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

[Hugh Dickins]

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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