Re: [PATCH 10 of 20] ipath - support for userspace apps using core driver

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On Fri, 2006-03-17 at 11:37 +1100, Nick Piggin wrote:

> But it doesn't look like dma_alloc_coherent is guaranteed to return
> memory allocated from the regular page allocator, nor even memory
> backed by a struct page.

Hmm.  Which of the implementations that you've seen will return
something not backed by a struct page?  On the half dozen arches I've
looked at (i386/x86_64, powerpc, sparc64, ia64, mips), every one uses
either kmalloc, __get_free_pages, or __alloc_pages at some point, and I
think they all have struct pages behind them.

> For example, I see one that returns kmalloc()ed memory. If the pages
> for the slab are already allocated then __GFP_COMP will not do anything
> there.

Bleh.  Perhaps I'm being dense here, but if I'm making a request of
non-zero order and the slab has already been allocated, won't it be
populated with compound pages anyway?  Or will it have been allocated as
a single giant compound page, just handing me back individual hunks of
the appropriate size?

I ask this because I seemed to be getting compound pages out of
dma_alloc_coherent even when I *wasn't* passing in __GFP_COMP.  This is
apparently why PG_private was set on the individual pages I was getting
back.

	<b

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