[patch] mm: memcontrol: fix missed end-writeback accounting

[Johannes Weiner]

0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") changed page
migration to uncharge the old page right away.  The page is locked,
unmapped, truncated, and off the LRU.  But it could race with a
finishing writeback, which then doesn't get unaccounted properly:

test_clear_page_writeback()              migration
  acquire pc->mem_cgroup->move_lock
                                           wait_on_page_writeback()
  TestClearPageWriteback()
                                           mem_cgroup_migrate()
                                             clear PCG_USED
  if (PageCgroupUsed(pc))
    decrease memcg pages under writeback
  release pc->mem_cgroup->move_lock

One solution for this would be to simply remove the PageCgroupUsed()
check, as RCU protects the memcg anyway.

However, it's more robust to acknowledge that migration is really
modifying the charge state of alive pages in this case, and so it
should participate in the protocol specifically designed for this.

Fixes: 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: "3.17" <stable@xxxxxxxxxxxxxxx>
---
 mm/memcontrol.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3a203c7ec6c7..b35a44e9cd37 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6148,6 +6148,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
 			bool lrucare)
 {
 	struct page_cgroup *pc;
+	unsigned long flags;
 	int isolated;
 
 	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
@@ -6177,7 +6178,14 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
 	if (lrucare)
 		lock_page_lru(oldpage, &isolated);
 
+	/*
+	 * The page is locked, unmapped, truncated, and off the LRU,
+	 * but there might still be references, e.g. from finishing
+	 * writeback.  Follow the charge moving protocol here.
+	 */
+	move_lock_mem_cgroup(pc->mem_cgroup, &flags);
 	pc->flags = 0;
+	move_unlock_mem_cgroup(pc->mem_cgroup, &flags);
 
 	if (lrucare)
 		unlock_page_lru(oldpage, isolated);
-- 
2.1.2

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