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

[Harry Yoo (Oracle)]

On Fri, Mar 27, 2026 at 10:58:46AM +0500, 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.

Is it feasible to suppress the warning if dma_get_cache_alignment() is
smaller than L1_CACHE_BYTES?
 
> 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>

-- 
Cheers,
Harry / Hyeonggon