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

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Date:	Fri, 26 Oct 2012 13:15:20 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"mhocko@...e.cz" <mhocko@...e.cz>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v3] mm: thp: Set the accessed flag for old pages on
 access fault.

On Fri, Oct 26, 2012 at 10:07:15AM +0100, Will Deacon wrote:
> On Fri, Oct 26, 2012 at 08:44:35AM +0100, Kirill A. Shutemov wrote:
> > 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.
> 
> I appreciate the need to check whether the thing is splitting, but I thought
> that the pmd_same(*pmd, orig_pmd) check after taking the page_table_lock
> would be sufficient, because we know that the entry hasn't changed and that
> it wasn't splitting before we took the lock. This also mirrors the approach
> taken by do_huge_pmd_wp_page.
> 
> Is there something I'm missing?

Hm.. You're correct from my POV.

Acked-by: Kirill A. Shutemov <kirill@...temov.name>

I think the check in do_huge_pmd_prot_none() is redundant. It only add
latency. I'll prepare patch to remove it.

-- 
 Kirill A. Shutemov
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