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

Darrick J. Wong
2023-08-09T22:39:23Z

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
>