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

Matthew Wilcox
2023-08-10T15:22:33Z

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.
*/