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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Mar 25, 2025 at 02:40:00PM +0100, Petr Tesarik 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.
> 
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>
> ---
>  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);

You just rebooted the box if this happens, do you REALLY want to do
that?  People generally do not like their data lost :(

Why not just fix the callers, OR if this really isn't going to be
allowed, return an error and just fail the whole submission?  And stick
around to fix up all of the drivers that end up triggering this...

thanks,

greg k-h