fs: add documentation on fallocate hole punching
This patch simply adds documentation on how to handle the hole punching mode of
fallocate for any filesystem wishing to use it.
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 07a32b4..266d205 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -385,3 +385,12 @@
on many or all directory inodes on the way down a path walk (to check for
exec permission). These must now be rcu-walk aware (flags & IPERM_RCU). See
Documentation/filesystems/vfs.txt for more details.
+
+--
+[mandatory]
+ In ->fallocate() you must check the mode option passed in. If your
+filesystem does not support hole punching (deallocating space in the middle of a
+file) you must return -EOPNOTSUPP if FALLOC_FL_PUNCH_HOLE is set in mode.
+Currently you can only have FALLOC_FL_PUNCH_HOLE with FALLOC_FL_KEEP_SIZE set,
+so the i_size should not change when hole punching, even when puching the end of
+a file off.
