Re: [PATCH v2] dma-mapping: don't trace the DMA address when the allocation fails

[Sean Anderson <sean.anderson@linux.dev>]

On 9/7/26 08:01, Donggeun Yoo wrote:
> dma_alloc_attrs() passes *dma_handle to trace_dma_alloc() without
> checking whether the allocation succeeded. No backend writes it on
> failure: dma_direct_alloc(), iommu_dma_alloc() and the dma_map_ops
> instances assign it only on the path that returns a buffer. Callers
> usually pass an uninitialized automatic variable, so a failed allocation
> records whatever the stack held, next to the virt_addr=(null) that marks
> the record as an error:
> 
>   dma_alloc: dmatrace dir=BIDIRECTIONAL dma_addr=deadbeefdeadbeef
>   size=1099511627776 virt_addr=0000000000000000
> 
> The device coherent pool path reaches the same call: a non-zero return
> from dma_alloc_from_dev_coherent() means the request was handled, not
> that it succeeded, so cpu_addr is NULL and dma_handle is untouched once
> the pool runs out.
> 
> For an allocation event a NULL virt_addr already means the request
> failed, so the address field carries nothing. Report 0 for it in the
> event class rather than at each call site, which covers dma_alloc_pages()
> and dma_alloc_sgt_err() as well.
> 
> Fixes: 038eb433dc14 ("dma-mapping: add tracing for dma-mapping API calls")
> Fixes: 68b6dbf1f441 ("dma-mapping: trace more error paths")
> Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
> ---
> v2:
> - report 0 in the event class instead of splitting the two call sites,
>    as suggested by Marek. kernel/dma/mapping.c is untouched now, so
>    debug_dma_alloc_coherent() keeps being called for a failed allocation
>    and decides for itself what to do with it.
> - tested on x86_64 rather than compile-tested only.
> v1: https://lore.kernel.org/linux-iommu/20260905071919.30784-1-donggeunyoo.kernel@gmail.com/
> 
> Tested with a module that puts a known value in the caller's handle and
> then asks for 1 TiB from a 32-bit capable device, with the dma_alloc
> event enabled. Before, the record carries the caller's stale value; after,
> it carries 0:
> 
>   -dma_addr=deadbeefdeadbeef size=1099511627776 virt_addr=0000000000000000
>   +dma_addr=0 size=1099511627776 virt_addr=0000000000000000
> 
> A PAGE_SIZE request from the same device, as a control, reports its real
> address on both kernels, matching the handle the caller got back.
> 
> This changes what the record contains, not what the caller does:
> *dma_handle is still read to build the tracepoint arguments, since those
> are evaluated before the static branch.
> 
>   include/trace/events/dma.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
> index 9df02c1511de..b06d8f99922d 100644
> --- a/include/trace/events/dma.h
> +++ b/include/trace/events/dma.h
> @@ -134,7 +134,7 @@ DECLARE_EVENT_CLASS(dma_alloc_class,
>   	TP_fast_assign(
>   		__assign_str(device);
>   		__entry->virt_addr = virt_addr;
> -		__entry->dma_addr = dma_addr;
> +		__entry->dma_addr = virt_addr ? dma_addr : 0;
>   		__entry->size = size;
>   		__entry->flags = flags;
>   		__entry->dir = dir;
> 
> base-commit: cee9395acd8043be0644b25c34bfa86623f2b935

Reviewed-by: Sean Anderson <sean.anderson@linux.dev>