Re: [PATCH 2/4] char: xillybus: replace __get_free_pages() with kmalloc()

On Wed, Sep 02, 2026 at 01:41:54PM +0200, Eli Billauer wrote:

On 01/09/2026 19:46, Mike Rapoport wrote:

quoted

quoted

* Does vmalloc() guarantee that non-pageable physical RAM is allocated when
it returns?

It's not pageable in the sense of demand paging. Some architectures lazily
synchronize vmalloc page tables and this can cause page faults that will
take care of the page table synchronization.

Thanks for this clarification.

This sounds like a showstopper to me. Immediately after the allocation of
these buffers, data from the USB device can arrive at 400 MB/s. If the data
flow is throttled as a result of handling page faults while trying to write
the data to these buffers, this could lead to an overflow in the USB
device's own RAM buffers. This is because the USB device is an FPGA, where
RAM is an expensive resource.

So using vmalloc() may result in a malfunction in the data transport where
the __get_free_pages() would have been successful. This outweighs the
benefit of a somewhat higher probability of success in allocating very large
buffers, and surely the improved aesthetics of the kernel code.

kmalloc() still works, just like __get_free_pages()
 

Regards,
   Eli

-- 
Sincerely yours,
Mike.