ordering can deadlock if
+ * we hold the lock against page_stat move: so avoid it on anon.
+ */
+ if (!anon)
+ mem_cgroup_begin_update_page_stat(page, &locked, &flags);
+
/* page still mapped by someone else? */
if (!atomic_add_negative(-1, &page->_mapcount))
- return;
+ goto out;
/*
* Now that the last pte has gone, s390 must transfer dirty
@@ -1173,7 +1190,7 @@ void page_remove_rmap(struct page *page)
* not if it's in swapcache - there might be another pte slot
* containing the swap entry, but page not yet written to swap.
*/
- if ((!PageAnon(page) || PageSwapCache(page)) &&
+ if ((!anon || PageSwapCache(page)) &&
page_test_and_clear_dirty(page_to_pfn(page), 1))
set_page_dirty(page);
/*
@@ -1181,8 +1198,8 @@ void page_remove_rmap(struct page *page)
* and not charged by memcg for now.
*/
if (unlikely(PageHuge(page)))
- return;
- if (PageAnon(page)) {
+ goto out;
+ if (anon) {
mem_cgroup_uncharge_page(page);
if (!PageTransHuge(page))
__dec_zone_page_state(page, NR_ANON_PAGES);
@@ -1202,6 +1219,9 @@ void page_remove_rmap(struct page *page)
* Leaving it set also helps swapoff to reinstate ptes
* faster for those pages still in swapcache.
*/
+out:
+ if (!anon)
+ mem_cgroup_end_update_page_stat(page, &locked, &flags);
}
/*