Re: [BUGFIX][PATCH] add mem_cgroup_replace_page_cache.

[Johannes Weiner]

On Tue, Dec 06, 2011 at 12:39:23PM +0900, KAMEZAWA Hiroyuki wrote:
> 
> Hm, is this too naive ? better idea is welcome. 
> ==
> >From 33638351c5cd28af9f47f9ab1c44eeb1f63d9964 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Date: Tue, 6 Dec 2011 12:32:32 +0900
> Subject: [PATCH] memcg: add mem_cgroup_replace_page_cache() for fixing LRU issue.
> 
> commit ef6a3c6311 adds a function replace_page_cache_page(). This
> function replaces a page in radix-tree with a new page.
> At doing this, memory cgroup need to fix up the accounting information.
> memcg need to check PCG_USED bit etc.
> 
> In some(many?) case, 'newpage' is on LRU before calling replace_page_cache().
> So, memcg's LRU accounting information should be fixed, too.
> 
> This patch adds mem_cgroup_replace_page_cache() and removing old hooks.
> In that function, old pages will be unaccounted without touching res_counter
> and new page will be accounted to the memcg (of old page). At overwriting
> pc->mem_cgroup of newpage, take zone->lru_lock and avoid race with
> LRU handling.
> 
> Background:
>   replace_page_cache_page() is called by FUSE code in its splice() handling.
>   Here, 'newpage' is replacing oldpage but this newpage is not a newly allocated
>   page and may be on LRU. LRU mis-accounting will be critical for memory cgroup
>   because rmdir() checks the whole LRU is empty and there is no account leak.
>   If a page is on the other LRU than it should be, rmdir() will fail.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

I think this is okay.  It's a tiny bit unfortunate that the migration
code is more or less duplicated with some optimizations, but I fear
the other solutions would be more complex and thus not adequate as a
bug fix.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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