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

Jeff Moyer
2012-11-20T20:02:31Z

Jan Kara <jack@suse.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