From 691f3542799e17ad1a1dfba7288dc8ccb031510a Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Fri, 10 May 2013 09:57:00 +1000 Subject: [PATCH] mm: memcg: remove incorrect VM_BUG_ON for swap cache pages in uncharge 0c59b89 ("mm: memcg: push down PageSwapCache check into uncharge entry functions") added a VM_BUG_ON() on PageSwapCache in the uncharge path after checking that page flag once, assuming that the state is stable in all paths, but this is not the case and the condition triggers in user environments. An uncharge after the last page table reference to the page goes away can race with reclaim adding the page to swap cache. Swap cache pages are usually uncharged when they are freed after swapout, from a path that also handles swap usage accounting and memcg lifetime management. However, since the last page table reference is gone and thus no references to the swap slot left, the swap slot will be freed shortly when reclaim attempts to write the page to disk. The whole swap accounting is not even necessary. So while the race condition for which this VM_BUG_ON was added is real and actually existed all along, there are no negative effects. Remove the VM_BUG_ON again. Reported-by: Heiko Carstens Reported-by: Lingzhu Xiang Signed-off-by: Johannes Weiner Acked-by: Hugh Dickins Acked-by: Michal Hocko Cc: Signed-off-by: Andrew Morton --- mm/memcontrol.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index cb1c9dedf9b6..010d6c14129a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4108,8 +4108,6 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype, if (mem_cgroup_disabled()) return NULL; - VM_BUG_ON(PageSwapCache(page)); - if (PageTransHuge(page)) { nr_pages <<= compound_order(page); VM_BUG_ON(!PageTransHuge(page)); @@ -4205,6 +4203,18 @@ void mem_cgroup_uncharge_page(struct page *page) if (page_mapped(page)) return; VM_BUG_ON(page->mapping && !PageAnon(page)); + /* + * If the page is in swap cache, uncharge should be deferred + * to the swap path, which also properly accounts swap usage + * and handles memcg lifetime. + * + * Note that this check is not stable and reclaim may add the + * page to swap cache at any time after this. However, if the + * page is not in swap cache by the time page->mapcount hits + * 0, there won't be any page table references to the swap + * slot, and reclaim will free it and not actually write the + * page to disk. + */ if (PageSwapCache(page)) return; __mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_ANON, false); -- 2.39.5