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

Michael S. Tsirkin
Wed, 02 Mar 2022 13:33:04 -0000

On Wed, Mar 02, 2022 at 03:24:51PM +0200, Max Gurtovoy
wrote:
>
> On 3/2/2022 3:17 PM, Michael S. Tsirkin wrote:
> > On Wed, Mar 02, 2022 at 11:51:27AM +0200, Max Gurtovoy wrote:
> > > On 3/1/2022 5:43 PM, Michael S. Tsirkin 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?
> > > I don't think it's hard to create it. Just change some
registers in the
> > > device.
> > >
> > > But with the dynamic sgl allocation that I added recently, there
is no
> > > problem with this scenario.
> > Well the problem is device says it can't handle such large
descriptors,
> > I guess it works anyway, but it seems scary.
>
> I don't follow.
>
> The only problem this patch solves is when a virtio blk device reports
> larger value for max_discard_segments than max_segments.
>

No, the peroblem reported is when virtio blk device reports
max_segments < 256 but not max_discard_segments.
I would expect discard to follow max_segments restrictions then.

> Probably no such devices, but we need to be prepared.

Right, question is how to handle this.

> >
> > > This commit looks good to me, thanks Xie Yongji.
> > >
> > > Reviewed-by: Max Gurtovoy <mgurtovoy at nvidia.com>
> > >
> > > > > To fix it, let's simply
> > > > > remove the BUG_ON() which has become unnecessary after
commit
> > > > > 02746e26c39e("virtio-blk: avoid preallocating big
SGL for data").
> > > > > And the unused vblk->sg_elems can also be removed
together.
> > > > >
> > > > > Fixes: 1f23816b8eb8 ("virtio_blk: add discard and
write zeroes support")
> > > > > Suggested-by: Christoph Hellwig <hch at
infradead.org>
> > > > > Signed-off-by: Xie Yongji <xieyongji at
bytedance.com>
> > > > > ---
> > > > >    drivers/block/virtio_blk.c | 10 +---------
> > > > >    1 file changed, 1 insertion(+), 9 deletions(-)
> > > > >
> > > > > diff --git a/drivers/block/virtio_blk.c
b/drivers/block/virtio_blk.c
> > > > > index c443cd64fc9b..a43eb1813cec 100644
> > > > > --- a/drivers/block/virtio_blk.c
> > > > > +++ b/drivers/block/virtio_blk.c
> > > > > @@ -76,9 +76,6 @@ struct virtio_blk {
> > > > >    	 */
> > > > >    	refcount_t refs;
> > > > > -	/* What host tells us, plus 2 for header &
tailer. */
> > > > > -	unsigned int sg_elems;
> > > > > -
> > > > >    	/* Ida index - used to track minor number
allocations. */
> > > > >    	int index;
> > > > > @@ -322,8 +319,6 @@ static blk_status_t
virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> > > > >    	blk_status_t status;
> > > > >    	int err;
> > > > > -	BUG_ON(req->nr_phys_segments + 2 >
vblk->sg_elems);
> > > > > -
> > > > >    	status = virtblk_setup_cmd(vblk->vdev, req,
vbr);
> > > > >    	if (unlikely(status))
> > > > >    		return status;
> > > > > @@ -783,8 +778,6 @@ static int virtblk_probe(struct
virtio_device *vdev)
> > > > >    	/* Prevent integer overflows and honor max vq size
*/
> > > > >    	sg_elems = min_t(u32, sg_elems,
VIRTIO_BLK_MAX_SG_ELEMS - 2);
> > > > > -	/* We need extra sg elements at head and tail. */
> > > > > -	sg_elems += 2;
> > > > >    	vdev->priv = vblk = kmalloc(sizeof(*vblk),
GFP_KERNEL);
> > > > >    	if (!vblk) {
> > > > >    		err = -ENOMEM;
> > > > > @@ -796,7 +789,6 @@ static int virtblk_probe(struct
virtio_device *vdev)
> > > > >    	mutex_init(&vblk->vdev_mutex);
> > > > >    	vblk->vdev = vdev;
> > > > > -	vblk->sg_elems = sg_elems;
> > > > >    	INIT_WORK(&vblk->config_work,
virtblk_config_changed_work);
> > > > > @@ -853,7 +845,7 @@ static int virtblk_probe(struct
virtio_device *vdev)
> > > > >    		set_disk_ro(vblk->disk, 1);
> > > > >    	/* We can handle whatever the host told us to
handle. */
> > > > > -	blk_queue_max_segments(q, vblk->sg_elems-2);
> > > > > +	blk_queue_max_segments(q, sg_elems);
> > > > >    	/* No real sector limit. */
> > > > >    	blk_queue_max_hw_sectors(q, -1U);
> > > > > --
> > > > > 2.20.1