mm/hugetlb: pre-allocate pgtable pages for uffd wr-protects

commit fed15f1345dc8a7fc8baa81e8b55c3ba010d7f4b upstream.

Userfaultfd-wp uses pte markers to mark wr-protected pages for both shmem
and hugetlb.  Shmem has pre-allocation ready for markers, but hugetlb path
was overlooked.

Doing so by calling huge_pte_alloc() if the initial pgtable walk fails to
find the huge ptep.  It's possible that huge_pte_alloc() can fail with
high memory pressure, in that case stop the loop immediately and fail
silently.  This is not the most ideal solution but it matches with what we
do with shmem meanwhile it avoids the splat in dmesg.

Link: https://lkml.kernel.org/r/20230104225207.1066932-2-peterx@redhat.com
Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: James Houghton <jthoughton@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: James Houghton <jthoughton@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: <stable@vger.kernel.org>	[5.19+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3fc5a214f0b99ad24a38f88cf26f394dce026d58..52475c4262e45ef8ea06b27873cb7befc6092925 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6604,8 +6604,17 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
                spinlock_t *ptl;
                ptep = huge_pte_offset(mm, address, psize);
                if (!ptep) {
-                       address |= last_addr_mask;
-                       continue;
+                       if (!uffd_wp) {
+                               address |= last_addr_mask;
+                               continue;
+                       }
+                       /*
+                        * Userfaultfd wr-protect requires pgtable
+                        * pre-allocations to install pte markers.
+                        */
+                       ptep = huge_pte_alloc(mm, vma, address, psize);
+                       if (!ptep)
+                               break;
                }
                ptl = huge_pte_lock(h, mm, ptep);
                if (huge_pmd_unshare(mm, vma, address, ptep)) {