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

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Message-ID: <20250422043955.GA28077@lst.de>
Date: Tue, 22 Apr 2025 06:39:56 +0200
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...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>,
	Leon Romanovsky <leonro@...dia.com>
Subject: Re: [PATCH v8 24/24] nvme-pci: optimize single-segment handling

On Fri, Apr 18, 2025 at 09:47:54AM +0300, 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.

This needs to be folded into the earlier patches or split prep patches
instead of undoing work done earlier, preferably combined with a bit
of code movement so that the new nvme_try_setup_prp_simple stays in
the same place as before and the diff shows it reusing code.

E.g. change

"nvme-pci: use a better encoding for small prp pool allocations" to
already use the flags instead of my boolean, and maybe include 
abort in the flags instead of using a separate bool so that we
don't increase hte iod size.

Slot in a new patch after that that dropping the single SGL segment
fastpath if we think we don't need that, although if we need the PRP
one I suspect that one would still be very helpful as well.

Add a patch if we want the try_ version of, although when keeping
the optimization for SGLs as well that are will look a bit different.

I'm happy to give away my patch authorship credits if that helps with
the folding.