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SubjectRe: 2.6.25-rc7-git2: Reported regressions from 2.6.24
On Thu, 27 Mar 2008, Linus Torvalds wrote:

> Totally irrelevant.
> The page allocation path does
> if (gfp_flags & __GFP_ZERO)
> prep_zero_page(page, order, gfp_flags);
> and that will cause a warning REGARDLESS of whether the page is a HIGHMEM
> page or not.

prep_zero_page does:

static inline void prep_zero_page(struct page *page, int order, gfp_t
int i;

* clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
* and __GFP_HIGHMEM from hard or soft interrupt context.
VM_BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
for (i = 0; i < (1 << order); i++)
clear_highpage(page + i);

So we forbit __GFP_HIGHMEM and in_interrupt which makes sense. The simple
forwarding of large kmallocs to the page allocator as done by SLUB / SLOB
is fine.

Then clear_highpage calls additional checking functions that have
the effect of generally forbiding zeroing in interrupt context if
CONFIG_HIGHMEM is set. This is wrong and needs to be fixed.

> And the fact is, passing in GFP_ZERO from the SLUB code is a bug
> regardless, because it unnecessarily does the dual memset().

Well that is only the fallback path of __slab_alloc which is not triggered
here and not performance sensitive. We could clear the flag there but
that is irrevelant for this issue.

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