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

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

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@rowland.harvard.edu>
Suggested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Jeff Kirsher <Jeff.kirsher@gmail.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---

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
-- 
2.53.0