linux-ext4 - Re: [PATCH v4 4/8] ext4: introduce direct I/O read path using iomap infrastructure

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Date:   Wed, 9 Oct 2019 21:55:09 +1100
From:   Matthew Bobrowski <mbobrowski@...browski.org>
To:     Jan Kara <jack@...e.cz>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, david@...morbit.com, darrick.wong@...cle.com
Subject: Re: [PATCH v4 4/8] ext4: introduce direct I/O read path using iomap
 infrastructure

On Tue, Oct 08, 2019 at 12:52:07PM +0200, Jan Kara wrote:
> On Thu 03-10-19 21:34:00, Matthew Bobrowski wrote:
> > This patch introduces a new direct I/O read path that makes use of the
> > iomap infrastructure.
> > 
> > The new function ext4_dio_read_iter() is responsible for calling into
> > the iomap infrastructure via iomap_dio_rw(). If the read operation
> > being performed on the inode does not pass the preliminary checks
> > performed within ext4_dio_supported(), then we simply fallback to
> > buffered I/O in order to fulfil the request.
> > 
> > Existing direct I/O read buffer_head code has been removed as it's now
> > redundant.
> > 
> > Signed-off-by: Matthew Bobrowski <mbobrowski@...browski.org>
> 
> The patch looks good to me. Just one small nit below. With that fixed, you
> can add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>

Cool, I'll fix it!

> > +	/*
> > +	 * Get exclusion from truncate and other inode operations.
> > +	 */
> > +	if (!inode_trylock_shared(inode)) {
> > +		if (iocb->ki_flags & IOCB_NOWAIT)
> > +			return -EAGAIN;
> > +		inode_lock_shared(inode);
> > +	}
> 
> I've noticed here you actually introduce new trylock pattern - previously
> we had unconditional inode_lock_shared() in ext4_direct_IO_read(). So the
> cleanest would be to just use unconditional inode_lock_shared() here and
> then fixup IOCB_NOWAIT handling (I agree that was missing in the original
> code) in a separate patch.

Right, so I will just have an unconditional call to
inode_lock_shared() and in the patch that follows I will fix it up to
apply the new pattern.

> And the pattern should rather look like:
>
> 	if (iocb->ki_flags & IOCB_NOWAIT) {
> 		if (!inode_trylock_shared(inode))
> 			return -EAGAIN;
> 	} else {
> 		inode_lock_shared(inode);
> 	}
> 
> to avoid two atomical operations instead of one in the fast path. No need
> to repeat old mistakes when we know better :).

Yes, also agree.

--<M>--