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

[Johannes Weiner]

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@xxxxxxxxxx>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>