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

[Jan Kara <jack@suse.cz>]

  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.

> >   There are even some cases when the logic actually is broken (I've tested
> > it and it really does not work) - for example when you create an empty
> > file, the inode won't get written when you fsync it.
> >   So what we should IMHO do is to convert all ext?_mark_inode_dirty()
> > calls to simple mark_inode_dirty() (or even maybe introduce and use
> > mark_inode_dirty_datasync() where appropriate). It will cost us some more
> > CPU and stack space but if we optimize ext3_dirty_inode() for the case
> > where handle is already started, it shouldn't be too bad.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR