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xfs: document the invalidate_bdev call in invalidate_bdev

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

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Message-Id: <20230809220545.1308228-8-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christoph Hellwig 2023-08-09 15:05:39 -07:00 committed by Christian Brauner
parent 35a93b148b
commit 1a0a5dad67
1 changed files with 26 additions and 0 deletions
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26
fs/xfs/xfs_super.c
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@ -410,6 +410,32 @@ 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, so it needs to
* invalidate that pagecache on unmount. 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);