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

[Nick Piggin]

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.

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

--
SUSE Labs, Novell Inc.
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