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SubjectRe: [PATCH] mm: fix null pointer dereference in wait_iff_congested()
Hello Zlatko,

On Fri, Dec 28, 2012 at 03:16:38AM +0100, Zlatko Calusic wrote:
> From: Zlatko Calusic <zlatko.calusic@iskon.hr>
>
> The unintended consequence of commit 4ae0a48b is that
> wait_iff_congested() can now be called with NULL struct zone*
> producing kernel oops like this:

For good description, it would be better to write simple pseudo code
flow to show how NULL-zone pass into wait_iff_congested because
kswapd code flow is too complex.

As I see the code, we have following line above wait_iff_congested.

if (!unbalanced_zone || blah blah)
break;

How can NULL unbalanced_zone reach wait_iff_congested?

>
> BUG: unable to handle kernel NULL pointer dereference
> IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140
>
> This trivial patch fixes it.
>
> Reported-by: Zhouping Liu <zliu@redhat.com>
> Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
> ---
> mm/vmscan.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 02bcfa3..e55ce55 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2782,7 +2782,7 @@ loop_again:
> if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
> if (has_under_min_watermark_zone)
> count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
> - else
> + else if (unbalanced_zone)
> wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10);
> }
>
> --
> 1.8.1.rc3
>
> --
> Zlatko
>
> --
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--
Kind regards,
Minchan Kim


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