netdev - Re: [PATCH] virtio_net: Fix queue full check

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Date:	Thu, 4 Nov 2010 21:47:04 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>, yvugenfi@...hat.com
Subject: Re: [PATCH] virtio_net: Fix queue full check

"Michael S. Tsirkin" <mst@...hat.com> wrote on 11/04/2010 05:54:24 PM:

> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>
> I thought about this some more.  I think the original
> code is actually correct in returning ENOSPC: indirect
> buffers are nice, but it's a mistake
> to rely on them as a memory allocation might fail.
>
> And if you look at virtio-net, it is dropping packets
> under memory pressure which is not really a happy outcome:
> the packet will get freed, reallocated and we get another one,
> adding pressure on the allocator instead of releasing it
> until we free up some buffers.
>
> So I now think we should calculate the capacity
> assuming non-indirect entries, and if we manage to
> use indirect, all the better.
>
> So below is what I propose now - as a replacement for
> my original patch.  Krishna Kumar, Rusty, what do you think?
>
> Separately I'm also considering moving the
>    if (vq->num_free < out + in)
> check earlier in the function to keep all users honest,
> but need to check what the implications are for e.g. block.
> Thoughts on this?

This looks like the right thing to do.  Besides this, I
think virtio-net still needs to remove check for ENOMEM?
I will test this patch tomorrow.

Another question about add_recvbuf_small and
add_recvbuf_big - both call virtqueue_add_buf_gfp with
in+out > 1, and that can fail with -ENOSPC.  So try_fill_recv
gets -ENOSPC.  When that happens, oom is not set to true,
I thought it should have got set.  Is this a bug?

Thanks,

- KK

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