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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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