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Date:   Wed, 21 Apr 2021 10:43:26 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: [External] Re: [PATCH] mm: hugetlb: fix a race between
 memory-failure/soft_offline and gather_surplus_pages
Message-ID: <YH/lrlmkEb5b8LCo@dhcp22.suse.cz>
References: <20210421060259.67554-1-songmuchun@bytedance.com>
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 <20210421082103.GE22456@linux>
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On Wed 21-04-21 10:21:03, Oscar Salvador 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.

I have to say I have missed the HWPoison check so the this particular
scenario is not possible indeed.

> But it is true that we might need hugetlb lock, this needs some more
> thought.

yes, nobody should be touching to the reference count of hugetlb pool
pages out of the hugetlb proper.

> I will have a look. 

Thanks!
-- 
Michal Hocko
SUSE Labs