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

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Message-ID: <x49sj84hwl4.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 20 Nov 2012 15:02:15 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"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

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?

>> +
>> +	/*
>> +	 * 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.)

>> @@ -149,8 +209,11 @@ void ext4_add_complete_io(ext4_io_end_t *io_end)
>>  	struct workqueue_struct *wq;
>>  	unsigned long flags;
>>  
>> -	BUG_ON(!(io_end->flag & EXT4_IO_END_UNWRITTEN));
>> -	wq = EXT4_SB(io_end->inode->i_sb)->dio_unwritten_wq;
>> +	BUG_ON(!ext4_io_end_deferred(io_end));
>> +	if (io_end->flag & EXT4_IO_END_UNWRITTEN)
>> +		wq = EXT4_SB(io_end->inode->i_sb)->dio_unwritten_wq;
>> +	else
>> +		wq = EXT4_SB(io_end->inode->i_sb)->aio_dio_flush_wq;
>   Umm, I'd prefer if we used aio_dio_flush_wq when EXT4_IO_END_NEEDS_SYNC
> is set. That way slow syncing works will be always offloaded to a separate
> workqueue.

OK.

Thanks!
Jeff
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