linux-kernel - Re: [PATCH v6 00/16] dma-mapping: migrate to physical address-based API

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Message-ID: <20250923170936.GA2614310@nvidia.com>
Date: Tue, 23 Sep 2025 14:09:36 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Keith Busch <kbusch@...nel.org>
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>,
	David Hildenbrand <david@...hat.com>, 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 v6 00/16] dma-mapping: migrate to physical address-based
 API

On Sat, Sep 20, 2025 at 06:47:27PM -0600, Keith Busch wrote:
> On Sat, Sep 20, 2025 at 06:53:52PM +0300, Leon Romanovsky wrote:
> > On Fri, Sep 19, 2025 at 10:08:21AM -0600, Keith Busch wrote:
> > > On Fri, Sep 12, 2025 at 12:03:27PM +0300, Leon Romanovsky wrote:
> > > > On Fri, Sep 12, 2025 at 12:25:38AM +0200, Marek Szyprowski wrote:
> > > > > >
> > > > > > This series does the core code and modern flows. A followup series
> > > > > > will give the same treatment to the legacy dma_ops implementation.
> > > > > 
> > > > > Applied patches 1-13 into dma-mapping-for-next branch. Let's check if it 
> > > > > works fine in linux-next.
> > > > 
> > > > Thanks a lot.
> > > 
> > > Just fyi, when dma debug is enabled, we're seeing this new warning
> > > below. I have not had a chance to look into it yet, so I'm just
> > > reporting the observation.
> > 
> > Did you apply all patches or only Marek's branch?
> > I don't get this warning when I run my NVMe tests on current dmabuf-vfio branch.
> 
> This was the snapshot of linux-next from the 20250918 tag. It doesn't
> have the full patchset applied.
> 
> One other thing to note, this was runing on arm64 platform using smmu
> configured with 64k pages. If your iommu granule is 4k instead, we
> wouldn't use the blk_dma_map_direct path.

I spent some time looking to see if I could guess what this is and
came up empty. It seems most likely we are leaking a dma mapping
tracking somehow? The DMA API side is pretty simple here though..

Not sure the 64k/4k itself is a cause, but triggering the non-iova
flow is probably the issue.

Can you check the output of this debugfs:

/*
 * Dump mappings entries on user space via debugfs
 */
static int dump_show(struct seq_file *seq, void *v)

? If the system is idle and it has lots of entries that is probably
confirmation of the theory.

Jason