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

To: Jan Kara <jack@xxxxxxx>
Subject: Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous direct I/O requests
From: Jeff Moyer <jmoyer@xxxxxxxxxx>
Date: Tue, 20 Nov 2012 15:02:15 -0500
Cc: "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: <20121120100751.GB1408@xxxxxxxxxxxxx> (Jan Kara's message of "Tue, 20 Nov 2012 11:07:51 +0100")
References: <20121120074116.24645.36369.stgit@xxxxxxxxxxxxxxxxxxx> <20121120074131.24645.38489.stgit@xxxxxxxxxxxxxxxxxxx> <20121120100751.GB1408@xxxxxxxxxxxxx>
User-agent: Gnus/5.110011 (No Gnus v0.11) Emacs/23.1 (gnu/linux)
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?

>> +
>> +    /*
>> +     * 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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