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
