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Date:   Tue, 8 Oct 2019 12:52:07 +0200
From:   Jan Kara <jack@suse.cz>
To:     Matthew Bobrowski <mbobrowski@mbobrowski.org>
Cc:     tytso@mit.edu, jack@suse.cz, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        hch@infradead.org, david@fromorbit.com, darrick.wong@oracle.com
Subject: Re: [PATCH v4 4/8] ext4: introduce direct I/O read path using iomap
 infrastructure
Message-ID: <20191008105207.GG5078@quack2.suse.cz>
References: <cover.1570100361.git.mbobrowski@mbobrowski.org>
 <df2b8a10641ec8a0509f137dcc2db1d3cc6087f1.1570100361.git.mbobrowski@mbobrowski.org>
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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@mbobrowski.org>

The patch looks good to me. Just one small nit below. With that fixed, you
can add:

Reviewed-by: Jan Kara <jack@suse.cz>

> +	/*
> +	 * 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. 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 :).

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR