linux-kernel - Re: [PATCH v9 01/24] PCI/P2PDMA: Refactor the p2pdma mapping helpers

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Message-ID: <20250427072507.GB5848@unreal>
Date: Sun, 27 Apr 2025 10:25:07 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>, 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>,
	Matthew Wilcox <willy@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Kanchan Joshi <joshi.k@...sung.com>,
	Chaitanya Kulkarni <kch@...dia.com>
Subject: Re: [PATCH v9 01/24] PCI/P2PDMA: Refactor the p2pdma mapping helpers

On Fri, Apr 25, 2025 at 05:21:59PM -0700, Luis Chamberlain wrote:
> On Wed, Apr 23, 2025 at 11:12:52AM +0300, Leon Romanovsky wrote:
> > From: Christoph Hellwig <hch@....de>
> > 
> > The current scheme with a single helper to determine the P2P status
> > and map a scatterlist segment force users to always use the map_sg
> > helper to DMA map, which we're trying to get away from because they
> > are very cache inefficient.
> > 
> > Refactor the code so that there is a single helper that checks the P2P
> > state for a page, including the result that it is not a P2P page to
> > simplify the callers, and a second one to perform the address translation
> > for a bus mapped P2P transfer that does not depend on the scatterlist
> > structure.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> > Reviewed-by: Logan Gunthorpe <logang@...tatee.com>
> > Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
> > Tested-by: Jens Axboe <axboe@...nel.dk>
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> 
> Might make it easier for patch review to split off adding
> __pci_p2pdma_update_state() in a seprate patch first.

Original code __pci_p2pdma_update_state() had this code and was
dependent on SG, which we are removing in this patch.

       if (state->map == PCI_P2PDMA_MAP_BUS_ADDR) {
               sg->dma_address = sg_phys(sg) + state->bus_off;
               sg_dma_len(sg) = sg->length;
               sg_dma_mark_bus_address(sg);
       }

So to split, we would need to introduce new version of __pci_p2pdma_update_state(),
rename existing one to something like __pci_p2pdma_update_state2() and
remove it in next patch. Such pattern of adding and immediately deleting
code is not welcomed.

> Other than that, looks good.
> 
> Reviewed-by: Luis Chamberlain <mcgrof@...rel.org>

Thanks

> 
>   Luis