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

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Date:   Tue, 12 Jan 2021 18:13:02 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.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, 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?

>
> >               /*
> >                * Move PageHWPoison flag from head page to the raw error page,
> >                * which makes any subpages rather than the error page reusable.
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs