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

[Simon Jeons <simon.jeons@gmail.com>]

On Sun, 2013-01-27 at 15:12 -0800, Hugh Dickins wrote:
> On Sat, 26 Jan 2013, Simon Jeons wrote:
> > On Fri, 2013-01-25 at 18:03 -0800, Hugh Dickins wrote:
> > > +	while (!get_page_unless_zero(page)) {
> > > +		/*
> > > +		 * Another check for page->mapping != expected_mapping would
> > > +		 * work here too.  We have chosen the !PageSwapCache test to
> > > +		 * optimize the common case, when the page is or is about to
> > > +		 * be freed: PageSwapCache is cleared (under spin_lock_irq)
> > > +		 * in the freeze_refs section of __remove_mapping(); but Anon
> > > +		 * page->mapping reset to NULL later, in free_pages_prepare().
> > > +		 */
> > > +		if (!PageSwapCache(page))
> > > +			goto stale;
> > > +		cpu_relax();
> > > +	}
> > > +
> > > +	if (ACCESS_ONCE(page->mapping) != expected_mapping) {
> > >  		put_page(page);
> > >  		goto stale;
> > >  	}
> > > +
> > >  	if (locked) {
> > >  		lock_page(page);
> > > -		if (page->mapping != expected_mapping) {
> > > +		if (ACCESS_ONCE(page->mapping) != expected_mapping) {
> > >  			unlock_page(page);
> > >  			put_page(page);
> > >  			goto stale;
> > >  		}
> > >  	}
> > 
> > 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.



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