Re: [PATCH v2] dma-debug: suppress cacheline overlap warning when arch has no DMA alignment requirement

[Marek Szyprowski]

On 27.03.2026 13:41, Mikhail Gavrilov wrote:
> When CONFIG_DMA_API_DEBUG is enabled, the DMA debug infrastructure
> tracks active mappings per cacheline and warns if two different DMA
> mappings share the same cacheline ("cacheline tracking EEXIST,
> overlapping mappings aren't supported").
>
> On x86_64, ARCH_KMALLOC_MINALIGN defaults to 8, so small kmalloc
> allocations (e.g. the 8-byte hub->buffer and hub->status in the USB
> hub driver) frequently land in the same 64-byte cacheline.  When both
> are DMA-mapped, this triggers a false positive warning.
>
> This has been reported repeatedly since v5.14 (when the EEXIST check
> was added) across various USB host controllers and devices including
> xhci_hcd with USB hubs, USB audio devices, and USB ethernet adapters.
>
> The cacheline overlap is only a real concern on architectures that
> require DMA buffer alignment to cacheline boundaries (i.e. where
> ARCH_DMA_MINALIGN >= L1_CACHE_BYTES).  On architectures like x86_64
> where dma_get_cache_alignment() returns 1, the hardware is
> cache-coherent and overlapping cacheline mappings are harmless.
>
> Suppress the EEXIST warning when dma_get_cache_alignment() is less
> than L1_CACHE_BYTES, indicating the architecture does not require
> cacheline-aligned DMA buffers.
>
> Verified with a kernel module reproducer that performs two kmalloc(8)
> allocations back-to-back and DMA-maps both:
>
>   Before: allocations share a cacheline, EEXIST fires within ~50 pairs
>   After:  same cacheline pair found, but no warning emitted
>
> Fixes: 2b4bbc6231d7 ("dma-debug: report -EEXIST errors in add_dma_entry")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=215740
> Suggested-by: Harry Yoo <harry@xxxxxxxxxx>
> Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>

Applied to dma-mapping-fixes. Thanks!

> ---
>
> v1 -> v2:
>   - Moved fix from include/linux/slab.h (ARCH_KMALLOC_MINALIGN)
>     to kernel/dma/debug.c per Harry Yoo's suggestion.
>   - Instead of forcing cacheline-aligned allocations, suppress
>     the warning when the architecture has no DMA alignment
>     requirement (dma_get_cache_alignment() < L1_CACHE_BYTES).
>
> v1: https://lore.kernel.org/all/20260327055846.248829-1-mikhail.v.gavrilov@xxxxxxxxx/
>
> Reproducer module that triggers the bug reliably:
>   https://bugzilla.kernel.org/attachment.cgi?id=309769
>
>  kernel/dma/debug.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 0677918f06a8..1a725edbbbf6 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -615,6 +615,7 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
>  	} else if (rc == -EEXIST &&
>  		   !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
>  		   !(entry->is_cache_clean && overlap_cache_clean) &&
> +		   dma_get_cache_alignment() >= L1_CACHE_BYTES &&
>  		   !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
>  		     is_swiotlb_active(entry->dev))) {
>  		err_printk(entry->dev, entry,

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland