On Wed, 8 May 2013, Johannes Weiner wrote:
> 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 <heiko.carst...@de.ibm.com>
> Reported-by: Lingzhu Xiang <lxi...@redhat.com>
> Signed-off-by: Johannes Weiner <han...@cmpxchg.org>
> Cc: Hugh Dickins <hu...@google.com>
Acked-by: Hugh Dickins <hu...@google.com>
> Cc: Michal Hocko <mho...@suse.cz>
> Cc: sta...@vger.kernel.org
> ---
> mm/memcontrol.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index cb1c9de..010d6c1 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);
> --
> 1.7.11.7
>
>
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