Re: [PATCH v2] dma-mapping: move dma_map_resource() sanity check into debug code

[Leon Romanovsky]

On Mon, May 11, 2026 at 04:31:33PM +0800, Jianpeng Chang wrote:
> dma_map_resource() uses pfn_valid() to ensure the range is not RAM.
> However, pfn_valid() only checks for availability of the memory map for
> a PFN but it does not ensure that the PFN is actually backed by RAM. On
> ARM64 with SPARSEMEM (128MB section granularity), MMIO addresses that
> share a section with RAM will falsely trigger the WARN_ON_ONCE and cause
> dma_map_resource() to return DMA_MAPPING_ERROR.
> 
> This causes a WARNING on Raspberry Pi 4 during spi_bcm2835 probe because
> the SPI FIFO register (0xfe204004) falls in the same sparsemem section
> as the end of RAM (0xf8000000-0xfbffffff), both in section 31
> (0xf8000000-0xffffffff).
> 
> Move the sanity check from dma_map_resource() into debug_dma_map_phys()
> and replace the unreliable pfn_valid() with pfn_valid() &&
> !PageReserved(), which correctly identifies actual usable RAM without
> false positives for MMIO regions that happen to have struct pages.
> 
> Since dma_map_resource() is dma_map_phys(DMA_ATTR_MMIO), the check
> applies equally to both APIs. Any non-reserved page represents kernel
> memory to a sufficient degree that using DMA_ATTR_MMIO on it is almost
> certainly wrong and risks breaking coherency on non-coherent platforms.
> ZONE_DEVICE pages used for PCI P2P DMA (MEMORY_DEVICE_PCI_P2PDMA) have
> PageReserved set, so they will not trigger a false positive.
> 
> The check is now a WARN_ONCE that no longer blocks the mapping, since
> being unobtrusive is more important than being exhaustive for what is
> merely a debug sanity check.
> 
> Fixes: f7326196a781 ("dma-mapping: export new dma_*map_phys() interface")
> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@xxxxxxxxxxxxx>
> ---
> v2:
>   - move check to debug_dma_map_phys and replace pfn_valid() with
>     pfn_valid() && !PageReserved() as Robin suggested.
>   - update commit message to explain why PageReserved is safe for
>     ZONE_DEVICE PCI_P2PDMA pages
> v1: https://lore.kernel.org/all/20260507032120.4072283-1-jianpeng.chang.cn@xxxxxxxxxxxxx/
>  kernel/dma/debug.c   | 9 +++++++++
>  kernel/dma/mapping.c | 4 ----
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 1a725edbbbf6..180aa2c930b5 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -1239,6 +1239,15 @@ void debug_dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
>  	if (dma_mapping_error(dev, dma_addr))
>  		return;
>  
> +	if (attrs & DMA_ATTR_MMIO) {
> +		unsigned long pfn = PHYS_PFN(phys);
> +
> +		WARN_ONCE(pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn)),
> +			  "dma_map_resource called for RAM address %pa\n",
> +			  &phys);
> +		return;

I’m not comfortable with this return statement. It effectively disables  
DMA debugging for any caller that uses dma_map_phys(..., DMA_ATTR_MMIO),
like dma-buf, block layer, HMM, e.t.c

Thanks

> +	}
> +
>  	entry = dma_entry_alloc();
>  	if (!entry)
>  		return;
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 23ed8eb9233e..e6b07f160d20 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -365,10 +365,6 @@ EXPORT_SYMBOL(dma_unmap_sg_attrs);
>  dma_addr_t dma_map_resource(struct device *dev, phys_addr_t phys_addr,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {
> -	if (IS_ENABLED(CONFIG_DMA_API_DEBUG) &&
> -	    WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
> -		return DMA_MAPPING_ERROR;
> -
>  	return dma_map_phys(dev, phys_addr, size, dir, attrs | DMA_ATTR_MMIO);
>  }
>  EXPORT_SYMBOL(dma_map_resource);
> -- 
> 2.54.0
>