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SubjectRe: [BUGFIX][PATCH v2] add mem_cgroup_replace_page_cache.
On Thu, 8 Dec 2011 16:18:29 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> 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.
>
> Changelog: v1 -> v2
> - fixed mem_cgroup_disabled() check missing.
> - added comments.
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> include/linux/memcontrol.h | 6 ++++++
> mm/filemap.c | 18 ++----------------
> mm/memcontrol.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 52 insertions(+), 16 deletions(-)

It's a relatively intrusive patch and I'm a bit concerned about
feeding it into 3.2.

How serious is the bug, and which kernel version(s) do you think we
should fix it in?



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