[BUG] -mm: mm-only-i_size_write-debugging.patch

[Carsten Otte <cotte@de.ibm.com>]

I think that the warning introduced by the patch 
mm-only-i_size_write-debugging.patch is incorrect in the case I ran into:
[in fs/inode.c]
+void i_size_write(struct inode *inode, loff_t i_size)
+{
+       WARN_ON_ONCE(!mutex_is_locked(&inode->i_mutex));

I can reprocude triggering this warning by copying files to a 
read+write mounted ext2 filesystem (without -o xip).

The stack chain looks like this:
BUG: warning at fs/inode.c:1389/i_size_write()
Call Trace:
([<0000000000103502>] show_trace+0x166/0x16c)
  [<00000000001035ce>] show_stack+0xc6/0xf8
  [<000000000010362e>] dump_stack+0x2e/0x3c
  [<00000000001b6484>] i_size_write+0x78/0x84
  [<00000000001ca5ce>] generic_commit_write+0x62/0x84
  [<00000000002296ec>] ext2_commit_chunk+0x64/0xbc
  [<0000000000229b70>] ext2_make_empty+0x1e8/0x21c
  [<000000000022eeb2>] ext2_mkdir+0xea/0x19c
  [<00000000001a66d8>] vfs_mkdir+0x10c/0x1c4
  [<00000000001aa3ac>] sys_mkdirat+0xc8/0x120
  [<00000000001aa436>] sys_mkdir+0x32/0x40
  [<000000000010dc18>] sysc_tracego+0xe/0x14
  [<000002000012a3fa>] 0x2000012a3fa

I believe ext2 is save to write without i_mutex here: The inode is 
newly created by ext2_new_inode, and is not accessible to other 
contexts at this time. On the other hand, ext2_mkdir could grab the 
i_mutex of the newly created inode.

Any advice would be helpful.