Re: [PATCH] page_cgroup: fix horrid swap accounting regression

[Michal Hocko]

On Mon 05-03-12 20:52:55, Hugh Dickins 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.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Thanks, this one looks really nasty.

Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
> 
>  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;
>  }
>  
>  /**

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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