virtio-blk: Don't use MAX_DISCARD_SEGMENTS if max_discard_seg is zero

Currently the value of max_discard_segment will be set to
MAX_DISCARD_SEGMENTS (256) with no basis in hardware if device
set 0 to max_discard_seg in configuration space. It's incorrect
since the device might not be able to handle such large descriptors.
To fix it, let's follow max_segments restrictions in this case.

Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20220304100058.116-1-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c443cd64fc9b46051d978adb5bd95ce0b1d3a5fe..7fc2c8b970771ea2b5a0bc57cd139baa5ba4bc70 100644 (file)
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -925,9 +925,15 @@ static int virtblk_probe(struct virtio_device *vdev)
 
                virtio_cread(vdev, struct virtio_blk_config, max_discard_seg,
                             &v);
+
+               /*
+                * max_discard_seg == 0 is out of spec but we always
+                * handled it.
+                */
+               if (!v)
+                       v = sg_elems - 2;
                blk_queue_max_discard_segments(q,
-                                              min_not_zero(v,
-                                                           MAX_DISCARD_SEGMENTS));
+                                              min(v, MAX_DISCARD_SEGMENTS));
 
                blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
        }