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    SubjectRe: [Patch v2] mm: thp: grab the lock before manipulation defer list
    On Thu 09-01-20 22:30:54, Wei Yang wrote:
    > As all the other places, we grab the lock before manipulate the defer list.
    > Current implementation may face a race condition.
    >
    > For example, the potential race would be:
    >
    > CPU1 CPU2
    > mem_cgroup_move_account split_huge_page_to_list
    > !list_empty
    > lock
    > !list_empty
    > list_del
    > unlock
    > lock
    > # !list_empty might not hold anymore
    > list_del_init
    > unlock
    >
    > When this sequence happens, the list_del_init() in
    > mem_cgroup_move_account() would crash if CONFIG_DEBUG_LIST since the
    > page is already been removed by list_del in split_huge_page_to_list().
    >
    > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
    >
    > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
    > Acked-by: David Rientjes <rientjes@google.com>

    Thanks a lot for the changelog approvements!
    Acked-by: Michal Hocko <mhocko@suse.com>

    >
    > ---
    > v2:
    > * move check on compound outside suggested by Alexander
    > * an example of the race condition, suggested by Michal
    > ---
    > mm/memcontrol.c | 18 +++++++++++-------
    > 1 file changed, 11 insertions(+), 7 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index bc01423277c5..1492eefe4f3c 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -5368,10 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
    > }
    >
    > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > - if (compound && !list_empty(page_deferred_list(page))) {
    > + if (compound) {
    > spin_lock(&from->deferred_split_queue.split_queue_lock);
    > - list_del_init(page_deferred_list(page));
    > - from->deferred_split_queue.split_queue_len--;
    > + if (!list_empty(page_deferred_list(page))) {
    > + list_del_init(page_deferred_list(page));
    > + from->deferred_split_queue.split_queue_len--;
    > + }
    > spin_unlock(&from->deferred_split_queue.split_queue_lock);
    > }
    > #endif
    > @@ -5385,11 +5387,13 @@ static int mem_cgroup_move_account(struct page *page,
    > page->mem_cgroup = to;
    >
    > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > - if (compound && list_empty(page_deferred_list(page))) {
    > + if (compound) {
    > 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++;
    > + if (list_empty(page_deferred_list(page))) {
    > + 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);
    > }
    > #endif
    > --
    > 2.17.1

    --
    Michal Hocko
    SUSE Labs

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