Re: [PATCH v2 next] memcg: fix deadlock by avoiding stat lock whenanon

[Johannes Weiner]

On Wed, Feb 29, 2012 at 06:44:59PM -0800, Hugh Dickins wrote:
> Fix deadlock in "memcg: use new logic for page stat accounting".
> 
> page_remove_rmap() first calls mem_cgroup_begin_update_page_stat(),
> which may take move_lock_mem_cgroup(), unlocked at the end of
> page_remove_rmap() by mem_cgroup_end_update_page_stat().
> 
> The PageAnon case never needs to mem_cgroup_dec_page_stat(page,
> MEMCG_NR_FILE_MAPPED); but it often needs to mem_cgroup_uncharge_page(),
> which does lock_page_cgroup(), while holding that move_lock_mem_cgroup().
> Whereas mem_cgroup_move_account() calls move_lock_mem_cgroup() while
> holding lock_page_cgroup().
> 
> Since mem_cgroup_begin and end are unnecessary here for PageAnon,
> simply avoid the deadlock and wasted calls in that case.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Agreed, let's keep that lock ordering for now, and the comment makes
it clear.  Thanks!

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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