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Date:   Mon, 5 Dec 2022 15:52:51 -0800
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Subject: Re: [PATCH 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd
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To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Peter Xu <peterx@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        James Houghton <jthoughton@google.com>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Rik van Riel" <riel@surriel.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Muchun Song <songmuchun@bytedance.com>,
        "David Hildenbrand" <david@redhat.com>
References: <20221129193526.3588187-1-peterx@redhat.com>
 <20221129193526.3588187-9-peterx@redhat.com> <Y45/5ezH6uwn7Cfy@monkey>
From:   John Hubbard <jhubbard@nvidia.com>
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On 12/5/22 15:33, Mike Kravetz wrote:
> On 11/29/22 14:35, Peter Xu wrote:
>> Since walk_hugetlb_range() walks the pgtable, it needs the vma lock
>> to make sure the pgtable page will not be freed concurrently.
>>
>> Signed-off-by: Peter Xu <peterx@redhat.com>
>> ---
>>   mm/pagewalk.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>> index 7f1c9b274906..d98564a7be57 100644
>> --- a/mm/pagewalk.c
>> +++ b/mm/pagewalk.c
>> @@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
>>   	const struct mm_walk_ops *ops = walk->ops;
>>   	int err = 0;
>>   
>> +	hugetlb_vma_lock_read(vma);
>>   	do {
>>   		next = hugetlb_entry_end(h, addr, end);
>>   		pte = huge_pte_offset(walk->mm, addr & hmask, sz);
> 
> For each found pte, we will be calling mm_walk_ops->hugetlb_entry() with
> the vma_lock held.  I looked into the various hugetlb_entry routines, and
> I am not sure about hmm_vma_walk_hugetlb_entry.  It seems like it could
> possibly call hmm_vma_fault -> handle_mm_fault -> hugetlb_fault.  If this
> can happen, then we may have an issue as hugetlb_fault will also need to
> acquire the vma_lock in read mode.
> 
> I do not know the hmm code well enough to know if this may be an actual
> issue?

Oh, this sounds like a serious concern. If we add a new lock, and hold it
during callbacks that also need to take it, that's not going to work out,
right?

And yes, hmm_range_fault() and related things do a good job of revealing
this kind of deadlock. :)

thanks,
-- 
John Hubbard
NVIDIA