linux-kernel - Re: [PATCH v8 24/24] nvme-pci: optimize single-segment handling

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Message-ID: <20250418111949.GC199604@unreal>
Date: Fri, 18 Apr 2025 14:19:49 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Damien Le Moal <dlemoal@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>,
	Kanchan Joshi <joshi.k@...sung.com>, Jake Edge <jake@....net>,
	Jonathan Corbet <corbet@....net>, Jason Gunthorpe <jgg@...pe.ca>,
	Zhu Yanjun <zyjzyj2000@...il.com>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Chuck Lever <chuck.lever@...cle.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Nitesh Shetty <nj.shetty@...sung.com>
Subject: Re: [PATCH v8 24/24] nvme-pci: optimize single-segment handling

On Fri, Apr 18, 2025 at 05:02:38PM +0900, Damien Le Moal wrote:
> On 4/18/25 15:47, Leon Romanovsky wrote:
> > From: Kanchan Joshi <joshi.k@...sung.com>
> > 
> > blk_rq_dma_map API is costly for single-segment requests.
> > Avoid using it and map the bio_vec directly.
> > 
> > Signed-off-by: Kanchan Joshi <joshi.k@...sung.com>
> > Signed-off-by: Nitesh Shetty <nj.shetty@...sung.com>
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > ---
> >  drivers/nvme/host/pci.c | 65 +++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 60 insertions(+), 5 deletions(-)

<...>

> >  static void nvme_unmap_data(struct nvme_dev *dev, struct request *req)
> >  {
> >  	struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
> > +	unsigned int nr_segments = blk_rq_nr_phys_segments(req);
> > +	dma_addr_t dma_addr;
> > +
> > +	if (nr_segments == 1 && (iod->flags & IOD_SINGLE_SEGMENT)) {
> 
> nvme_pci_setup_prps() calls nvme_try_setup_prp_simple() which sets
> IOD_SINGLE_SEGMENT if and only if the req has a single phys segment. So why do
> you need to count the segments again here ? Looking at the flag only should be
> enough, no ?

Yes, you are right. There is no need in extra check of nr_segments and
it is enough to rely on IOD_SINGLE_SEGMENT.

Thanks