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On Tue, 24 Jul 2007, Andrew Morton wrote:

> __GFP_COMP I'm not so sure about.
> drivers/char/drm/drm_pci.c:drm_pci_alloc() (and other places like infiniband)
> pass it into dma_alloc_coherent() which some architectures implement via slab. umm,
> arch/arm/mm/consistent.c is one such.

Should drm_pci_alloc really aright in setting __GFP_COMP?
dma_alloc_coherent does not set __GFP_COMP for other higher order allocs
and expects to be able to operate on the page structs indepedently. That
is not the case for a compound page.

Creates a really interesting case for SLAB. Slab did not use __GFP_COMP in
order to be able to allow the use page->private (No longer an issue since
the 2.6.22 cleanups and avoiding the use of page->private for the compound

Now the __GFP_COMP flag is passed through for any higher order page alloc
(such as a kmalloc allocation > PAGE_SIZE). Then we may have allocated one
slab that is a compound page amoung others higher order pages allocated
without __GFP_COMP. May have caused rare and strange failures in 2.6.21
and earlier because of the concurrent page->private use in compound head
pages and arch pages.

SLUB will always use __GFP_COMP so the pages are consistent regardless if
__GFP_COMP is passed in or not.

The strange scenarios come about by expecting a page allocation when
sometimes we just substitute a slab alloc.

We could filter __GFP_COMP out to avoid the BUG()? Or deal with it on a
case by case basis?

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