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Date: Fri, 9 Jan 2026 06:58:52 +0100
From: Christoph Hellwig <hch@lst.de>
To: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
	Pavel Begunkov <asml.silence@gmail.com>,
	linux-block@vger.kernel.org, io-uring@vger.kernel.org,
	Vishal Verma <vishal1.verma@intel.com>, tushar.gohad@intel.com,
	Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@kernel.dk>,
	Sagi Grimberg <sagi@grimberg.me>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-media@vger.kernel.org,
	dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org
Subject: Re: [RFC v2 01/11] file: add callback for pre-mapping dmabuf
Message-ID: <20260109055852.GA5063@lst.de>
References: <7b2017f4-02a3-482a-a173-bb16b895c0cb@amd.com> <20251204110709.GA22971@lst.de> <0571ca61-7b17-4167-83eb-4269bd0459fe@amd.com> <20251204131025.GA26860@lst.de> <aVnFnzRYWC_Y5zHg@fedora> <754b4cc9-20ab-4d87-85bf-eb56be058856@amd.com> <20260107160151.GA21887@lst.de> <aV8UJvkt7VGzHjxS@fedora> <20260108101703.GA24709@lst.de> <aWBjsa2RZ_uaO9Ns@fedora>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Fri, Jan 09, 2026 at 10:10:57AM +0800, Ming Lei wrote:
> On Thu, Jan 08, 2026 at 11:17:03AM +0100, Christoph Hellwig wrote:
> > On Thu, Jan 08, 2026 at 10:19:18AM +0800, Ming Lei wrote:
> > > > The feature is in no way nvme specific.  nvme is just the initial
> > > > underlying driver.  It makes total sense to support this for any high
> > > > performance block device, and to pass it through file systems.
> > > 
> > > But why does FS care the dma buffer attachment? Since high performance
> > > host controller is exactly the dma buffer attachment point.
> > 
> > I can't parse what you're trying to say here.
> 
> dma buffer attachment is simply none of FS's business.

The file systems should indeed never do a dma buffer attachment itself,
but that's not the point.

> > But even when not stacking, the registration still needs to go
> > through the file system even for a single device, never mind multiple
> > controlled by the file system.
> 
> dma_buf can have multiple importers, so why does it have to go through FS for
> single device only?
>
> If the registered buffer is attached to single device before going
> through FS, it can not support stacking block device, and it can't or not
> easily to use for multiple block device, no matter if they are behind same
> host controller or multiple.

Because the file system, or the file_operations instance to be more
specific, is the only entity that known what block device(s) or other DMA
capable device(s) like (R)NIC a file maps to.