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

On Wed, Aug 09, 2023 at 03:39:23PM -0700, Darrick J. Wong wrote:

On Wed, Aug 09, 2023 at 03:05:39PM -0700, Christoph Hellwig wrote:

quoted

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

      ^ seemingly?

quoted

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>

Fixed in-tree.