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Date:   Tue, 11 Jul 2023 11:01:59 -0700
From:   Mike Kravetz <mike.kravetz@oracle.com>
To:     Jiaqi Yan <jiaqiyan@google.com>
Cc:     Miaohe Lin <linmiaohe@huawei.com>, akpm@linux-foundation.org,
        naoya.horiguchi@nec.com, songmuchun@bytedance.com,
        shy828301@gmail.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, duenwen@google.com,
        axelrasmussen@google.com, jthoughton@google.com
Subject: Re: [PATCH v3 2/4] mm/hwpoison: check if a subpage of a hugetlb
 folio is raw HWPOISON
Message-ID: <20230711180159.GA3887@monkey>
References: <20230707201904.953262-1-jiaqiyan@google.com>
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On 07/11/23 10:05, Jiaqi Yan wrote:
> On Mon, Jul 10, 2023 at 8:16 AM Jiaqi Yan <jiaqiyan@google.com> wrote:
> > On Fri, Jul 7, 2023 at 7:57 PM Miaohe Lin <linmiaohe@huawei.com> wrote:
> > > On 2023/7/8 4:19, Jiaqi Yan wrote:
> > >
> > > > +             if (subpage == p->page) {
> > > > +                     ret = true;
> > > > +                     break;
> > > > +             }
> > > > +     }
> > > > +
> > > > +     return ret;
> > > >  }
> > >
> > > It seems there's a race between __is_raw_hwp_subpage and unpoison_memory:
> > >   unpoison_memory               __is_raw_hwp_subpage
> > >                                   if (!folio_test_hwpoison(folio)) -- hwpoison is set
> > >     folio_free_raw_hwp            llist_for_each_entry_safe raw_hwp_list
> > >       llist_del_all                 ..
> > >     folio_test_clear_hwpoison
> > >
> >
> > Thanks Miaohe for raising this concern.
> >
> > > But __is_raw_hwp_subpage is used in hugetlbfs, unpoison_memory couldn't reach here because there's a
> > > folio_mapping == NULL check before folio_free_raw_hwp.
> >
> > I agree. But in near future I do want to make __is_raw_hwp_subpage
> > work for shared-mapping hugetlb, so it would be nice to work with
> > unpoison_memory. It doesn't seem to me that holding mf_mutex in
> > __is_raw_hwp_subpage is nice or even absolutely correct. Let me think
> > if I can come up with something in v4.
> 
> At my 2nd thought, if __is_raw_hwp_subpage simply takes mf_mutex
> before llist_for_each_entry, it will introduce a deadlock:
> 
> unpoison_memory                       __is_raw_hwp_subpage
>   held mf_mutex                         held hugetlb_lock
>   get_hwpoison_hugetlb_folio            attempts mf_mutex
>     attempts hugetlb lock
> 
> Not for this patch series, but for future, is it a good idea to make
> mf_mutex available to hugetlb code? Then enforce the order of locking
> to be mf_mutex first, hugetlb_lock second? I believe this is the
> current locking pattern / order for try_memory_failure_hugetlb.

I think only holding mf_mutex in __is_raw_hwp_subpage would be sufficient
to prevent races with unpoison_memory.  memory failure code needs to take
both mf_mutex and hugetlb_lock.  The hugetlb lock is to prevent hugetlb
page state changes.  IIUC, __is_raw_hwp_subpage is only taking hugetlb_lock
to prevent races with memory failure code.

Of course, I could be missing something as there are subtle issues with
locking in the memory failure code.
-- 
Mike Kravetz