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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/27/26 15:09, Marek Szyprowski wrote:
> 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@arm.com>
>>
>> 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.

So what about Harry's proposal [1]? Mikhail seems to be on board? [2]

It seems it would achieve the goal that enabling DMA_API_DEBUG doesn't
change the kernel behavior? But I don't know this area too well so
maybe there's a catch.

[1] https://lore.kernel.org/all/acYlxRBhSMcwBnja@hyeyoo/
[2] https://lore.kernel.org/all/CABXGCsO_C8%2B%2B4%2BoPfZ%2BbQyrBnEGy5JFpXHkGNpfy%2B8%3D5BvVNfg@mail.gmail.com/

> 
>> 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