fs: introduce new truncate sequence

Introduce a new truncate calling sequence into fs/mm subsystems. Rather than
setattr > vmtruncate > truncate, have filesystems call their truncate sequence
from ->setattr if filesystem specific operations are required. vmtruncate is
deprecated, and truncate_pagecache and inode_newsize_ok helpers introduced
previously should be used.

simple_setattr is introduced for simple in-ram filesystems to implement
the new truncate sequence. Eventually all filesystems should be converted
to implement a setattr, and the default code in notify_change should go
away.

simple_setsize is also introduced to perform just the ATTR_SIZE portion
of simple_setattr (ie. changing i_size and trimming pagecache).

To implement the new truncate sequence:
- filesystem specific manipulations (eg freeing blocks) must be done in
  the setattr method rather than ->truncate.
- vmtruncate can not be used by core code to trim blocks past i_size in
  the event of write failure after allocation, so this must be performed
  in the fs code.
- convert usage of helpers block_write_begin, nobh_write_begin,
  cont_write_begin, and *blockdev_direct_IO* to use _newtrunc postfixed
  variants. These avoid calling vmtruncate to trim blocks (see previous).
- inode_setattr should not be used. generic_setattr is a new function
  to be used to copy simple attributes into the generic inode.
- make use of the better opportunity to handle errors with the new sequence.

Big problem with the previous calling sequence: the filesystem is not called
until i_size has already changed.  This means it is not allowed to fail the
call, and also it does not know what the previous i_size was. Also, generic
code calling vmtruncate to truncate allocated blocks in case of error had
no good way to return a meaningful error (or, for example, atomically handle
block deallocation).

Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 5 insertions, 5 deletions

diff --git a/mm/truncate.c b/mm/truncate.c
index f42675a3615d..937571b8b233 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -548,18 +548,18 @@ EXPORT_SYMBOL(truncate_pagecache);
  * NOTE! We have to be ready to update the memory sharing
  * between the file and the memory map for a potential last
  * incomplete page.  Ugly, but necessary.
+ *
+ * This function is deprecated and simple_setsize or truncate_pagecache
+ * should be used instead.
  */
 int vmtruncate(struct inode *inode, loff_t offset)
 {
-	loff_t oldsize;
 	int error;
 
-	error = inode_newsize_ok(inode, offset);
+	error = simple_setsize(inode, offset);
 	if (error)
 		return error;
-	oldsize = inode->i_size;
-	i_size_write(inode, offset);
-	truncate_pagecache(inode, oldsize, offset);
+
 	if (inode->i_op->truncate)
 		inode->i_op->truncate(inode);