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

Darrick J. Wong
2023-08-10T15:57:06Z

On Thu, Aug 10, 2023 at 04:22:15PM +0100, Matthew Wilcox wrote:
> 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.

"XFS' buffer cache is not coherent with the bdev's page cache."
?

--D

> */