linux-kernel - Re: [PATCH 7/11] ksm: make KSM page migration possible

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Message-ID: <alpine.LNX.2.00.1301271936071.896@eggly.anvils>
Date:	Sun, 27 Jan 2013 19:44:56 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Simon Jeons <simon.jeons@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Holasek <pholasek@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/11] ksm: make KSM page migration possible

On Sun, 27 Jan 2013, Simon Jeons wrote:
> On Sun, 2013-01-27 at 15:12 -0800, Hugh Dickins wrote:
> > On Sat, 26 Jan 2013, Simon Jeons wrote:
> > > 
> > > Could you explain why need check page->mapping twice after get page?
> > 
> > Once for the !locked case, which should not return page if mapping changed.
> > Once for the locked case, which should not return page if mapping changed.
> > We could use "else", but that wouldn't be an improvement.
> 
> But for locked case, page->mapping will be check twice.

Thrice.

I'm beginning to wonder: you do realize that page->mapping is volatile,
from the point of view of get_ksm_page()?  That is the whole point of
why get_ksm_page() exists.

I can see that the word "volatile" is not obviously used here - it's
tucked away inside the ACCESS_ONCE() - but I thought the descriptions
of races and barriers made that obvious.

If the comments here haven't helped enough, please take a look at
git commit 4035c07a8959 "ksm: take keyhole reference to page".
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