Re: [PATCH 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

John Hubbard
2022-12-05T23:53:12Z

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