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