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.

Link: https://lkml.kernel.org/r/20230227173632.3292573-28-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 70f4b1222e5d021ebc20457c1bdd119218aa0f99..55ac9cdfd3983d4e5c4f1aebe6ea5a58fd69b92c 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5266,6 +5266,15 @@ retry:
        if (!vma_start_read(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)) {
+               vma_end_read(vma);
+               goto inval;
+       }
+
        /* Check since vm_start/vm_end might change before we lock the VMA */
        if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
                vma_end_read(vma);