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SubjectRe: [PATCH v2 next] memcg: fix deadlock by avoiding stat lock when anon
On Wed, 29 Feb 2012 18:44:59 -0800 (PST)
Hugh Dickins <hughd@google.com> wrote:

> Fix deadlock in "memcg: use new logic for page stat accounting".
>
> page_remove_rmap() first calls mem_cgroup_begin_update_page_stat(),
> which may take move_lock_mem_cgroup(), unlocked at the end of
> page_remove_rmap() by mem_cgroup_end_update_page_stat().
>
> The PageAnon case never needs to mem_cgroup_dec_page_stat(page,
> MEMCG_NR_FILE_MAPPED); but it often needs to mem_cgroup_uncharge_page(),
> which does lock_page_cgroup(), while holding that move_lock_mem_cgroup().
> Whereas mem_cgroup_move_account() calls move_lock_mem_cgroup() while
> holding lock_page_cgroup().
>
> Since mem_cgroup_begin and end are unnecessary here for PageAnon,
> simply avoid the deadlock and wasted calls in that case.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Thank you and I'm sorry.

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>


> ---
> v2: added comment in the code so it's not thought just an optimization.
>
> mm/rmap.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>
> --- 3.3-rc5-next/mm/rmap.c 2012-02-26 23:51:46.506050210 -0800
> +++ linux/mm/rmap.c 2012-02-29 17:55:42.868665736 -0800
> @@ -1166,10 +1166,18 @@ void page_add_file_rmap(struct page *pag
> */
> void page_remove_rmap(struct page *page)
> {
> + bool anon = PageAnon(page);
> bool locked;
> unsigned long flags;
>
> - mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> + /*
> + * The anon case has no mem_cgroup page_stat to update; but may
> + * uncharge_page() below, where the lock ordering can deadlock if
> + * we hold the lock against page_stat move: so avoid it on anon.
> + */
> + if (!anon)
> + mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> +
> /* page still mapped by someone else? */
> if (!atomic_add_negative(-1, &page->_mapcount))
> goto out;
> @@ -1181,7 +1189,7 @@ void page_remove_rmap(struct page *page)
> * not if it's in swapcache - there might be another pte slot
> * containing the swap entry, but page not yet written to swap.
> */
> - if ((!PageAnon(page) || PageSwapCache(page)) &&
> + if ((!anon || PageSwapCache(page)) &&
> page_test_and_clear_dirty(page_to_pfn(page), 1))
> set_page_dirty(page);
> /*
> @@ -1190,7 +1198,7 @@ void page_remove_rmap(struct page *page)
> */
> if (unlikely(PageHuge(page)))
> goto out;
> - if (PageAnon(page)) {
> + if (anon) {
> mem_cgroup_uncharge_page(page);
> if (!PageTransHuge(page))
> __dec_zone_page_state(page, NR_ANON_PAGES);
> @@ -1211,7 +1219,8 @@ void page_remove_rmap(struct page *page)
> * faster for those pages still in swapcache.
> */
> out:
> - mem_cgroup_end_update_page_stat(page, &locked, &flags);
> + if (!anon)
> + mem_cgroup_end_update_page_stat(page, &locked, &flags);
> }
>
> /*
> --
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