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

Christoph Hellwig
2023-08-09T22:05:55Z

Copy and paste the commit message from Darrick into a comment to explain
the seemly odd invalidate_bdev in xfs_shutdown_devices.

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
+ * 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