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

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Message-ID: <20210107083902.GB13207@dhcp22.suse.cz>
Date:   Thu, 7 Jan 2021 09:39:02 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v2 4/6] mm: hugetlb: add return -EAGAIN
 for dissolve_free_huge_page

On Thu 07-01-21 11:11:41, Muchun Song wrote:
> On Thu, Jan 7, 2021 at 1:07 AM Michal Hocko <mhocko@...e.com> 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@...edance.com>
> > > ---
> > >  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.

-- 
Michal Hocko
SUSE Labs