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: <20210112123738.GQ22493@dhcp22.suse.cz>
Date:   Tue, 12 Jan 2021 13:37:38 +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 19:43:21, Muchun Song wrote:
> On Tue, Jan 12, 2021 at 7:17 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > 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?
> 
> If there is a race with somebody else freeing the page, the page
> can be freed to the hugepage pool not the buddy allocator. Do
> you think that this page is dissolved?

OK, I see what you mean. Effectively the page would be in a limbo, not
yet in the pool nor in the allocator but it can find its way to the
either of the two. But I still dislike returning a failure because that
would mean e.g. memory hotplug to fail. Can you simply retry inside this
code path (drop the lock, cond_resched and retry)?
-- 
Michal Hocko
SUSE Labs