linux-ext4 - Re: [PATCH v7 11/11] ext4: introduce direct I/O write using iomap infrastructure

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Message-ID: <20191105205712.GB1739@bobrowski>
Date:   Wed, 6 Nov 2019 07:57:13 +1100
From:   Matthew Bobrowski <mbobrowski@...browski.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     jack@...e.cz, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, riteshh@...ux.ibm.com
Subject: Re: [PATCH v7 11/11] ext4: introduce direct I/O write using iomap
 infrastructure

On Tue, Nov 05, 2019 at 11:28:55AM -0500, Theodore Y. Ts'o wrote:
> On Tue, Nov 05, 2019 at 11:02:39PM +1100, Matthew Bobrowski wrote:
> > +	ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
> > +			   is_sync_kiocb(iocb) || unaligned_aio || extend);
> > +
> > +	if (extend)
> > +		ret = ext4_handle_inode_extension(inode, offset, ret, count);
> > +
> 
> Can we do a slight optimization here like this?
> 
> 	ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
> 			   is_sync_kiocb(iocb) || unaligned_aio || extend);
> 
> 	if (extend && ret != -EBIOCQUEUED)
> 		ret = ext4_handle_inode_extension(inode, offset, ret, count);
> 
> 
> If iomap_dio_rw() returns -EBIOCQUEUED, there's no need to do any of
> the ext4_handle_inode_extension --- in particular, there's no need to
> call ext4_truncate_failed_write(), which has a bunch of extra
> overhead, including taking and releasing i_data_sem.

Hm, but for extension, unaligned asynchronous IO, or synchronous IO
cases, 'wait_for_completion' within iomap_dio_rw() is set to true and
as a result we'd never expect to receive -EIOCBQUEUED from
iomap_dio_rw()?

So, with that said, would the above change be necessary seeing as
though we'd never expect ret == -EIOCBQUEUED when extend == true?

Maybe I'm missing something?

/M