linux-ext4 - Re: [RFC PATCH] Filesystem Journal Notifications

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Message-ID: <20080916230552.GB13446@apaithan-desktop.sanmateo.corp.akamai.com>
Date:	Tue, 16 Sep 2008 16:05:52 -0700
From:	Abhijit Paithankar <apaithan@...mai.com>
To:	Andreas Dilger <adilger@....com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Jamie Lokier <jamie@...reable.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [RFC PATCH] Filesystem Journal Notifications

Hi,

Thanks for your feedback.
My comments are inline.

* Andreas Dilger (adilger@....com) wrote:
> On Sep 15, 2008  12:36 -0700, Abhijit Paithankar wrote:
> > In this version of the patch I've done away with most of the file-system
> > specific changes. The file system now only needs to send out a journal
> > commit notification.
> 
> Is there a reason NOT to use the journal commit callback mechanism that
> I posted?  This would only require registering a single callback for
> each transaction for the inotify, but has the benefit of being completely
> generic and can be used for other commit notifiers in the future.
> 

In the current patch if two applications want to be notified on the journal
commit event they could register for IN_JOURNAL_COMMIT on the same inode,
which could be the root inode of the file system.

There is no limit on the number of applications that could register on the
root inode for journal commit notifications. The only limitation is that
they all have to register on the same inode for that partition.

The journal commit callback mechanism could be viable way to notify
journal commits. However it is not clear to me how an application would
register the callback.

I suspect we need to have some other mechanism to register that for the
application.


> Most of the rest of my comments are related to following the Linux
> coding style.  Please also generate your patch with "diff -up" so
> that the context includes the function name.
> 
> > +void journal_notify_commit (journal_t *journal)
> > +{
> 
> No space between the function name and the '(', journal_notify_commit(
ok, will fix.

> 
> > +	struct super_block *sb = journal->j_private;
> > +	if (sb->s_priv_inode && sb->s_journal_cookie)
> > +		fsnotify_journal_commit(sb);
> > +}
> 
> Please add an empty line between variable declarations and the code.
ack.

> 
> > +void jbd2_journal_notify_commit (journal_t *journal)
> > +{
> > +	struct super_block *sb = journal->j_private;
> > +	if (sb->s_priv_inode && sb->s_journal_cookie)
> > +		fsnotify_journal_commit(sb);
> > +}
> 
> Same comments as above.
ack.

> 
> > +		if (journal_cookie) {
> > +			size_t len, rem, event_size = sizeof(struct inotify_event);
> 
> Wrap at 80 columns, likely putting "size_t event_size = ..." on a new line.
sure.

> 
> >  static void inotify_dev_queue_event(struct inotify_watch *w, u32 wd, u32 mask,
> > +	if (mask & IN_JOURNAL_COMMIT) {
> > +		if (!(w->inode) || !(w->inode->i_sb)
> > +			|| !(w->inode->i_sb->s_priv_inode)
> > +			|| !(w->inode->i_sb->s_journal_cookie))
> > +			goto out;
> > +	}
> 
> Operators like "||" should go at the end of the line, and the new line
> should be aligned with the opening parenthesis:
> 
> 		if (!(w->inode) || !(w->inode->i_sb) ||
> 		    !(w->inode->i_sb->s_priv_inode) ||
> 		    !(w->inode->i_sb->s_journal_cookie))
> 
will do.

> > +	journal_notify_commit(journal);
> >  	jbd_debug(1, "JBD: commit %d complete, head %d\n",
> >  		  journal->j_commit_sequence, journal->j_tail_sequence);
> 
> Note that this is also a "late" notification.  At phase 7 the transaction
> is complete and committed to disk, the rest of journal_commit_transaction()
> is just doing cleanup and background processing.
Yes, will move it up.

> 
> > +/* This is a journal commit event for file systems. These events are sent
> > + * from a journaling filesystem or the journal itself.
> > + *
> > + * IN_JOURNAL_COMMIT is special in that only one file per journaling
> > + * filesystem partition (super-block) is allowed. Registering for this
> > + * event with multiple files will overwrite previous registrations.
> 
> This is a major limitation of your implementation - what if 2 applications
> want to be notified of this event?  Using the journal_callback_set() API
> in the patch I sent out would allow an arbitrary number of applications
> to monitor the commit.

Any number of applications can register for the journal event as long as they
register on the same inode.

It is not clear to me how using journal_callback_set API would change that if
we use inotify to register callbacks.

Thanks!
Abhijit
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