linux-kernel - Re: [PATCH 1/2] numa, mm: drop redundant check in do_huge_pmd_numa

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Date:	Fri, 26 Oct 2012 16:57:50 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-mm@...ck.org, Will Deacon <will.deacon@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] numa, mm: drop redundant check in
 do_huge_pmd_numa_page()

On Fri, Oct 26, 2012 at 03:43:12PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-10-26 at 16:41 +0300, Kirill A. Shutemov wrote:
> > On Fri, Oct 26, 2012 at 03:08:05PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2012-10-26 at 15:54 +0300, Kirill A. Shutemov wrote:
> > > > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > > > 
> > > > We check if the pmd entry is the same as on pmd_trans_huge() in
> > > > handle_mm_fault(). That's enough.
> > > > 
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > 
> > > Ah indeed, Will mentioned something like this on IRC as well, I hadn't
> > > gotten around to looking at it -- now have, thanks!
> > > 
> > > Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > 
> > > That said, where in handle_mm_fault() do we wait for a split to
> > > complete? We have a pmd_trans_huge() && !pmd_trans_splitting(), so a
> > > fault on a currently splitting pmd will fall through.
> > > 
> > > Is it the return from the fault on unlikely(pmd_trans_huge()) ?
> > 
> > Yes, this code will catch it:
> > 
> > 	/* if an huge pmd materialized from under us just retry later */
> > 	if (unlikely(pmd_trans_huge(*pmd)))
> > 		return 0;
> > 
> > If the pmd is under splitting it's still a pmd_trans_huge().
> 
> OK, so then we simply keep taking the same fault until the split is
> complete? Wouldn't it be better to wait for it instead of spin on
> faults?

IIUC, on next fault we will wait split the page in fallow_page().

-- 
 Kirill A. Shutemov

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