Re: [PATCH v2] usb: core: warn if a GFP zone flag is passed to hcd_buffer_alloc()

[Petr Tesarik]

On Tue, 25 Mar 2025 14:40:00 +0100
Petr Tesarik <ptesarik@xxxxxxxx> wrote:

> Remove a misleading comment and issue a warning if a zone modifier is
> specified when allocating a hcd buffer.
> 
> There is no valid use case for a GFP zone modifier in hcd_buffer_alloc():
> - PIO mode can use any kernel-addressable memory
> - dma_alloc_coherent() ignores memory zone bits
> 
> This function is called by usb_alloc_coherent() and indirectly by
> usb_submit_urb(). Despite the comment, no in-tree users currently pass
> GFP_DMA.

Let me provide a bit of background on this patch. My actual goal is to
remove the DMA zone; I'm now going through core code and removing
GFP_DMA references that somehow look incorrect to me.

I hope this preparation makes it easier to review the removal of
GFP_DMA later.

Petr T

> Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx>
> ---
>  drivers/usb/core/buffer.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c
> index 87230869e1fa..10844cd42e66 100644
> --- a/drivers/usb/core/buffer.c
> +++ b/drivers/usb/core/buffer.c
> @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd)
>  }
>  
>  
> -/* sometimes alloc/free could use kmalloc with GFP_DMA, for
> - * better sharing and to leverage mm/slab.c intelligence.
> - */
> -
>  void *hcd_buffer_alloc(
>  	struct usb_bus		*bus,
>  	size_t			size,
> @@ -128,6 +124,12 @@ void *hcd_buffer_alloc(
>  	if (hcd->localmem_pool)
>  		return gen_pool_dma_alloc(hcd->localmem_pool, size, dma);
>  
> +	/*
> +	 * Zone modifiers are ignored by DMA API, and PIO should always use
> +	 * GFP_KERNEL.
> +	 */
> +	WARN_ON_ONCE(mem_flags & GFP_ZONEMASK);
> +
>  	/* some USB hosts just use PIO */
>  	if (!hcd_uses_dma(hcd)) {
>  		*dma = ~(dma_addr_t) 0;