linux-kernel - Re: [PATCH v2] mm: vmscan: add VM_BUG_ON on illegal return values from scan

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Message-ID: <CAOJsxLGivq0p1j4Axykdz-O8FtYfn=M1BfLEnc=q-fjxA2Yonw@mail.gmail.com>
Date:	Tue, 21 May 2013 10:17:15 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Oskar Andero <oskar.andero@...ymobile.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Radovan Lekanovic <radovan.lekanovic@...ymobile.com>,
	David Rientjes <rientjes@...gle.com>,
	Glauber Costa <glommer@...nvz.org>,
	Dave Chinner <dchinner@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] mm: vmscan: add VM_BUG_ON on illegal return values
 from scan_objects

On Tue, May 21, 2013 at 10:13 AM, Oskar Andero
<oskar.andero@...ymobile.com> wrote:
> Add a VM_BUG_ON to catch any illegal value from the shrinkers. It's a
> potential bug if scan_objects returns a negative other than -1 and
> would lead to undefined behaviour.
>
> Cc: Glauber Costa <glommer@...nvz.org>
> Cc: Dave Chinner <dchinner@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Oskar Andero <oskar.andero@...ymobile.com>
> ---
>  mm/vmscan.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6bac41e..63fec86 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -293,6 +293,7 @@ shrink_slab_one(struct shrinker *shrinker, struct shrink_control *shrinkctl,
>                 ret = shrinker->scan_objects(shrinker, shrinkctl);
>                 if (ret == -1)
>                         break;
> +               VM_BUG_ON(ret < -1);

It seems to me relaxing the shrinker API restrictions and changing the
"ret == -1" to "ret < 0" would be a much more robust approach...

>                 freed += ret;
>
>                 count_vm_events(SLABS_SCANNED, nr_to_scan);
> --
> 1.8.1.5
>
> --
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