linux-kernel - Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on access fault.

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Message-ID: <20121026074435.GA871@shutemov.name>
Date:	Fri, 26 Oct 2012 10:44:35 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, mhocko@...e.cz, peterz@...radead.org,
	akpm@...ux-foundation.org, Chris Metcalf <cmetcalf@...era.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on
 access fault.

On Thu, Oct 25, 2012 at 05:44:31PM +0100, Will Deacon wrote:
> On x86 memory accesses to pages without the ACCESSED flag set result in the
> ACCESSED flag being set automatically. With the ARM architecture a page access
> fault is raised instead (and it will continue to be raised until the ACCESSED
> flag is set for the appropriate PTE/PMD).
> 
> For normal memory pages, handle_pte_fault will call pte_mkyoung (effectively
> setting the ACCESSED flag). For transparent huge pages, pmd_mkyoung will only
> be called for a write fault.
> 
> This patch ensures that faults on transparent hugepages which do not result
> in a CoW update the access flags for the faulting pmd.
> 
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
> 
> Ok chaps, I rebased this thing onto today's next (which basically
> necessitated a rewrite) so I've reluctantly dropped my acks and kindly
> ask if you could eyeball the new code, especially where the locking is
> concerned. In the numa code (do_huge_pmd_prot_none), Peter checks again
> that the page is not splitting, but I can't see why that is required.

In handle_mm_fault() we check if the pmd is under splitting without
page_table_lock. It's kind of speculative cheap check. We need to re-check
if the PMD is really not under splitting after taking page_table_lock.

See section "Locking in hugepage aware code" in Documentation/vm/transhuge.txt

-- 
 Kirill A. Shutemov
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