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

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Message-ID: <20250422113643.GE48485@unreal>
Date: Tue, 22 Apr 2025 14:36:43 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, 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 Tue, Apr 22, 2025 at 06:39:56AM +0200, Christoph Hellwig wrote:
> 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 uploaded new tag dma-split-Apr-22 with the changes. At the end, I
decided to keep SGL optimized path too, but didn't create separate
patch for try_* variants as they require block iterator, which is
introduced in conversion patch only.

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

I kept you as an author. Please advise if I need to change it to be someone else.

Thanks