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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