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

[Marek Szyprowski]

Hi

On 27.03.2026 13:26, Catalin Marinas wrote:
> + Marek, Robin

Thanks for adding me to the loop.

> 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.
>>
>> 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.
> This indeed has come up regularly in the past years.
>
>> +/*
>> + * 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
> TL;DR: I think this is fine:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>
> I'm not sure that's the best way to hide the warning but there
> are no great solutions either. On one hand, we want the DMA debug to
> capture potential problems on architectures it's not running on. OTOH,
> we also want to avoid false positives on coherent architectures/devices.
> I don't think reconciling the two requirements is easy.
>
> When DMA_API_DEBUG is enabled, the above will change the x86 behaviour
> that could have implications beyond DMA (e.g. may not catch some buffer
> overflow because it's within L1_CACHE_BYTES). Similarly for non-coherent
> architectures that select DMA_BOUNCE_UNALIGNED_KMALLOC (arm64 and riscv
> currently). arm64 defines ARCH_DMA_MINALIGN to 128 but
> ARCH_KMALLOC_MINALIGN to 8 (why 128 is larger than L1_CACHE_BYTES is
> another matter but let's ignore it for now).

IMHO enabling DMA_API_DEBUG should not change the kernel behavior, so I 
would prefer fixing this in DMA-debug code somehow.

> More of a thinking out loud, we have:
>
> 1. Coherent architectures - alignment doesn't matter
>
> 2. Non-coherent architectures with:
>     a) Sufficiently large ARCH_KMALLOC_MINALIGN
>     b) Small ARCH_KMALLOC_MINALIGN but DMA_BOUNCE_UNALIGNED_KMALLOC
>     c) Broken config - forgot to set ARCH_DMA_MINALIGN or bouncing
>
> We can ignore (2.c), the aim of the DMA debug is to catch wrong uses in
> drivers. If drivers is the only goal, the above change will do when
> running on (1) or (2.a) hardware - it will catch sub-L1_CACHE_BYTES
> buffers from drivers while assuming kmalloc() machinery is safe.
> However, if running on (2.b) it won't catch anything that may be
> problematic on (2.a) since the DMA debug ignores the overlap.
>
> We could make DMA_BOUNCE_UNALIGNED_KMALLOC dependent on !DMA_API_DEBUG
> but it would be nice to be able to sanity-check the bouncing logic.
> Well, it wasn't checking it before and with commit 03521c892bb8
> ("dma-debug: don't report false positives with
> DMA_BOUNCE_UNALIGNED_KMALLOC"), we made this clear that overlapping will
> be ignored.
>
> Irrespective of whether we disable bouncing with DMA_API_DEBUG, maybe we
> could replace the above commit with:
>
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 3928a509c44c..488045ef6245 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -175,7 +175,7 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
>   	if (!is_mmio)
>   		kmsan_handle_dma(phys, size, dir);
>   	trace_dma_map_phys(dev, phys, addr, size, dir, attrs);
> -	debug_dma_map_phys(dev, phys, size, dir, addr, attrs);
> +	debug_dma_map_phys(dev, dma_to_phys(addr), size, dir, addr, attrs);
>
>   	return addr;
>   }
>
> Anyway, this I think is unrelated to the proposed change affecting x86,
> more of a how to make the DMA API debugging more useful when running on
> arm64 or riscv.

This is not enough, there is also a dma_map_sg_attrs() path.

I've reverted 03521c892bb8 and added the following change:

diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c index 
55e7ca8ceb86..bbada41143ea 100644 --- a/kernel/dma/debug.c +++ 
b/kernel/dma/debug.c @@ -18,6 +18,7 @@ #include <linux/uaccess.h> 
#include <linux/export.h> #include <linux/device.h> +#include 
<linux/dma-direct.h> #include <linux/types.h> #include <linux/sched.h> 
#include <linux/ctype.h> @@ -1241,7 +1242,8 @@ void 
debug_dma_map_phys(struct device *dev, phys_addr_t phys, size_t size, 
entry->dev = dev; entry->type = dma_debug_phy; - entry->paddr = phys; + 
entry->paddr = IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) ? + 
dma_to_phys(dev, dma_addr) : phys; entry->dev_addr = dma_addr; 
entry->size = size; entry->direction = direction; @@ -1335,7 +1337,9 @@ 
void debug_dma_map_sg(struct device *dev, struct scatterlist *sg, 
entry->type = dma_debug_sg; entry->dev = dev; - entry->paddr = 
sg_phys(s); + entry->paddr = + 
IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) ? + dma_to_phys(dev, 
sg_dma_address(s)) : sg_phys(s); entry->size = sg_dma_len(s); 
entry->dev_addr = sg_dma_address(s); entry->direction = direction;

thenran my tests on ARM64 and RV64 boards. Only one new warning has been 
reported (I didn't analyze it yet), so this might be indeed a better 
solution than skipping overlapping cache lines warnings when 
DMA_BOUNCE_UNALIGNED_KMALLOC is set.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland