On Mon, Sep 10, 2012 at 04:02:39PM +0200, Eric Dumazet wrote:
> On Mon, 2012-09-10 at 16:13 +0300, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > H. Peter Anvin doesn't like huge zero page which sticks in memory forever
> > after the first allocation. Here's implementation of lockless refcounting
> > for huge zero page.
> >
> ...
>
> > +static unsigned long get_huge_zero_page(void)
> > +{
> > + struct page *zero_page;
> > +retry:
> > + if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
> > + return ACCESS_ONCE(huge_zero_pfn);
> > +
> > + zero_page = alloc_pages(GFP_TRANSHUGE | __GFP_ZERO, HPAGE_PMD_ORDER);
> > + if (!zero_page)
> > + return 0;
> > + if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
> > + __free_page(zero_page);
> > + goto retry;
> > + }
>
> This might break if preemption can happen here ?
>
> The second thread might loop forever because huge_zero_refcount is 0,
> and huge_zero_pfn not zero.
I fail to see why the second thread might loop forever. Long time yes, but
forever?
Yes, disabling preemption before alloc_pages() and enabling after
atomic_set() looks reasonable. Thanks.
>
> If preemption already disabled, a comment would be nice.
>
>
> > +
> > + /* We take additional reference here. It will be put back by shinker */
>
> typo : shrinker
Thx.
> > + atomic_set(&huge_zero_refcount, 2);
> > + return ACCESS_ONCE(huge_zero_pfn);
> > +}
> > +
>
>
>
--
Kirill A. Shutemov
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