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?
> 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
