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Date: Tue, 9 Jan 2024 19:21:45 +0200
From: Ido Schimmel <idosch@idosch.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: joro@8bytes.org, will@kernel.org, iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org, zhangzekun11@huawei.com,
	john.g.garry@oracle.com, dheerajkumar.srivastava@amd.com,
	jsnitsel@redhat.com
Subject: Re: [PATCH v3 0/2] iommu/iova: Make the rcache depot properly
 flexible
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 <c9cf02b5-7add-46ea-8db1-46fdce191c1c@arm.com>
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In-Reply-To: <c9cf02b5-7add-46ea-8db1-46fdce191c1c@arm.com>

Hi Robin,

Thanks for the reply.

On Mon, Jan 08, 2024 at 05:35:26PM +0000, Robin Murphy wrote:
> Hmm, we've got what looks to be a set of magazines forming a plausible depot
> list (or at least the tail end of one):
> 
> ffff8881411f9000 -> ffff8881261c1000
> 
> ffff8881261c1000 -> ffff88812be26400
> 
> ffff88812be26400 -> ffff8188392ec000
> 
> ffff8188392ec000 -> ffff8881a5301000
> 
> ffff8881a5301000 -> NULL
> 
> which I guess has somehow become detached from its rcache->depot without
> being freed properly? However I'm struggling to see any conceivable way that
> could happen which wouldn't already be more severely broken in other ways as
> well (i.e. either general memory corruption or someone somehow still trying
> to use the IOVA domain while it's being torn down).

The machine is running a debug kernel that among other things has KASAN
enabled, but there are no traces in the kernel log so there is no memory
corruption that I'm aware of.

> Out of curiosity, does reverting just patch #2 alone make a difference?

Will try and let you know.

> And is your workload doing anything "interesting" in relation to IOVA
> domain lifetimes, like creating and destroying SR-IOV virtual
> functions, changing IOMMU domain types via sysfs, or using that
> horrible vdpa thing, or are you seeing this purely from regular driver
> DMA API usage?

The machine is running networking related tests, but it is not using
SR-IOV, VMs or VDPA so there shouldn't be anything "interesting" as far
as IOMMU is concerned.

The two networking drivers on the machine are "igb" for the management
port and "mlxsw" for the data ports (the machine is a physical switch).
I believe the DMA API usage in the latter is quite basic and I don't
recall any DMA related problems with this driver since it was first
accepted upstream in 2015.

Thanks