linux-kernel - Re: [PATCH] loop: change queue block size to match when using DIO.

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Date:   Fri, 30 Aug 2019 08:50:24 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Martijn Coenen <maco@...roid.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundatio.org,
        kernel-team@...roid.com, narayan@...gle.com, dariofreni@...gle.com,
        ioffe@...gle.com, jiyong@...gle.com, maco@...gle.com
Subject: Re: [PATCH] loop: change queue block size to match when using DIO.

On Wed, Aug 28, 2019 at 12:32:29PM +0200, Martijn Coenen wrote:
> The loop driver assumes that if the passed in fd is opened with
> O_DIRECT, the caller wants to use direct I/O on the loop device.
> However, if the underlying filesystem has a different block size than
> the loop block queue, direct I/O can't be enabled. Instead of requiring
> userspace to manually change the blocksize and re-enable direct I/O,
> just change the queue block size to match.

Why can't we enable the block device in that case?  All the usual
block filesystems support 512 byte aligned direct I/O with a 4k
file system block size (as long as the underlying block device
sector size is also 512 bytes).