Re: [PATCH next] memcg: fix deadlock by avoiding stat lock when anon

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Feb 28, 2012 at 09:26:56PM -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@google.com>

Eek.

Saving the begin/end_update_page_stat() calls for the anon case where
we know in advance we don't need them is one thing, but this also
hides a dependencies that even eludes lockdep behind what looks like a
minor optimization of the anon case.

Wouldn't this be more robust if we turned the ordering inside out in
move_account instead?

> ---
> 
>  mm/rmap.c |   11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> --- 3.3-rc5-next/mm/rmap.c	2012-02-26 23:51:46.506050210 -0800
> +++ linux/mm/rmap.c	2012-02-27 20:25:56.423324211 -0800
> @@ -1166,10 +1166,12 @@ void page_add_file_rmap(struct page *pag
>   */
>  void page_remove_rmap(struct page *page)
>  {
> +	bool anon = PageAnon(page);
>  	bool locked;
>  	unsigned long flags;
>  
> -	mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> +	if (!anon)
> +		mem_cgroup_begin_update_page_stat(page, &locked, &flags);
>  	/* page still mapped by someone else? */
>  	if (!atomic_add_negative(-1, &page->_mapcount))
>  		goto out;
> @@ -1181,7 +1183,7 @@ void page_remove_rmap(struct page *page)
>  	 * not if it's in swapcache - there might be another pte slot
>  	 * containing the swap entry, but page not yet written to swap.
>  	 */
> -	if ((!PageAnon(page) || PageSwapCache(page)) &&
> +	if ((!anon || PageSwapCache(page)) &&
>  	    page_test_and_clear_dirty(page_to_pfn(page), 1))
>  		set_page_dirty(page);
>  	/*
> @@ -1190,7 +1192,7 @@ void page_remove_rmap(struct page *page)
>  	 */
>  	if (unlikely(PageHuge(page)))
>  		goto out;
> -	if (PageAnon(page)) {
> +	if (anon) {
>  		mem_cgroup_uncharge_page(page);
>  		if (!PageTransHuge(page))
>  			__dec_zone_page_state(page, NR_ANON_PAGES);
> @@ -1211,7 +1213,8 @@ void page_remove_rmap(struct page *page)
>  	 * faster for those pages still in swapcache.
>  	 */
>  out:
> -	mem_cgroup_end_update_page_stat(page, &locked, &flags);
> +	if (!anon)
> +		mem_cgroup_end_update_page_stat(page, &locked, &flags);
>  }
>  
>  /*

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