Re: [patch 3/3] mm: fault handler to replace nopage and populate

[Jeff Garzik <jeff@garzik.org>]

Nick Piggin wrote:
> Nonlinear mappings are (AFAIKS) simply a virtual memory concept that
> encodes the virtual address -> file offset differently from linear
> mappings.
> 
> I can't see why the filesystem/pagecache code should need to know anything
> about it, except for the fact that the ->nopage handler didn't quite pass
> down enough information (ie. pgoff). But it is more logical to pass pgoff
> rather than have the ->nopage function calculate it itself anyway. And
> having the nopage handler install the pte itself is sort of nasty.
> 
> This patch introduces a new fault handler that replaces ->nopage and ->populate
> and (hopefully) ->page_mkwrite. Most of the old mechanism is still in place
> so there is a lot of duplication and nice cleanups that can be removed if
> everyone switches over.
> 
> The rationale for doing this in the first place is that nonlinear mappings
> are subject to the pagefault vs invalidate/truncate race too, and it seemed
> stupid to duplicate the synchronisation logic rather than just consolidate
> the two.
> 
> Comments?

That's pretty nice.

Back when I was writing [the now slated for death] 
sound/oss/via82xxx_audio.c driver, Linus suggested that I implement 
->nopage() for accessing the mmap'able DMA'd audio buffers, rather than 
using remap_pfn_range().  It worked out very nicely, because it allowed 
the sound driver to retrieve $N pages for the mmap'able buffer (passed 
as an s/g list to the hardware) rather than requiring a single humongous 
buffer returned by pci_alloc_consistent().

And although probably not your primary motivation, your change does IMO 
improve this area of the kernel.

	Jeff