Re: [PATCH 1/4] USB: usbfs: replace __get_free_page() with kmalloc()

[Alan Stern]

On Mon, Aug 31, 2026 at 03:27:07PM +0200, Oliver Neukum wrote:
> On 30.08.26 10:10, Mike Rapoport (Microsoft) wrote:
> > do_proc_control() allocates a temporary buffer for the data stage of a
> > control transfer issued from userspace.
> > 
> > This buffer can be allocated with kmalloc() as there's nothing special
> > about it to go directly to the page allocator.
> 
> You are breaking the calculation. A page has a size of exactly
> PAGE_SIZE. A kmalloced object of PAGE_SIZE is larger. That is
> a bad idea.

By "the calculation", are you referring to the 
usbfs_increase_memory_usage() call in do_proc_control()?

The overhead in kmalloc allocations doesn't really matter for this 
purpose -- it can be considered a rounding error.  Besides, we don't 
know how big the overhead is, so we can't account for it.  You can see 
that in the same call, we don't try to account for the overhead involved 
in usb_alloc_urb() or kmalloc_obj() either.

I'd say there's nothing wrong with this patch.

Alan Stern