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

[Max Gurtovoy <mgurtovoy@xxxxxxxxxx>]

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.

This commit looks good to me, thanks Xie Yongji.

Reviewed-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>

> > 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@xxxxxxxxxxxxx>
> > Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
> > ---
> >   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