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

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Message-ID: <20121026143454.GA10898@otc-wbsnb-06>
Date:	Fri, 26 Oct 2012 17:34:54 +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 04:07:44PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-10-26 at 16:57 +0300, Kirill A. Shutemov wrote:
> > > > 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(). 
> 
> What follow_page()?, a regular hardware page-fault will not call
> follow_page() afaict, we do a down_read(), find_vma() and call
> handle_mm_fault() -- with a lot of error and corner case checking in
> between.

Yeah, you're right. Then, it seems we're spinning on the fault until the
page is splitted.

I'm not sure how long spliting takes and if splitting itself can fix some
fault reason.

-- 
 Kirill A. Shutemov

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