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    SubjectRe: [Patch v4] mm: thp: remove the defer list related code since this will not happen
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    On 1/17/20 4:57 PM, Yang Shi wrote:
    >
    >
    > On 1/17/20 3:38 PM, Wei Yang wrote:
    >> If compound is true, this means it is a PMD mapped THP. Which implies
    >> the page is not linked to any defer list. So the first code chunk will
    >> not be executed.
    >>
    >> Also with this reason, it would not be proper to add this page to a
    >> defer list. So the second code chunk is not correct.
    >>
    >> Based on this, we should remove the defer list related code.
    >>
    >> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg
    >> aware")
    >>
    >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
    >> Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    >> Cc: <stable@vger.kernel.org>    [5.4+]
    >>
    >> ---
    >> v4:
    >>    * finally we identified the related code is not necessary and not
    >>      correct, just remove it
    >>    * thanks to Kirill T first spot some problem
    >
    > Thanks for debugging and figuring this out. Acked-by: Yang Shi
    > <yang.shi@linux.alibaba.com>

    BTW, the patch itself is fine, but the subject looks really confusing.
    It sounds like we would remove all deferred list code. I'd suggest
    rephrase it to:

    mm: thp: don't need care deferred split queue in memcg charge move path

    >
    >> v3:
    >>    * remove all review/ack tag since rewrite the changelog
    >>    * use deferred_split_huge_page as the example of race
    >>    * add cc stable 5.4+ tag as suggested by David Rientjes
    >>
    >> 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, 18 deletions(-)
    >>
    >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    >> index 6c83cf4ed970..27c231bf4565 100644
    >> --- a/mm/memcontrol.c
    >> +++ b/mm/memcontrol.c
    >> @@ -5340,14 +5340,6 @@ static int mem_cgroup_move_account(struct page
    >> *page,
    >>           __mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
    >>       }
    >>   -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    >> -    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);
    >> -    }
    >> -#endif
    >>       /*
    >>        * It is safe to change page->mem_cgroup here because the page
    >>        * is referenced, charged, and isolated - we can't race with
    >> @@ -5357,16 +5349,6 @@ static int mem_cgroup_move_account(struct page
    >> *page,
    >>       /* caller should have done css_get */
    >>       page->mem_cgroup = to;
    >>   -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    >> -    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);
    >> -    }
    >> -#endif
    >> -
    >>       spin_unlock_irqrestore(&from->move_lock, flags);
    >>         ret = 0;
    >

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