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

[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@kernel.org>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---

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@gmail.com/

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

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

[Marek Szyprowski]

On 27.03.2026 13:41, 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.
>
> 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@kernel.org>
> Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

Applied to dma-mapping-fixes. Thanks!

> ---
>
> 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@gmail.com/
>
> 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,

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

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

[Robin Murphy]

On 2026-03-30 8:44 am, Marek Szyprowski wrote:
> On 27.03.2026 13:41, 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.
>>
>> 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.

Really the value of this check is for mappings of structure members or 
array elements which have no inherent guarantee of being individually 
aligned to ARCH_DMA_MINALIGN, and while x86 can also get away with that, 
it represents a genuine issue for non-coherent architectures. USB's 
mapping of a dedicated tiny allocation is, if anything, rather the 
special case.

TBH I'd be inclined to have CONFIG_DMA_DEBUG raise ARCH_DMA_MINALIGN as 
appropriate such that genuine false-positives can't happen, rather than 
effectively defeat the whole check, but I suppose that might carry a 
risk of tripping up arch code that doesn't expect it... Oh well.

Thanks,
Robin.

>> 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@kernel.org>
>> Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
>> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
> 
> Applied to dma-mapping-fixes. Thanks!
> 
>> ---
>>
>> 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@gmail.com/
>>
>> 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,
> 
> Best regards

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

[Andy Shevchenko]

On Wed, Apr 1, 2026 at 3:11 PM Robin Murphy <robin.murphy@arm.com> wrote:
> On 2026-03-30 8:44 am, Marek Szyprowski wrote:
> > On 27.03.2026 13:41, Mikhail Gavrilov wrote:

...

> TBH I'd be inclined to have CONFIG_DMA_DEBUG raise ARCH_DMA_MINALIGN as
> appropriate such that genuine false-positives can't happen, rather than
> effectively defeat the whole check,

I dunno if you read v1 thread, where I proposed to unroll the check
and use pr_debug_once() for the cases which we expect not to panic,
but would be good to have a track of.


-- 
With Best Regards,
Andy Shevchenko

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

[Robin Murphy]

On 2026-04-01 2:25 pm, Andy Shevchenko wrote:
> On Wed, Apr 1, 2026 at 3:11 PM Robin Murphy <robin.murphy@arm.com> wrote:
>> On 2026-03-30 8:44 am, Marek Szyprowski wrote:
>>> On 27.03.2026 13:41, Mikhail Gavrilov wrote:
> 
> ...
> 
>> TBH I'd be inclined to have CONFIG_DMA_DEBUG raise ARCH_DMA_MINALIGN as
>> appropriate such that genuine false-positives can't happen, rather than
>> effectively defeat the whole check,
> 
> I dunno if you read v1 thread, where I proposed to unroll the check
> and use pr_debug_once() for the cases which we expect not to panic,
> but would be good to have a track of.

I had not seen v1, as I took the last 3 days off and hadn't got that far 
up my inbox yet - I guess it's at least reassuring to have reached 
similar conclusions independently :)

The fundamental issue here is that dma-debug doesn't realistically have 
a way to know whether the thing being mapped is intentionally a whole 
dedicated kmalloc allocation - where we can trust SLUB (and 
DMA_BOUNCE_UNALIGNED_KMALLOC if appropriate) to do the right thing 
across different systems - or just something which might happen to line 
up by coincidence on someone's development machine, but for portability 
they definitely do still need to take explicit care about (e.g. struct 
devres::data).

Certainly some milder and/or opt-in reporting of 
potential-false-positive overlaps wouldn't be a terrible idea, if 
someone wants to have a crack at it.

Thanks,
Robin.

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

[Marek Szyprowski]

On 01.04.2026 17:26, Robin Murphy wrote:
> On 2026-04-01 2:25 pm, Andy Shevchenko wrote:
>> On Wed, Apr 1, 2026 at 3:11 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>> On 2026-03-30 8:44 am, Marek Szyprowski wrote:
>>>> On 27.03.2026 13:41, Mikhail Gavrilov wrote:
>>
>> ...
>>
>>> TBH I'd be inclined to have CONFIG_DMA_DEBUG raise ARCH_DMA_MINALIGN as
>>> appropriate such that genuine false-positives can't happen, rather than
>>> effectively defeat the whole check,
>>
>> I dunno if you read v1 thread, where I proposed to unroll the check
>> and use pr_debug_once() for the cases which we expect not to panic,
>> but would be good to have a track of.
>
> I had not seen v1, as I took the last 3 days off and hadn't got that far up my inbox yet - I guess it's at least reassuring to have reached similar conclusions independently :)
>
> The fundamental issue here is that dma-debug doesn't realistically have a way to know whether the thing being mapped is intentionally a whole dedicated kmalloc allocation - where we can trust SLUB (and DMA_BOUNCE_UNALIGNED_KMALLOC if appropriate) to do the right thing across different systems - or just something which might happen to line up by coincidence on someone's development machine, but for portability they definitely do still need to take explicit care about (e.g. struct devres::data).


