Re: [PATCH 07/13] xfs: document the invalidate_bdev call in invalidate_bdev

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 09, 2023 at 03:05:39PM -0700, Christoph Hellwig wrote:
> +	/*
> +	 * Udev is triggered whenever anyone closes a block device or unmounts
> +	 * a file systemm on a block device.
> +	 * The default udev rules invoke blkid to read the fs super and create
> +	 * symlinks to the bdev under /dev/disk.  For this, it uses buffered
> +	 * reads through the page cache.
> +	 *
> +	 * xfs_db also uses buffered reads to examine metadata.  There is no
> +	 * coordination between xfs_db and udev, which means that they can run
> +	 * concurrently.  Note there is no coordination between the kernel and
> +	 * blkid either.
> +	 *
> +	 * On a system with 64k pages, the page cache can cache the superblock
> +	 * and the root inode (and hence the root directory) with the same 64k
> +	 * page.  If udev spawns blkid after the mkfs and the system is busy
> +	 * enough that it is still running when xfs_db starts up, they'll both
> +	 * read from the same page in the pagecache.
> +	 *
> +	 * The unmount writes updated inode metadata to disk directly.  The XFS
> +	 * buffer cache does not use the bdev pagecache, nor does it invalidate
> +	 * the pagecache on umount.  If the above scenario occurs, the pagecache
> +	 * no longer reflects what's on disk, xfs_db reads the stale metadata,
> +	 * and fails to find /a.  Most of the time this succeeds because closing
> +	 * a bdev invalidates the page cache, but when processes race, everyone
> +	 * loses.
> +	 */
>  	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp) {
>  		blkdev_issue_flush(mp->m_logdev_targp->bt_bdev);
>  		invalidate_bdev(mp->m_logdev_targp->bt_bdev);

While I have no complaints with this as a commit message, it's just too
verbose for an inline comment, IMO.  Something pithier and more generic
would seem appropriate.  How about:

	/*
	 * Prevent userspace (eg blkid or xfs_db) from seeing stale data.
	 * XFS is not coherent with the bdev's page cache.
	 */