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

[Nick Piggin]

Marc Singer wrote:
> 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.

I guess you could do it a number of ways. Maybe having GFP_USERMAP
set __GFP_USERMAP|__GFP_COMP, and the arm dma memory allocator can
strip the __GFP_COMP.

If you get an explicit __GFP_COMP passed down, the allocator doesn't
know whether that was because they want a user mappable area, or
really want a compound page (in which case, stripping __GFP_COMP is
the wrong thing to do).

> > 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.

Hacks usually are ;) It would get you working though.

If you want to write a patch that would be appreciated, that would be
appreciated.

--
SUSE Labs, Novell Inc.
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