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

[Pedro Falcato <pfalcato@suse.de>]

On Wed, Sep 02, 2026 at 01:41:54PM +0200, Eli Billauer wrote:
> On 01/09/2026 19:46, Mike Rapoport wrote:
> > > * 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

Note that these page faults are supposed to be uncommon in nature. They will
only happen in architectures where the top-level page tables are not
pre-populated, and when they weren't populated before. On a 64-bit architecture
that would be something like every 512 GiB of address space :)

> 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.
> 
> Regards,
>    Eli
> 

-- 
Pedro