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 
head).

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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