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

[Eli Billauer]

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 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