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SubjectRe: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous
On Wed, Jun 01, 2011 at 09:15:29PM +0200, Andrea Arcangeli wrote:
> On Wed, Jun 01, 2011 at 06:58:09PM +0100, Mel Gorman wrote:
> > Umm, HIGHMEM4G implies a two-level pagetable layout so where are
> > things like _PAGE_BIT_SPLITTING being set when THP is enabled?
>
> They should be set on the pgd, pud_offset/pgd_offset will just bypass.
> The splitting bit shouldn't be special about it, the present bit
> should work the same.

This comment is misleading at best then.

#define _PAGE_BIT_SPLITTING _PAGE_BIT_UNUSED1 /* only valid on a PSE pmd */

At the PGD level, it can have PSE set obviously but it's not a
PMD. I confess I haven't checked the manual to see if it's safe to
use _PAGE_BIT_UNUSED1 like this so am taking your word for it. I
found that the bug is far harder to reproduce with 3 pagetable levels
than with 2 but that is just timing. So far it has proven impossible
on x86-64 at least within 27 hours so that has me looking at how
pagetable management between x86 and x86-64 differ.

Barriers are a big different between how 32-bit !SMP and X86-64 but
don't know yet which one is relevant or if this is even the right
direction.

--
Mel Gorman
SUSE Labs


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