From: Sha Zhengju <handai.szj@xxxxxxxxxx> While doing memcg page stat accounting, we need to access page_cgroup members including pc->mem_cgroup and pc->flags, so there are 3 candidates which have potential race conditions with it: move account, charge, uncharge. But page stat and uncharge can also take it easy because the former is done before the page is deleted from radix-tree and the later is after the delete, so they will be serialized by some other locks(like page lock). So the races among them will be solved by: stat move charge uncharge stat X move lock no race no race move X lock_page_cgroup lock_page_cgroup charge X lock_page_cgroup uncharge X Signed-off-by: Sha Zhengju <handai.szj@xxxxxxxxxx> --- mm/memcontrol.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index fe4f123..b31513e 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2317,9 +2317,10 @@ static bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask, * are no race with "charge". * * Considering "uncharge", we know that memcg doesn't clear pc->mem_cgroup - * at "uncharge" intentionally. So, we always see valid pc->mem_cgroup even - * if there are race with "uncharge". Statistics itself is properly handled - * by flags. + * at "uncharge" intentionally but clear PageCgroupUsed flag for that page. + * Besides, the file-stat operations happen before a page is deleted from + * radix-tree while uncharge is after the delete. So there are no race with + * "uncharge" too. * * Considering "move", this is an only case we see a race. To make the race * small, we check mm->moving_account and detect there are possibility of race -- 1.7.9.5 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>