On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:

On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:

quoted

On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin [off-list ref] wrote:

quoted

On Mon, Feb 28, 2022 at 02:57:20PM +0800, Xie Yongji wrote:

quoted

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.

We should cover only for a buggy devices.

The situation that max_discard_seg > seg_max should be fine.

Thus the bellow can be added to this patch:

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c443cd64fc9b..3e372b97fe10 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
                 virtio_cread(vdev, struct virtio_blk_config, 

max_discard_seg,
                              &v);
                 blk_queue_max_discard_segments(q,
-                                              min_not_zero(v,
- MAX_DISCARD_SEGMENTS));
+                                              min_t(u32, (v ? v : 
sg_elems),
+ MAX_DISCARD_SEGMENTS));

                 blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
         }