Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Feb 28, 2022 at 02:57:20PM +0800, Xie Yongji wrote:
> > > Currently we have a BUG_ON() to make sure the number of sg
> > > list does not exceed queue_max_segments() in virtio_queue_rq().
> > > However, the block layer uses queue_max_discard_segments()
> > > instead of queue_max_segments() to limit the sg list for
> > > discard requests. So the BUG_ON() might be triggered if
> > > virtio-blk device reports a larger value for max discard
> > > segment than queue_max_segments().
> >
> > Hmm the spec does not say what should happen if max_discard_seg
> > exceeds seg_max. Is this the config you have in mind? how do you
> > create it?
> >
> 
> One example: the device doesn't specify the value of max_discard_seg
> in the config space, then the virtio-blk driver will use
> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> BUG_ON() if the seg_max is less than 256.
> 
> While the spec didn't say what should happen if max_discard_seg
> exceeds seg_max, it also doesn't explicitly prohibit this
> configuration. So I think we should at least not panic the kernel in
> this case.
> 
> Thanks,
> Yongji

Oh that last one sounds like a bug, I think it should be
min(MAX_DISCARD_SEGMENTS, seg_max)

When max_discard_seg and seg_max both exist, that's a different question. We can
- do min(max_discard_seg, seg_max)
- fail probe
- clear the relevant feature flag

I feel we need a better plan than submitting an invalid request to device.

-- 
MST