linux-ext4 - Re: [RFC PATCH v2 1/7] statx: add I/O alignment information

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Date:   Wed, 15 Jun 2022 23:07:17 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Keith Busch <kbusch@...nel.org>
Subject: Re: [RFC PATCH v2 1/7] statx: add I/O alignment information

On Wed, Jun 15, 2022 at 05:04:57PM -0700, Eric Biggers wrote:
> One more thing.  I'm trying to add support for STATX_DIOALIGN on block devices.
> Unfortunately I don't think it is going to work, at all, since the inode is for
> the device node and not the block device itself.  This is true even after the
> file is opened (I previously thought that at least that case would work).

For an open file the block device inode is pointed to by
file->f_mapping->host.

> Were you expecting that this would work on block devices?  It seems they will
> need a different API -- a new BLK* ioctl, or files in /sys/block/$dev/queue.

blkdev_get_no_open on inode->i_rdev gets you the block device, which
then has bdev->bd_inode point to the underlying block device, although
for a block device those limit probably would be retrieved not from
the inode but the gendisk / request_queue anyway.