Re: fsync on ext[34] working only by an accident

[Jan Kara <jack@suse.cz>]

On Thu 10-09-09 14:45:51, Aneesh Kumar K.V wrote:
> On Thu, Sep 10, 2009 at 02:34:49PM +0530, Aneesh Kumar K.V wrote:
> > On Thu, Sep 10, 2009 at 10:50:56AM +0200, Jan Kara wrote:
> > >   Hi,
> > > 
> > > On Thu 10-09-09 12:16:05, Aneesh Kumar K.V wrote:
> > > > On Tue, Sep 08, 2009 at 03:26:01PM +0200, Jan Kara wrote:
> > > > >   When looking at how ext3/4 handles fsync, I've realized I don't
> > > > > understand how writing out inode on fsync can work. The problem is that
> > > > > ext3/4 mostly calls ext?_mark_inode_dirty() which actually does *not* dirty
> > > > > the inode. It just copies the in-memory inode content to disk buffer.
> > > > > So in particular the inode looks clean to VFS and our check in
> > > > > ext?_sync_file() shouldn't trigger.
> > > > >   The only obvious case when we call mark_inode_dirty() is from write_end
> > > > > functions when we update i_size but that's clearly not enough. Now I did
> > > > > some research why things seem to be actually working. The trick is that
> > > > > when allocating block, we call vfs_dq_alloc_block() which calls
> > > > > mark_inode_dirty(). But that's all what's keeping our fsync / writeout
> > > > > logic from breaking!
> > > > 
> > > > ext4_handle_dirty_metadata should do mark_inode_dirty right ?
> > > > __ext4_handle_dirty_metadata -> mark_buffer_dirty ->__set_page_dirty
> > > > -> __mark_inode_dirty ->  list_move(&inode->i_list, &sb->s_dirty);
> > >   ext4_handle_dirty_metadata() marks the buffer dirty only when we do not
> > > have a journal (BTW, the inode that gets dirtied in the nojournal case
> > > is the block-device one, not the one whose metadata we mark as dirty, so
> > > it won't work there either - but Google guys are working on this I think).
> > > With a journal the function just calls jbd2_journal_dirty_metadata which
> > > does nothing with the inode.
> > 
> > When we don't have a journal handle we do that as a part of journal commit
> > right ? __jbd2_journal_temp_unlink_buffer -> mark_buffer_dirty  
> > 
> > I guess fsync only requires the meta data update to be in journal ?
> > 
> 
> Adding the file inode to the sb->s_dirty is done through block_write_end ?
> Why do you mention above that it is not "clearly not enough" ?
  Where? I don't see block_write_end() marking the inode dirty anywhere...
It calls __block_commit_write() and that dirties only buffers, not the
inode.
								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR