linux-kernel - Re: [patch 03/11] mm: shmem: do not try to uncharge known swapcache pages

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Date:	Mon, 9 Jul 2012 13:37:39 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 03/11] mm: shmem: do not try to uncharge known swapcache
 pages

On Mon, 9 Jul 2012, Michal Hocko wrote:
> 
> Maybe I am missing something but who does the uncharge from:
> shmem_unuse
>   mem_cgroup_cache_charge
>   shmem_unuse_inode
>     shmem_add_to_page_cache

There isn't any special uncharge for shmem_unuse(): once the swapcache
page is matched up with its memcg, it will get uncharged by one of the
usual routes to swapcache_free() when the page is freed: maybe in the
call from __remove_mapping(), maybe when free_page_and_swap_cache()
ends up calling it.

Perhaps you're worrying about error (or unfound) paths in shmem_unuse()?
By the time we make the charge, we know for sure that it's a shmem page,
and make the charge appropriately; in racy cases it might get uncharged
again in the delete_from_swap_cache().  Can the unfound case occur these
days?  I'd have to think more deeply to answer that, but the charge will
not go missing.

Hugh
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