* Re: [PATCH] nvme-pci: fix single segment detection [not found] <20190509110409.19647-1-hch@lst.de> @ 2019-05-09 11:24 ` Ming Lei 2019-05-09 12:34 ` Christoph Hellwig 0 siblings, 1 reply; 4+ messages in thread From: Ming Lei @ 2019-05-09 11:24 UTC (permalink / raw) To: Christoph Hellwig Cc: keith.busch, sagi, axboe, linux-nvme, linux-block, Jens Axboe On Thu, May 09, 2019 at 01:04:09PM +0200, Christoph Hellwig wrote: > blk_rq_nr_phys_segments isn't exactly accurate at the request/bio > level, so work around that fact with a few crude sanity checks. To > be fixed for real in the block layer soon. > > Fixes: 297910571f08 ("nvme-pci: optimize mapping single segment requests using SGLs") > Signed-off-by: Christoph Hellwig <hch@lst.de> > --- > drivers/nvme/host/pci.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c > index 2a8708c9ac18..9a4253be2723 100644 > --- a/drivers/nvme/host/pci.c > +++ b/drivers/nvme/host/pci.c > @@ -824,7 +824,15 @@ static blk_status_t nvme_map_data(struct nvme_dev *dev, struct request *req, > blk_status_t ret = BLK_STS_RESOURCE; > int nr_mapped; > > - if (blk_rq_nr_phys_segments(req) == 1) { > + /* > + * Crude hack to work around the block layer accounting for the fact > + * that the SG mapping can merge multiple physically contigous segments > + * in blk_rq_nr_phys_segments() despite the fact that not everyone is > + * mapping the request to a scatterlist. To be fixed for real in the > + * block layer eventually.. > + */ > + if (blk_rq_nr_phys_segments(req) == 1 && > + !req->bio->bi_next && req->bio->bi_vcnt == 1) { > struct bio_vec bv = req_bvec(req); I'd suggest to fix block layer instead of working around the issue here, then any driver may benefit from the fix. Especially checking bio->bi_vcnt is just a hack, and drivers should never use .bi_vcnt. Thanks, Ming ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] nvme-pci: fix single segment detection 2019-05-09 11:24 ` [PATCH] nvme-pci: fix single segment detection Ming Lei @ 2019-05-09 12:34 ` Christoph Hellwig 2019-05-09 13:31 ` Ming Lei 0 siblings, 1 reply; 4+ messages in thread From: Christoph Hellwig @ 2019-05-09 12:34 UTC (permalink / raw) To: Ming Lei Cc: Christoph Hellwig, keith.busch, sagi, axboe, linux-nvme, linux-block, Jens Axboe On Thu, May 09, 2019 at 07:24:12PM +0800, Ming Lei wrote: > I'd suggest to fix block layer instead of working around the issue here, > then any driver may benefit from the fix. That is my plan, and I started on it. But the fix isn't trivial, and will probably take a while and be invasive. > Especially checking bio->bi_vcnt is just a hack, and drivers should > never use .bi_vcnt. That is why it is explicitly commented as a hack. But a good enough one to still speed up typical 4K I/Os - one a bio has been cloned chances are very high we don't care about the fast path any more. ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] nvme-pci: fix single segment detection 2019-05-09 12:34 ` Christoph Hellwig @ 2019-05-09 13:31 ` Ming Lei 2019-05-13 6:26 ` Christoph Hellwig 0 siblings, 1 reply; 4+ messages in thread From: Ming Lei @ 2019-05-09 13:31 UTC (permalink / raw) To: Christoph Hellwig Cc: keith.busch, sagi, axboe, linux-nvme, linux-block, Jens Axboe On Thu, May 09, 2019 at 02:34:06PM +0200, Christoph Hellwig wrote: > On Thu, May 09, 2019 at 07:24:12PM +0800, Ming Lei wrote: > > I'd suggest to fix block layer instead of working around the issue here, > > then any driver may benefit from the fix. > > That is my plan, and I started on it. But the fix isn't trivial, and > will probably take a while and be invasive. > > > Especially checking bio->bi_vcnt is just a hack, and drivers should > > never use .bi_vcnt. > > That is why it is explicitly commented as a hack. But a good enough > one to still speed up typical 4K I/Os - one a bio has been cloned > chances are very high we don't care about the fast path any more. NVMe hasn't max segment size limit, so fair enough for typical 4K IO workload: Reviewed-by: Ming Lei <ming.lei@redhat.com> Thanks, Ming ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] nvme-pci: fix single segment detection 2019-05-09 13:31 ` Ming Lei @ 2019-05-13 6:26 ` Christoph Hellwig 0 siblings, 0 replies; 4+ messages in thread From: Christoph Hellwig @ 2019-05-13 6:26 UTC (permalink / raw) To: Ming Lei Cc: Christoph Hellwig, keith.busch, sagi, axboe, linux-nvme, linux-block, Jens Axboe So actually, after running out ways to make any of my complex block layer fix ideas work I have a simple fix for the block layer now. Lets see if is acceptable.. ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2019-05-13 6:26 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- [not found] <20190509110409.19647-1-hch@lst.de> 2019-05-09 11:24 ` [PATCH] nvme-pci: fix single segment detection Ming Lei 2019-05-09 12:34 ` Christoph Hellwig 2019-05-09 13:31 ` Ming Lei 2019-05-13 6:26 ` Christoph Hellwig
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