Re: 2.6.35 Regression/oops from virtio: return ENOMEM on out of memory patch

[Chris Mason <chris.mason@oracle.com>]

On Mon, Jul 19, 2010 at 06:16:10PM +0300, Michael S. Tsirkin wrote:
> On Mon, Jul 19, 2010 at 11:02:16AM -0400, Chris Mason wrote:
> > Hi everyone,
> > 
> > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=686d363786a53ed28ee875b84ef24e6d5126ef6f
> > 
> > I've been having problems with my long running stress runs and tracked
> > it down to the above commit.  Under load I get a couple of GFP_ATOMIC
> > allocation failures from virtio per day (not really surprising), and in
> > the past it would carry on happily.
> > 
> > Now I get the atomic allocation failure followed by this:
> > 
> > BUG: unable to handle kernel paging request at ffff88087c37e458
> > IP: [<ffffffff812e3752>] virtqueue_add_buf_gfp+0x305/0x353
> > 
> > (Full oops below).
> > 
> > Looking at virtqueue_add_buf_gfp, it does:
> > 
> >         /* If the host supports indirect descriptor tables, and we have multiple 
> >          * buffers, then go indirect. FIXME: tune this threshold */
> >         if (vq->indirect && (out + in) > 1 && vq->num_free) {
> >                 head = vring_add_indirect(vq, sg, out, in, gfp);
> >                 if (head != vq->vring.num)
> >                         goto add_head;
> >         }
> > [ ... ]
> > 
> > add_head:
> >         /* Set token. */
> >         vq->data[head] = data;
> > 
> > Since vring_add_indirect is returning -ENOMEM, head is -ENOMEM and things
> > go bad pretty quickly.  Full oops below, afraid I don't know the virtio
> > code well enough to provide the clean and obvious fix (outside of
> > reverting) at this late rc.
> 
> Good catch! Can you verify this fix please?
> 
> virtio: fix oops on OOM
> 
> virtio ring was changed to return an error code on OOM,
> but one caller was missed and still checks for vq->vring.num.
> The fix is just to check for <0 error code.
> 
> Long term it might make sense to change goto add_head to
> just return an error on oom instead, but let's apply
> a minimal fix for 2.6.35.

I haven't been able to trigger the allocation failure since putting this in,
but it has run well for the last two days.  I'll let you know if there
are problems.

-chris