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DateFri, 28 Mar 2008 12:25:57 -0700 (PDT)
FromLinus Torvalds <>
SubjectRe: 2.6.25-rc7-git2: Reported regressions from 2.6.24
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On Fri, 28 Mar 2008, Christoph Lameter wrote:

prep_zero_page does:

static inline void prep_zero_page(struct page *page, int order, gfp_t gfp_flags) {
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);

.. and clear_highpage() does:

void *kaddr = kmap_atomic(page, KM_USER0);
clear_page(kaddr);

.. where kmap_atomic() on x86 does:

kmap_atomic() ->
kmap_atomic_prot() ->
debug_kmap_atomic_prot() ->
	      if (in_irq())
	        WARN_ON_ONCE()
none of which are at all conditional on __GFP_HIGHMEM.
But none of this is relevant. The warning possibly didn't even come from slub, it just made me look at it - because *something* is doing GFP_ATOMIC together with __GFP_ZERO, and it became obvious that SLUB is one potential cause of that.

And the SLUB case simply isn't valid!

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.

No. Dammit, the bug is in SLUB.

If SLUB *ever* calls the page allocator with __GFP_ZERO set, it's a bug, and that has nothing to do with GFP_ATOMIC or anything else. Because SLUB uses its own logic for clearing the result.

Why cannot you just admit it?

Now, _outside_ of SLUB there appear to be other users too, and those users need to either be fixed or we need to allow __GFP_ZERO togethe with GFP_ATOMIC. But the fact is, SLUB had a really stupid bug that it shouldn't have had.

Linus



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