Re: [External] Re: [PATCH v5 3/5] mm: hugetlb: fix a race between freeing and dissolving the page

[Michal Hocko]

On Thu 14-01-21 21:47:36, Muchun Song wrote:
> On Thu, Jan 14, 2021 at 9:20 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
[...]
> > > @@ -1770,6 +1789,28 @@ int dissolve_free_huge_page(struct page *page)
> > >               int nid = page_to_nid(head);
> > >               if (h->free_huge_pages - h->resv_huge_pages == 0)
> > >                       goto out;
> > > +
> > > +             /*
> > > +              * We should make sure that the page is already on the free list
> > > +              * when it is dissolved.
> > > +              */
> > > +             if (unlikely(!PageHugeFreed(head))) {
> > > +                     spin_unlock(&hugetlb_lock);
> > > +
> > > +                     /*
> > > +                      * Theoretically, we should return -EBUSY when we
> > > +                      * encounter this race. In fact, we have a chance
> > > +                      * to successfully dissolve the page if we do a
> > > +                      * retry. Because the race window is quite small.
> > > +                      * If we seize this opportunity, it is an optimization
> > > +                      * for increasing the success rate of dissolving page.
> > > +                      */
> > > +                     while (PageHeadHuge(head) && !PageHugeFreed(head))
> > > +                             cond_resched();
> >
> > Sorry, I should have raised that when replying to the previous version
> > already but we have focused more on other things. Is there any special
> > reason that you didn't simply
> >         if (!PageHugeFreed(head)) {
> >                 spin_unlock(&hugetlb_lock);
> >                 cond_resched();
> >                 goto retry;
> >         }
> >
> > This would be less code and a very slight advantage would be that the
> > waiter might get blocked on the spin lock while the concurrent freeing
> > is happening. But maybe you wanted to avoid exactly this contention?
> > Please put your thinking into the changelog.
> 
> I want to avoid the lock contention. I will add this reason
> to the changelog. Thanks.

Please also explain why it matters and whether an unintended contention
is a real problem.
-- 
Michal Hocko
SUSE Labs