Messages in this thread | | | Date | Mon, 10 Jul 2006 16:59:48 +1000 | From | Nick Piggin <> | Subject | Re: DMA memory, split_page, BUG_ON(PageCompound()), sound |
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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.
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