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
