linux-ext4 - Re: [PATCH 2/5] jbd2: optimize jbd2_journal_force

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Date:	Thu, 18 Apr 2013 20:07:27 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/5] jbd2: optimize jbd2_journal_force_commit

On Wed 17-04-13 11:39:27, Dmitry Monakhov wrote:
> On Mon, 15 Apr 2013 14:29:13 +0200, Jan Kara <jack@...e.cz> wrote:
> > On Sun 14-04-13 23:01:34, Dmitry Monakhov wrote:
> > > Current implementation of jbd2_journal_force_commit() is suboptimal because
> > > result in empty and useless commits. But callers just want to force and wait
> > > any unfinished commits. We already has jbd2_journal_force_commit_nested()
> > > which does exactly what we want, except we are guaranteed that we do not hold
> > > journal transaction open.
> >   Umm, I have a questions regarding this patch:
> > Grep shows there are just two places in the code which use
> > ext4_force_commit() (and thus jbd2_journal_force_commit()). These are
> > ext4_write_inode() and ext4_sync_file() (in data=journal mode). The first
> > callsite can use _nested() variant immediately as we even assert there's
> > no handle started. The second call site can use the _nested variant as well
> > because if we had the transaction started when entering ext4_sync_file() we
> > would have serious problems (lock inversion, deadlocks in !data=journal
> > modes) anyway. So IMO there's no need for !nested variant at all (at least
> > in ext4, ocfs2 uses it as well, IMHO it can be converted as well but that's
> > a different topic). Thoughts?
> I'm not sure that I completely understand what you meant, but it seems
> incorrect to use jbd2_journal_force_commit_nested() in 
> ext4_write_inode() and ext4_sync_file(). Because nested variant has
> probabilistic behavior, It may skip real transaction commit if we hold
> a transaction running.  ext4_write_inode() and ext4_sync_file() 
> are the functions where we demand deterministic behavior. If we silently
> miss real transaction commit because current->journal_info != NULL (due
> to some bugs) this breaks data integrity assumptions and it is better to
> make it loud and trigger a BUGON.
  I see. I was confused by the fact that 'nested' argument got used only in
the assertion but now I see why that is.

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