btrfs: avoid monopolizing a core when activating a swap file

commit 2c8507c upstream.

During swap activation we iterate over the extents of a file and we can
have many thousands of them, so we can end up in a busy loop monopolizing
a core. Avoid this by doing a voluntary reschedule after processing each
extent.

CC: [email protected] # 5.4+
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: fdmanana

fs/btrfs/inode.c

@@ -7117,6 +7117,8 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 			ret = -EAGAIN;
 			goto out;
 		}
+
+		cond_resched();
 	}
 
 	if (file_extent)