Right, the debug code cannot distinguish them. I'm still convinced that increasing
ARCH_DMA_MINALIGN when DEBUG is enabled is not a good idea. I also agree that the
current exceptions for DMA_BOUNCE_UNALIGNED_KMALLOC and dma_get_cache_alignment() <
L1_CACHE_BYTES are wider than really needed. What we can do about it?

For the (dma_get_cache_alignment() < L1_CACHE_BYTES) case I came up with the idea
of reducing the check only to the dma_get_cache_alignment()-size overlapping:


diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index 86f87e43438c..9bf4b5c368f9 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -418,13 +418,13 @@ static void hash_bucket_del(struct dma_debug_entry *entry)
 static RADIX_TREE(dma_active_cacheline, GFP_ATOMIC | __GFP_NOWARN);
 static DEFINE_SPINLOCK(radix_lock);
 #define ACTIVE_CACHELINE_MAX_OVERLAP ((1 << RADIX_TREE_MAX_TAGS) - 1)
-#define CACHELINE_PER_PAGE_SHIFT (PAGE_SHIFT - L1_CACHE_SHIFT)
-#define CACHELINES_PER_PAGE (1 << CACHELINE_PER_PAGE_SHIFT)

 static phys_addr_t to_cacheline_number(struct dma_debug_entry *entry)
 {
-       return ((entry->paddr >> PAGE_SHIFT) << CACHELINE_PER_PAGE_SHIFT) +
-               (offset_in_page(entry->paddr) >> L1_CACHE_SHIFT);
+       if (dma_get_cache_alignment() >= L1_CACHE_BYTES)
+               return entry->paddr >> L1_CACHE_SHIFT;
+       else
+               return entry->paddr >> ilog2(dma_get_cache_alignment());
 }

 static int active_cacheline_read_overlap(phys_addr_t cln)


For the remaining cases (DMA_BOUNCE_UNALIGNED_KMALLOC mainly), based on the Catalin's
suggestion, I came up with the following idea:

diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index 0677918f06a8..f9b6a989ac15 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>
@@ -590,6 +591,14 @@ static int dump_show(struct seq_file *seq, void *v)
 }
 DEFINE_SHOW_ATTRIBUTE(dump);

+static inline void dma_debug_fixup_bounced(struct dma_debug_entry *entry)
+{
+       if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
+           dma_kmalloc_needs_bounce(entry->dev, entry->size, entry->direction) &&
+           is_swiotlb_active(entry->dev))
+               entry->paddr = dma_to_phys(entry->dev, entry->dev_addr);
+}
+
 /*
  * Wrapper function for adding an entry to the hash.
  * This function takes care of locking itself.
@@ -601,6 +610,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
        unsigned long flags;
        int rc;

+       dma_debug_fixup_bounced(entry);
+
        entry->is_cache_clean = attrs & (DMA_ATTR_DEBUGGING_IGNORE_CACHELINES |
                                         DMA_ATTR_REQUIRE_COHERENT);

@@ -614,9 +625,7 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
                global_disable = true;
        } else if (rc == -EEXIST &&
                   !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
-                  !(entry->is_cache_clean && overlap_cache_clean) &&
-                  !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
-                    is_swiotlb_active(entry->dev))) {
+                  !(entry->is_cache_clean && overlap_cache_clean)) {
                err_printk(entry->dev, entry,
                        "cacheline tracking EEXIST, overlapping mappings aren't supported\n");
        }
@@ -981,6 +990,8 @@ static void check_unmap(struct dma_debug_entry *ref)
        struct hash_bucket *bucket;
        unsigned long flags;

+       dma_debug_fixup_bounced(ref);
+
        bucket = get_hash_bucket(ref, &flags);
        entry = bucket_find_exact(bucket, ref);


What do You think about it? Both at least allows to detect errors like multiple,
incompatible mappings of the same memory.


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