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

[John Hubbard]

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@xxxxxxxxxx>
> > ---
> >   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