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

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Aug 09, 2023 at 03:05:39PM -0700, Christoph Hellwig wrote:
> Copy and paste the commit message from Darrick into a comment to explain
> the seemly odd invalidate_bdev in xfs_shutdown_devices.

      ^ seemingly?
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_super.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 4ae3b01ed038c7..c169beb0d8cab3 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -399,6 +399,32 @@ STATIC void
>  xfs_shutdown_devices(
>  	struct xfs_mount	*mp)
>  {
> +	/*
> +	 * 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

This sentence reads a little strangely, since "nor does it invalidate"
would seem to conflict with the invalidate_bdev call below.  I suggest
changing the verb a bit:

"The XFS buffer cache does not use the bdev pagecache, so it needs to
invalidate that pagecache on unmount."

With those two things changed,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D


> +	 * 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);
> -- 
> 2.39.2
>