Re: DMA memory, split_page, BUG_ON(PageCompound()), sound

[Marc Singer <elf@buici.com>]

On Sun, Jul 09, 2006 at 01:26:06PM +1000, Nick Piggin wrote:
> Marc Singer wrote:
> >I'm investigating why I am triggering a BUG_ON in split_page() when I
> >use the sound subsystems dma memory allocation aide.
> >
> >The crux of the problem appears to be that snd_malloc_dev_pages()
> >passes __GFP_COMP into dma_alloc_coherent().  On the ARM and several
> >other architectures, the dma allocation code calls split_page () with
> >pages allocated with this flag which, in turn, triggers the BUG_ON()
> >check for the CompoundPage flag.
> >
> >So, the questions are these: Who is doing the wrong thing?  Should the
> >snd_malloc_dev_pages() call drop the __GFP_COMP flag?  Should
> >split_page() allow the page to be compound?  Should __GFP_COMP be 0 on
> >ARM and other architectures that don't support compound pages?
> 
> I personally never liked the explicit __GFP_COMP going in everywhere,
> and would have much preferred a GFP_USERMAP, which the architecture /
> allocator could satisfy as they liked.

Thus, the __GFP_COMP bit would be part of another flags such that it
is set on x86 (or any other architecture that supports it) and clear
on those that don't.

> As a hack, you can make arm's dma_alloc_coherent() drop __GFP_COMP,
> which should work.

There are many architectures that have this problem, so I suspect that
such a patch would be unappreciated.