linux-kernel - Re: [PATCH] page_cgroup: fix horrid swap accounting regression

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Message-ID: <CAA_GA1cQ4i8Rtrh6OBrgw-dPc7dqnoJbbPRBrBoawihtgn2zLA@mail.gmail.com>
Date:	Tue, 6 Mar 2012 13:03:00 +0800
From:	Bob Liu <lliubbo@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] page_cgroup: fix horrid swap accounting regression

Hi Hugh,

On Tue, Mar 6, 2012 at 12:52 PM, Hugh Dickins <hughd@...gle.com> wrote:
> Why is memcg's swap accounting so broken?  Insane counts, wrong ownership,
> unfreeable structures, which later get freed and then accessed after free.
>
> Turns out to be a tiny a little 3.3-rc1 regression in 9fb4b7cc0724
> "page_cgroup: add helper function to get swap_cgroup": the helper
> function (actually named lookup_swap_cgroup()) returns an address
> using void* arithmetic, but the structure in question is a short.
>

Sorry for my mistake.

> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>
>  mm/page_cgroup.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> --- 3.3-rc6/mm/page_cgroup.c    2012-01-20 08:42:35.320020840 -0800
> +++ linux/mm/page_cgroup.c      2012-03-05 19:51:13.535372098 -0800
> @@ -379,13 +379,15 @@ static struct swap_cgroup *lookup_swap_c
>        pgoff_t offset = swp_offset(ent);
>        struct swap_cgroup_ctrl *ctrl;
>        struct page *mappage;
> +       struct swap_cgroup *sc;
>
>        ctrl = &swap_cgroup_ctrl[swp_type(ent)];
>        if (ctrlp)
>                *ctrlp = ctrl;
>
>        mappage = ctrl->map[offset / SC_PER_PAGE];
> -       return page_address(mappage) + offset % SC_PER_PAGE;
> +       sc = page_address(mappage);
> +       return sc + offset % SC_PER_PAGE;
>  }
>
>  /**

Reviewed-by: Bob Liu <lliubbo@...il.com>

-- 
Regards,
--Bob
--
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