Re: [PATCH] iommu/dma: Rate-limit WARN in iommu_dma_unmap_phys()

[Robin Murphy <robin.murphy@arm.com>]

On 2026-02-11 3:13 pm, Breno Leitao wrote:
> When a PCI error (e.g. AER error or DPC containment) marks the PCI
> channel as frozen or permanently failed, the IOMMU mappings for the
> device may already be torn down. If a driver continues processing
> completions in this state, every call to dma_unmap_page() triggers a
> WARN_ON in iommu_dma_unmap_phys().

That is definitely a major bug in the caller that needs fixing rather 
than papering over. You're lucky you do have an IOMMU that needs a valid 
mapping before it can get as far as the "corrupting memory" part of 
dma_unmap_phys() being called inappropriately.

Thanks,
Robin.

> In a real-world crash scenario on an NVIDIA Grace (ARM64) platform, a
> DPC event froze the PCI channel and the mlx5 NAPI poll continued
> processing error CQEs, calling dma_unmap for each pending WQE. With
> dozens of pending WQEs, the resulting WARN_ON storm monopolized the CPU
> in softirq context for over 23 seconds, triggering a soft lockup panic.
> 
> Replace WARN_ON(!phys) with WARN_RATELIMIT() to cap the warning output
> at the kernel's default rate limit (10 messages per 5 seconds), while
> still providing visibility into the failure with the device name in the
> message.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
> ---
> I initially attempted to fix this in the driver itself, but that approach
> doesn't appear to be optimal, given the mappings can go away at any
> time, which is impossible to check at any time. Please see the discussion at:
> 
> https://lore.kernel.org/all/20260209-mlx5_iommu-v1-1-b17ae501aeb2@debian.org/
> ---
>   drivers/iommu/dma-iommu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index c92088855450a..3cb5948eafe86 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1239,7 +1239,8 @@ void iommu_dma_unmap_phys(struct device *dev, dma_addr_t dma_handle,
>   	}
>   
>   	phys = iommu_iova_to_phys(iommu_get_dma_domain(dev), dma_handle);
> -	if (WARN_ON(!phys))
> +	if (WARN_RATELIMIT(!phys, "iova_to_phys translation failed for dev %s\n",
> +			   dev_name(dev)))
>   		return;
>   
>   	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) && !dev_is_dma_coherent(dev))
> 
> ---
> base-commit: f884ff9142ee4b741a88030d77feede84f51fd4f
> change-id: 20260211-dma_io_mmu-519b73988134
> 
> Best regards,
> --
> Breno Leitao <leitao@debian.org>
>