On Wed 13-01-21 18:14:55, Muchun Song wrote:
> On Wed, Jan 13, 2021 at 5:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Wed 13-01-21 13:22:07, Muchun Song wrote:
> > > There is a race between dissolve_free_huge_page() and put_page().
> > > 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.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > ---
> > > mm/hugetlb.c | 20 ++++++++++++++++++--
> > > 1 file changed, 18 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 4a9011e12175..898e4ea43e13 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -1772,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page)
> > > {
> > > int rc = -EBUSY;
> > >
> > > +retry:
> > > /* Not to disrupt normal path by vainly holding hugetlb_lock */
> > > if (!PageHuge(page))
> > > return 0;
> > > @@ -1793,8 +1794,23 @@ int dissolve_free_huge_page(struct page *page)
> > > * We should make sure that the page is already on the free list
> > > * when it is dissolved.
> > > */
> > > - if (unlikely(!PageHugeFreed(head)))
> > > - goto out;
> > > + 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();
> > > + cpu_relax();
> > > + }
> > > + goto retry;
> >
> > OK, so you have done the retry here. Please fold it into the previous
> > patch. Also do we need cpu_relax on top of cond_resched as well?
>
> Because the previous patch is a bugfix and should be backprt to the other
> stable tree, right?
Yes, it is a bugfix but it arguably opens another issue so the follow up
patch should better be applied along with it.
> I just want the fix patch to be small enough.
> So I do the retry in this patch. If you do not agree with this. I
> will fold this into the previous patch.
>
> Do you mean this?
>
> cpu_relax();
> cond_resched();
> cpu_relax();
No, I am questiong the use of cpu_relax. What is the point?
--
Michal Hocko
SUSE Labs
