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

To: Jeff Moyer <jmoyer@xxxxxxxxxx>
Subject: Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous direct I/O requests
From: Jan Kara <jack@xxxxxxx>
Date: Wed, 21 Nov 2012 01:56:26 +0100
Cc: Jan Kara <jack@xxxxxxx>, "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>, axboe@xxxxxxxxx, tytso@xxxxxxx, david@xxxxxxxxxxxxx, bpm@xxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <x49sj84hwl4.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
References: <20121120074116.24645.36369.stgit@xxxxxxxxxxxxxxxxxxx> <20121120074131.24645.38489.stgit@xxxxxxxxxxxxxxxxxxx> <20121120100751.GB1408@xxxxxxxxxxxxx> <x49sj84hwl4.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue 20-11-12 15:02:15, Jeff Moyer wrote:
> Jan Kara <jack@xxxxxxx> 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@xxxxxxx>
SUSE Labs, CR

Attachment: 0001-ext4-Reduce-i_mutex-usage-in-ext4_file_sync.patch
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