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

On Wed, 2 Sep 2026 13:41:54 +0200
Eli Billauer [off-list ref] 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.

IIUC The faults don't usually happen.
But if one cpu write the address somewhere and another cpu picks it up
immediately and access the buffer you can get a fault.
Even if they do happen they are unlikely to be worse than normal
interrupt latency.

If you are trying to receive data at very high speed I'd have thought that
you'd want the USB data written directly into you buffer list.
(Much the same way as most ethernet hardware handles rx frames.)
I realise the the usb kernel code isn't really written for that sort
of access (some of usbnet could do with it).

David 

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