[Bug 215740] kernel warning: DMA-API: xhci_hcd: cacheline tracking EEXIST, overlapping mappings aren't supported

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=215740

Mikhail (mikhail.v.gavrilov@gmail.com) changed:

           What    |Removed                     |Added
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                 CC|                            |mikhail.v.gavrilov@gmail.co
                   |                            |m

--- Comment #51 from Mikhail (mikhail.v.gavrilov@gmail.com) ---
Created attachment 309769
  --> https://bugzilla.kernel.org/attachment.cgi?id=309769&action=edit
dma_cacheline_eexist_repro.mod.c

I hit this exact warning on 7.0-rc5 with an ASUS USB Audio device
(0b05:1a5c) on AMD B650E (ROG STRIX B650E-I) + xhci_hcd 0000:0c:00.0.
Same stack trace through snd_usb_ctl_msg -> usb_control_msg ->
usb_hcd_map_urb_for_dma -> dma_map_phys -> add_dma_entry -> EEXIST.

I tested Guenter's patch from comment #43 (ARCH_KMALLOC_MINALIGN =
L1_CACHE_BYTES when CONFIG_DMA_API_DEBUG) and can confirm it fixes
the issue.

I also wrote a kernel module reproducer that triggers the bug reliably
on demand, without needing to wait for a specific USB device to be
plugged in during boot. The module does exactly what hub_configure()
does: two kmalloc(8) allocations back-to-back, then DMA-maps both.
On x86_64 with ARCH_KMALLOC_MINALIGN=8, they land in the same
64-byte cacheline within ~50 attempts, triggering the EEXIST warning.

Before fix:
  dma_repro: pair 53: buf_a=ffff8881c8f9eb80 buf_b=ffff8881c8f9eba0
             -- SAME cacheline 119793582!
  DMA-API: cacheline tracking EEXIST, overlapping mappings aren't
           supported

After fix (ARCH_KMALLOC_MINALIGN >= L1_CACHE_BYTES):
  dma_repro: no same-cacheline pair found in 64 attempts
             (ARCH_KMALLOC_MINALIGN may already be >= cache_line_size)

I can submit a formal patch based on Guenter's fix if nobody objects.

Reproducer module source attached.

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