diff options
authorJohannes Weiner <>2013-05-24 15:55:15 -0700
committerLinus Torvalds <>2013-05-24 16:22:51 -0700
commit28ccddf7952c496df2a51ce5aee4f2a058a98bab (patch)
parent26549c8d36a64d9130e4c0f32412be7ba6180923 (diff)
mm: memcg: remove incorrect VM_BUG_ON for swap cache pages in uncharge
Commit 0c59b89c81ea ("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 <> Signed-off-by: Linus Torvalds <>
1 files changed, 12 insertions, 2 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cb1c9dedf9b..010d6c14129 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);
@@ -4205,6 +4203,18 @@ void mem_cgroup_uncharge_page(struct page *page)
if (page_mapped(page))
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))
__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_ANON, false);