[PATCH 27/41] mm/mmap: prevent pagefault handler from racing with mmu_notifier registration

[Suren Baghdasaryan]

Page fault handlers might need to fire MMU notifications while a new
notifier is being registered. Modify mm_take_all_locks to write-lock all
VMAs and prevent this race with fault handlers that would hold VMA locks.
VMAs are locked before i_mmap_rwsem and anon_vma to keep the same
locking order as in page fault handlers.

Signed-off-by: Suren Baghdasaryan <surenb at google.com>
---
 mm/mmap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 30c7d1c5206e..a256deca0bc0 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3566,6 +3566,7 @@ static void vm_lock_mapping(struct mm_struct *mm, struct address_space *mapping)
  * of mm/rmap.c:
  *   - all hugetlbfs_i_mmap_rwsem_key locks (aka mapping->i_mmap_rwsem for
  *     hugetlb mapping);
+ *   - all vmas marked locked
  *   - all i_mmap_rwsem locks;
  *   - all anon_vma->rwseml
  *
@@ -3591,6 +3592,7 @@ int mm_take_all_locks(struct mm_struct *mm)
 	mas_for_each(&mas, vma, ULONG_MAX) {
 		if (signal_pending(current))
 			goto out_unlock;
+		vma_write_lock(vma);
 		if (vma->vm_file && vma->vm_file->f_mapping &&
 				is_vm_hugetlb_page(vma))
 			vm_lock_mapping(mm, vma->vm_file->f_mapping);
@@ -3677,6 +3679,7 @@ void mm_drop_all_locks(struct mm_struct *mm)
 		if (vma->vm_file && vma->vm_file->f_mapping)
 			vm_unlock_mapping(vma->vm_file->f_mapping);
 	}
+	vma_write_unlock_mm(mm);
 
 	mutex_unlock(&mm_all_locks_mutex);
 }
-- 
2.39.0