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

[Mikhail Gavrilov]

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>
---

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,
-- 
2.53.0