Re: [PATCH] mm/slab: align kmalloc to cacheline when DMA API debugging is active

[Guenter Roeck]

On 3/26/26 22:58, 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.
>
> Raise ARCH_KMALLOC_MINALIGN to L1_CACHE_BYTES when CONFIG_DMA_API_DEBUG
> is enabled, ensuring each kmalloc allocation occupies its own cacheline
> and eliminating the false positive.
>
> 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:  64 pairs allocated, all in separate cachelines, no warning
>
> Fixes: 2b4bbc6231d7 ("dma-debug: report -EEXIST errors in add_dma_entry")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=215740
> Suggested-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Suggested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Tested-by: Jeff Kirsher <Jeff.kirsher@xxxxxxxxx>
> Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>

Thanks a lot for taking care of this!

FWIW:

Reviewed-by: Guenter Roeck <linux@xxxxxxxxxxxx>

> ---
>
> Reproducer module that triggers the bug reliably:
>    https://bugzilla.kernel.org/attachment.cgi?id=309769
>
>   include/linux/slab.h | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 15a60b501b95..f044956e17c1 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -536,6 +536,19 @@ static inline bool kmem_dump_obj(void *object) { return false; }
>   #endif
>   #endif
>   
> +/*
> + * Align memory allocations to cache lines if DMA API debugging is active
> + * to avoid false positive DMA overlapping error messages.
> + */
> +#ifdef CONFIG_DMA_API_DEBUG
> +#ifndef ARCH_KMALLOC_MINALIGN
> +#define ARCH_KMALLOC_MINALIGN  L1_CACHE_BYTES
> +#elif ARCH_KMALLOC_MINALIGN < L1_CACHE_BYTES
> +#undef ARCH_KMALLOC_MINALIGN
> +#define ARCH_KMALLOC_MINALIGN  L1_CACHE_BYTES
> +#endif
> +#endif
> +
>   #ifndef ARCH_KMALLOC_MINALIGN
>   #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
>   #elif ARCH_KMALLOC_MINALIGN > 8