linux-kernel - Re: [PATCH v4 15/16] block-dma: properly take MMIO path

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Message-ID: <aLCpqI-VQ7KeB6DL@kbusch-mbp>
Date: Thu, 28 Aug 2025 13:10:32 -0600
From: Keith Busch <kbusch@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Danilo Krummrich <dakr@...nel.org>,
	iommu@...ts.linux.dev, Jason Wang <jasowang@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Jonathan Corbet <corbet@....net>, Juergen Gross <jgross@...e.com>,
	kasan-dev@...glegroups.com, linux-block@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvme@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, linux-trace-kernel@...r.kernel.org,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Robin Murphy <robin.murphy@....com>, rust-for-linux@...r.kernel.org,
	Sagi Grimberg <sagi@...mberg.me>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux.dev, Will Deacon <will@...nel.org>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v4 15/16] block-dma: properly take MMIO path

On Thu, Aug 28, 2025 at 03:41:15PM -0300, Jason Gunthorpe wrote:
> On Thu, Aug 28, 2025 at 11:15:20AM -0600, Keith Busch wrote:
> > 
> > I don't think that was ever the case. Metadata is allocated
> > independently of the data payload, usually by the kernel in
> > bio_integrity_prep() just before dispatching the request. The bio may
> > have a p2p data payload, but the integrity metadata is just a kmalloc
> > buf in that path.
> 
> Then you should do two dma mapping operations today, that is how the
> API was built. You shouldn't mix P2P and non P2P within a single
> operation right now..

Data and metadata are mapped as separate operations. They're just
different parts of one blk-mq request.