linux-kernel - Re: [External] Re: [PATCH] mm: hugetlb: fix a race between memory-failure/soft_offline and gather_surplus

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Message-ID: <CAMZfGtXsXViJTWrMoECCaCD+2mF7zB5n61aFJvVgpGTaUVEC4A@mail.gmail.com>
Date:   Wed, 21 Apr 2021 16:41:10 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [External] Re: [PATCH] mm: hugetlb: fix a race between
 memory-failure/soft_offline and gather_surplus_pages

On Wed, Apr 21, 2021 at 4:21 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Wed, Apr 21, 2021 at 04:15:00PM +0800, Muchun Song wrote:
> > > The hwpoison side of this looks really suspicious to me. It shouldn't
> > > really touch the reference count of hugetlb pages without being very
> > > careful (and having hugetlb_lock held). What would happen if the
> > > reference count was increased after the page has been enqueed into the
> > > pool? This can just blow up later.
> >
> > If the page has been enqueued into the pool, then the page can be
> > allocated to other users. The page reference count will be reset to
> > 1 in the dequeue_huge_page_node_exact(). Then memory-failure
> > will free the page because of put_page(). This is wrong. Because
> > there is another user.
>
> Note that dequeue_huge_page_node_exact() will not hand over any pages
> which are poisoned, so in this case it will not be allocated.

But softoffline does not set page hwpoison before
__get_hwpoison_page(). So the page still can be
allocated. Right?

> But it is true that we might need hugetlb lock, this needs some more
> thought.
>
> I will have a look.
>
> --
> Oscar Salvador
> SUSE L3