From: Matthew Wilcox (Oracle) willy@infradead.org
commit cb995f4eeba9d268fd4b56c2423ad6c1d1ea1b82 upstream.
If a read races with an invalidation followed by another read, it is possible for a folio to be replaced with a higher-order folio. If that happens, we'll see a sibling entry for the new folio in the next iteration of the loop. This manifests as a NULL pointer dereference while holding the RCU read lock.
Handle this by simply returning. The next call will find the new folio and handle it correctly. The other ways of handling this rare race are more complex and it's just not worth it.
Reported-by: Dave Chinner david@fromorbit.com Reported-by: Brian Foster bfoster@redhat.com Debugged-by: Brian Foster bfoster@redhat.com Tested-by: Brian Foster bfoster@redhat.com Reviewed-by: Brian Foster bfoster@redhat.com Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") Cc: stable@vger.kernel.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/filemap.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/mm/filemap.c +++ b/mm/filemap.c @@ -2385,6 +2385,8 @@ static void filemap_get_read_batch(struc continue; if (xas.xa_index > max || xa_is_value(folio)) break; + if (xa_is_sibling(folio)) + break; if (!folio_try_get_rcu(folio)) goto retry;