On Thu, May 16, 2019 at 09:17:04PM +0800, Ming Lei wrote:
> ll_merge_requests_fn() is only called from attempt_merge() in case
> that ELEVATOR_BACK_MERGE is returned from blk_try_req_merge(). However,
> for discard merge of both virtio_blk and nvme, ELEVATOR_DISCARD_MERGE is
> always returned from blk_try_req_merge() in attempt_merge(), so looks
> ll_merge_requests_fn() shouldn't be called for virtio_blk/nvme's discard
> request. Just wondering if you may explain a bit how the change on
> ll_merge_requests_fn() in this patch makes a difference on the above
> two commits?
Good question. I've seen virtio overwriting its range, but I think
this might have been been with a series to actually decrement
nr_phys_segments for all cases where we can merge the tail and front
bvecs. So mainline probably doesn't see it unless someone calls
blk_recalc_rq_segments due to a partial completion or when using
dm-multipath. Thinking of it at least the latter seems like a real
possibily, although a rather unlikely use case.
> > Fixes: 297910571f08 ("nvme-pci: optimize mapping single segment requests using SGLs")
>
> I guess it should be dff824b2aadb ("nvme-pci: optimize mapping of small
> single segment requests").
Indeed.
