linux-kernel - Re: [External] Re: [PATCH v3 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

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Message-ID: <20210112111712.GN22493@dhcp22.suse.cz>
Date:   Tue, 12 Jan 2021 12:17:12 +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>,
        linux- stable <stable@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v3 3/6] mm: hugetlb: fix a race between
 freeing and dissolving the page

On Tue 12-01-21 18:13:02, Muchun Song wrote:
> On Tue, Jan 12, 2021 at 6:02 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Sun 10-01-21 20:40:14, Muchun Song wrote:
> > [...]
> > > @@ -1770,6 +1788,14 @@ 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)))
> > > +                     goto out;
> > > +
> >
> > Do you really want to report EBUSY in this case? This doesn't make much
> > sense to me TBH. I believe you want to return 0 same as when you race
> > and the page is no longer PageHuge.
> 
> Return 0 is wrong. Because the page is not freed to the buddy allocator.
> IIUC, dissolve_free_huge_page returns 0 when the page is already freed
> to the buddy allocator. Right?

0 is return when the page is either dissolved or it doesn't need
dissolving. If there is a race with somebody else freeing the page then
there is nothing to dissolve. Under which condition it makes sense to
report the failure and/or retry dissolving?
-- 
Michal Hocko
SUSE Labs