Re: [PATCH v20 17/20] mm/swap.c: serialize memcg changes in pagevec_lru_move_fn

Johannes Weiner
2020-11-02T15:22:34Z

On Thu, Oct 29, 2020 at 06:45:02PM +0800, Alex Shi wrote:
> Hugh Dickins' found a memcg change bug on original version:
> If we want to change the pgdat->lru_lock to memcg's lruvec lock, we have
> to serialize mem_cgroup_move_account during pagevec_lru_move_fn. The
> possible bad scenario would like:
>
> cpu 0 cpu 1
> lruvec = mem_cgroup_page_lruvec()
> if (!isolate_lru_page())
> mem_cgroup_move_account
>
> spin_lock_irqsave(&lruvec->lru_lock <== wrong lock.
>
> So we need TestClearPageLRU to block isolate_lru_page(), that serializes
> the memcg change. and then removing the PageLRU check in move_fn callee
> as the consequence.
>
> __pagevec_lru_add_fn() is different from the others, because the pages
> it deals with are, by definition, not yet on the lru. TestClearPageLRU
> is not needed and would not work, so __pagevec_lru_add() goes its own
> way.
>
> Reported-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Acked-by: Hugh Dickins <hughd@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org

Acked-by: Johannes Weiner <hannes@cmpxchg.org>