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

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