Re: [PATCH] block: bios with an offset are always gappy

[Christoph Hellwig <hch@lst.de>]

On Thu, Apr 13, 2017 at 10:06:29AM +0200, Johannes Thumshirn wrote:
> Doing a mkfs.btrfs on a (qemu emulated) PCIe NVMe causes a kernel panic
> in nvme_setup_prps() because the dma_len will drop below zero but the
> length not.

I think we should also turns this into a WARN_ON_ONCE + error return..

But do you have an exact btrfsprogs version and command line?  I do a lot
of testing that involves mkfs.btrfs on nvme and haven't seen it..

> A git bisect tracked the behaviour down to commit 729204ef49ec ("block:
> relax check on sg gap"). Since commit 729204ef49ec a bio's offsets are not
> taken into account in the decision if the bio will gap any more. Restore
> the old behavior of checking bio offsets as well for the decision if a
> bio will gap.
> 
> Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
> Fixes: 729204ef49ec ("block: relax check on sg gap")
> Cc: Ming Lei <ming.lei@redhat.com>
> ---
>  include/linux/blkdev.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 7548f332121a..a03b7196209e 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1677,11 +1677,14 @@ static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
>  {
>  	if (bio_has_data(prev) && queue_virt_boundary(q)) {
>  		struct bio_vec pb, nb;
> +		bool offset;
>  
>  		bio_get_last_bvec(prev, &pb);
>  		bio_get_first_bvec(next, &nb);
>  
> -		if (!bios_segs_mergeable(q, prev, &pb, &nb))
> +		offset = pb.bv_offset || nb.bv_offset;
> +
> +		if (offset || !bios_segs_mergeable(q, prev, &pb, &nb))
>  			return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
>  	}

I think the code in NVMe (and potentially the other drivers using
virt_queue_boundary) is bogus.  All of them are actually fine with
gaps in the protocol, as long as the gaps are aligned to said boundary.

So I suspect what we really need is to fix up NVMe, and after that
we could even relax the above check, to not check for offset but
offset & queue_virt_boundary(q).