mm: handle userfaults under VMA lock

Enable handle_userfault to operate under VMA lock by releasing VMA lock
instead of mmap_lock and retrying.  Note that FAULT_FLAG_RETRY_NOWAIT
should never be used when handling faults under per-VMA lock protection
because that would break the assumption that lock is dropped on retry.

[[email protected]: fix a lockdep issue in vma_assert_write_locked]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Acked-by: Peter Xu <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Laurent Dufour <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Punit Agrawal <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 1 insertions, 8 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 5748a41c164c..2c6f45d18b73 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5471,14 +5471,7 @@ retry:
 	 * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
 	 * from its anon_vma.
 	 */
-	if (vma_is_anonymous(vma) && !vma->anon_vma)
-		goto inval_end_read;
-
-	/*
-	 * 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))
+	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
 		goto inval_end_read;
 
 	/* Check since vm_start/vm_end might change before we lock the VMA */