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    SubjectRe: [RFC PATCH] mm: thp: grab the lock before manipulation defer list
    On Fri 03-01-20 22:34:07, Wei Yang wrote:
    > As all the other places, we grab the lock before manipulate the defer list.
    > Current implementation may face a race condition.

    Please always make sure to describe the effect of the change. Why a racy
    list_empty check matters?

    > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
    >
    > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
    >
    > ---
    > I notice the difference during code reading and just confused about the
    > difference. No specific test is done since limited knowledge about cgroup.
    >
    > Maybe I miss something important?
    > ---
    > mm/memcontrol.c | 8 ++++----
    > 1 file changed, 4 insertions(+), 4 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index bc01423277c5..62b7ec34ef1a 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -5368,12 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
    > }
    >
    > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > + spin_lock(&from->deferred_split_queue.split_queue_lock);
    > if (compound && !list_empty(page_deferred_list(page))) {
    > - spin_lock(&from->deferred_split_queue.split_queue_lock);
    > list_del_init(page_deferred_list(page));
    > from->deferred_split_queue.split_queue_len--;
    > - spin_unlock(&from->deferred_split_queue.split_queue_lock);
    > }
    > + spin_unlock(&from->deferred_split_queue.split_queue_lock);
    > #endif
    > /*
    > * It is safe to change page->mem_cgroup here because the page
    > @@ -5385,13 +5385,13 @@ static int mem_cgroup_move_account(struct page *page,
    > page->mem_cgroup = to;
    >
    > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > + spin_lock(&to->deferred_split_queue.split_queue_lock);
    > if (compound && list_empty(page_deferred_list(page))) {
    > - spin_lock(&to->deferred_split_queue.split_queue_lock);
    > list_add_tail(page_deferred_list(page),
    > &to->deferred_split_queue.split_queue);
    > to->deferred_split_queue.split_queue_len++;
    > - spin_unlock(&to->deferred_split_queue.split_queue_lock);
    > }
    > + spin_unlock(&to->deferred_split_queue.split_queue_lock);
    > #endif
    >
    > spin_unlock_irqrestore(&from->move_lock, flags);
    > --
    > 2.17.1

    --
    Michal Hocko
    SUSE Labs

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