linux-kernel - Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous direct I/O requests

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Date:	Wed, 21 Nov 2012 01:56:26 +0100
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>,
	"Darrick J. Wong" <darrick.wong@...cle.com>, axboe@...nel.dk,
	tytso@....edu, david@...morbit.com, bpm@....com,
	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	hch@...radead.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com
Subject: Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous
 direct I/O requests

On Tue 20-11-12 15:02:15, Jeff Moyer wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> >> @@ -1279,6 +1280,9 @@ struct ext4_sb_info {
> >>  	/* workqueue for dio unwritten */
> >>  	struct workqueue_struct *dio_unwritten_wq;
> >>  
> >> +	/* workqueue for aio+dio+o_sync disk cache flushing */
> >> +	struct workqueue_struct *aio_dio_flush_wq;
> >> +
> >   Umm, I'm not completely decided whether we really need a separate
> > workqueue. But it doesn't cost too much so I guess it makes some sense -
> > fsync() is rather heavy so syncing won't starve extent conversion...
> 
> I'm assuming you'd like me to convert the names from flush to fsync,
> yes?
  Would be nicer, yes.

> >> +
> >> +	/*
> >> +	 * If we are running in nojournal mode, just flush the disk
> >> +	 * cache and return.
> >> +	 */
> >> +	if (!journal)
> >> +		return blkdev_issue_flush(inode->i_sb->s_bdev, GFP_NOIO, NULL);
> >   And this is wrong as well - you need to do work similar to what
> > ext4_sync_file() does. Actually it would be *much* better if these two
> > sites used the same helper function. Which also poses an interesting
> > question about locking - do we need i_mutex or not? Forcing a transaction
> > commit is definitely OK without it, similarly as grabbing transaction ids
> > from inode or ext4_should_journal_data() test. __sync_inode() call seems
> > to be OK without i_mutex as well so I believe we can just get rid of it
> > (getting i_mutex from the workqueue is a locking nightmare we don't want to
> > return to).
> 
> Just to be clear, are you saying you would like me to remove the
> mutex_lock/unlock pair from ext4_sync_file?  (I had already factored out
> the common code between this new code path and the fsync path in my tree.)
  Yes, after some thinking I came to that conclusion. We actually need to
keep i_mutex around ext4_flush_unwritten_io() to avoid livelocks but the
rest doesn't need it. The change should be definitely a separate patch just
in case there's something subtle I missed and we need to bisect in
future... I've attached a patch for that so that blame for bugs goes my way
;) Compile tested only so far. I'll give it some more testing overnight.

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

View attachment "0001-ext4-Reduce-i_mutex-usage-in-ext4_file_sync.patch" of type "text/x-patch" (1863 bytes)