Re: [External] Re: [PATCH v2 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

[Michal Hocko]

On Thu 07-01-21 17:01:16, Muchun Song wrote:
> On Thu, Jan 7, 2021 at 4:39 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Thu 07-01-21 11:11:41, Muchun Song wrote:
> > > On Thu, Jan 7, 2021 at 1:07 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > >
> > > > On Wed 06-01-21 16:47:37, Muchun Song wrote:
> > > > > When dissolve_free_huge_page() races with __free_huge_page(), we can
> > > > > do a retry. Because the race window is small.
> > > >
> > > > Is this a bug fix or mere optimization. I have hard time to tell from
> > > > the description.
> > >
> > > It is optimization. Thanks.
> > >
> > > >
> > > > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > > > > ---
> > > > >  mm/hugetlb.c | 26 +++++++++++++++++++++-----
> > > > >  1 file changed, 21 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > > [...]
> > > > > @@ -1825,6 +1828,14 @@ int dissolve_free_huge_page(struct page *page)
> > > > >       }
> > > > >  out:
> > > > >       spin_unlock(&hugetlb_lock);
> > > > > +
> > > > > +     /*
> > > > > +      * If the freeing of the HugeTLB page is put on a work queue, we should
> > > > > +      * flush the work before retrying.
> > > > > +      */
> > > > > +     if (unlikely(rc == -EAGAIN))
> > > > > +             flush_work(&free_hpage_work);
> > > >
> > > > Is it safe to wait for the work to finish from this context?
> > >
> > > Yes. It is safe.
> >
> > Please expand on why in the changelog. Same for the optimization
> > including some numbers showing it really helps.
> 
> OK. Changelog should be updated. Do you agree the race window
> is quite small?

Yes, the race is very rare and the window itself should be relatively
small. This doesn't really answer the question about blocking though.

> If so, why is it not an optimization? Don’t we dissolve
> the page as successfully as possible when we call
> dissolve_free_huge_page()? I am confused about numbers showing.
> Because this is not a performance optimization, but an increase in
> the success rate of dissolving.

And it is a very theoretical one, right? Can you even trigger it? What
would happen if the race is lost? Is it serious? This all would be part
of the changelog ideally. This is a tricky area that spans hugetlb,
memory hotplug and hwpoisoning. All of them tricky. 
-- 
Michal Hocko
SUSE Labs