mm: prevent userfaults to be handled under per-vma lock

Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
handling under VMA lock and retry holding mmap_lock. This can be handled
more gracefully in the future.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Peter Xu <peterx@redhat.com>

diff --git a/mm/memory.c b/mm/memory.c
index f528d0ea3671b85812c54421903e83ae4bbb4013..e618481d6b528b99146afedc929fb810a5f46f17 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5267,6 +5267,13 @@ retry:
        if (!vma_is_anonymous(vma))
                goto inval;
 
+       /*
+       * Due to the possibility of userfault handler dropping mmap_lock, avoid
+       * it for now and fall back to page fault handling under mmap_lock.
+       */
+       if (userfaultfd_armed(vma))
+               goto inval;
+
        if (!vma_read_trylock(vma)) {
                count_vm_vma_lock_event(VMA_LOCK_ABORT);
                goto inval;