linux-kernel - Re: [PATCH 1/2] USB: core: add a memory pool to urb for host-controller private data

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Message-ID: <b07c7d9f-a20a-49bc-adbc-d43326b2d2df@suse.com>
Date: Tue, 13 May 2025 10:46:39 +0200
From: Oliver Neukum <oneukum@...e.com>
To: David Wang <00107082@....com>, Oliver Neukum <oneukum@...e.com>
Cc: mathias.nyman@...el.com, gregkh@...uxfoundation.org,
 stern@...land.harvard.edu, surenb@...gle.com, kent.overstreet@...ux.dev,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] USB: core: add a memory pool to urb for
 host-controller private data

On 13.05.25 10:23, David Wang wrote:

Hi,
  > Thanks for reviewing this.  The memory flag thing do raise concern.
> I think I can make adjustment:  realloc the memory if flag changed.

I am sorry. I have been unclear. Here comes a detailed explanation:

What we call "gfp_t" is a combination of flags. They describe

A - the type of memory (always valid)
B - the way the memory can be allocated (valid only at a specific time)

The URB is a generic data structure to be processed by the CPU, _not_
the HC. It is always generic kernel memory. Flags of type A make no sense
to pass.
In fact you may not know for which device an URB will be used when you
allocate it. The only valid mem_flags you can pass to usb_alloc_urb()
are GFP_KERNEL, GFP_NOIO or GFP_ATOMIC.

If you need to reallocate memory for private data you _must_ use
the flags passed with usb_submit_urb(). A HCD can modify them by adding
flags of type A, but you cannot change flags of type B.
For example, if usb_alloc_urb() used GFP_KERNEL to allocate the URB,
but uses GFP_ATOMIC in usb_submit_urb(), you will deadlock if you save
and reuse the GFP_KERNEL.

	HTH
		Oliver