Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race

[Muchun Song]

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? 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();

If yes, IMHO, I don't think it is necessary.

>
> > +             }
> >
> >               /*
> >                * Move PageHWPoison flag from head page to the raw error page,
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs