Commit a61ed3ce authored by Johannes Weiner's avatar Johannes Weiner Committed by Linus Torvalds

memcg: close race between charge and putback

There is a potential race between a thread charging a page and another
thread putting it back to the LRU list:

  charge:                         putback:
  SetPageCgroupUsed               SetPageLRU
  PageLRU && add to memcg LRU     PageCgroupUsed && add to memcg LRU

The order of setting one flag and checking the other is crucial, otherwise
the charge may observe !PageLRU while the putback observes !PageCgroupUsed
and the page is not linked to the memcg LRU at all.

Global memory pressure may fix this by trying to isolate and putback the
page for reclaim, where that putback would link it to the memcg LRU again.
 Without that, the memory cgroup is undeletable due to a charge whose
physical page can not be found and moved out.
Signed-off-by: default avatarJohannes Weiner <jweiner@redhat.com>
Cc: Ying Han <yinghan@google.com>
Acked-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 9b272977
......@@ -993,6 +993,16 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
return;
pc = lookup_page_cgroup(page);
VM_BUG_ON(PageCgroupAcctLRU(pc));
/*
* putback: charge:
* SetPageLRU SetPageCgroupUsed
* smp_mb smp_mb
* PageCgroupUsed && add to memcg LRU PageLRU && add to memcg LRU
*
* Ensure that one of the two sides adds the page to the memcg
* LRU during a race.
*/
smp_mb();
if (!PageCgroupUsed(pc))
return;
/* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
......@@ -1044,7 +1054,16 @@ static void mem_cgroup_lru_add_after_commit(struct page *page)
unsigned long flags;
struct zone *zone = page_zone(page);
struct page_cgroup *pc = lookup_page_cgroup(page);
/*
* putback: charge:
* SetPageLRU SetPageCgroupUsed
* smp_mb smp_mb
* PageCgroupUsed && add to memcg LRU PageLRU && add to memcg LRU
*
* Ensure that one of the two sides adds the page to the memcg
* LRU during a race.
*/
smp_mb();
/* taking care of that the page is added to LRU while we commit it */
if (likely(!PageLRU(page)))
return;
......
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