Re: [PATCH] ext4: take i_mutex in ext4_symlink to eliminate a warning from ext4_truncate

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Wed, Mar 27, 2013 at 11:12:48AM -0400, Theodore Ts'o wrote:
> On Wed, Mar 27, 2013 at 11:19:22PM +0800, Zheng Liu wrote:
> > > > Otherwise, couldn't we end up with problems where a failed write calls
> > > > ext4_truncate() without i_data_sem(), and that races with something
> > > > else --- say, a punch or truncate call to that same inode?
> > 
> > Let me think about it.  I need to take a close look at it.
> 
> Note that I'm not so concerned when we are creating symlink --- you
> are quite right in pointing out in that case the inode isn't in the
> namespace yet, so that prevents races --- but also what might happen
> in an ENOSPC write(2) failure racing against a punch/truncate call.
> 
> But again, this is why I added the warning --- it was to find these
> edge cases that we might not have considered.  :-)

ext4_truncate_failed_write() is called by the following functions:
 - ext4_ind_direct_IO
 - ext4_convert_inline_data_to_extent
 - ext4_da_convert_inline_data_to_extent
 - ext4_write_begin
 - ext4_write_end
 - ext4_journalled_write_end
 - ext4_da_write_begin

All these functions are protected by i_mutex.  So we can serialize it
with truncate/punch hole.

Regards,
                                                - Zheng