xfs: implement live inode scan for scrub

This patch implements a live file scanner for online fsck functions that
require the ability to walk a filesystem to gather metadata records and
stay informed about metadata changes to files that have already been
visited.

The iscan structure consists of two inode number cursors: one to track
which inode we want to visit next, and a second one to track which
inodes have already been visited.  This second cursor is key to
capturing live updates to files previously scanned while the main thread
continues scanning -- any inode greater than this value hasn't been
scanned and can go on its way; any other update must be incorporated
into the collected data.  It is critical for the scanning thraad to hold
exclusive access on the inode until after marking the inode visited.

This new code is split out as a separate patch from its initial user for
the sake of enabling the author to move patches around his tree with
ease.  The intended usage model for this code is roughly:

	xchk_iscan_start(iscan);
	while ((error = xchk_iscan_advance(sc, iscan)) == 1) {
		struct xfs_inode	*ip;

		error = xchk_iscan_iget(sc, iscan, &ip);
		if (error == -EAGAIN)
			continue;
		if (error)
			break;

		xfs_ilock(ip, ...);
		/* capture inode metadata */
		xchk_iscan_mark_visited(iscan, ip);
		xfs_iunlock(ip, ...);

		xfs_irele(ip);
	}
	xchk_iscan_stop(iscan);

Hook functions for live updates can then do:

	if (xchk_iscan_want_live_update(...))
		/* update the captured inode metadata */

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 62 insertions, 0 deletions

diff --git a/fs/xfs/scrub/iscan.h b/fs/xfs/scrub/iscan.h
new file mode 100644
index 000000000000..d9839414a103
--- /dev/null
+++ b/fs/xfs/scrub/iscan.h
@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2021 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#ifndef __XFS_SCRUB_ISCAN_H__
+#define __XFS_SCRUB_ISCAN_H__
+
+struct xchk_iscan {
+	/* Lock to protect the scan cursor. */
+	struct mutex		lock;
+
+	/* This is the inode that is being scanned. */
+	xfs_ino_t		cursor_ino;
+
+	/*
+	 * This is the last inode that we've successfully scanned, either
+	 * because the caller scanned it, or we moved the cursor past an empty
+	 * part of the inode address space.  Scan callers should only use the
+	 * xchk_iscan_visit function to modify this.
+	 */
+	xfs_ino_t		__visited_ino;
+
+	/* Operational state of the livescan. */
+	unsigned long		__opstate;
+
+	/* Number of times to try iget calls for any inode. */
+	unsigned int		iget_tries;
+
+	/* Wait this many jiffies for an iget retry. */
+	unsigned int		iget_retry_delay;
+
+	/* Number of tries remaining for iget of cursor_ino.  Do not modify. */
+	unsigned int		__cursor_tries;
+};
+
+/* Set if the scan has been aborted due to some event in the fs. */
+#define XCHK_ISCAN_OPSTATE_ABORTED	(1)
+
+static inline bool
+xchk_iscan_aborted(const struct xchk_iscan *iscan)
+{
+	return test_bit(XCHK_ISCAN_OPSTATE_ABORTED, &iscan->__opstate);
+}
+
+static inline void
+xchk_iscan_abort(struct xchk_iscan *iscan)
+{
+	set_bit(XCHK_ISCAN_OPSTATE_ABORTED, &iscan->__opstate);
+}
+
+void xchk_iscan_start(struct xchk_iscan *iscan);
+void xchk_iscan_finish(struct xchk_iscan *iscan);
+
+int xchk_iscan_advance(struct xfs_scrub *sc, struct xchk_iscan *iscan);
+int xchk_iscan_iget(struct xfs_scrub *sc, struct xchk_iscan *iscan,
+		struct xfs_inode **ipp);
+
+void xchk_iscan_mark_visited(struct xchk_iscan *iscan, struct xfs_inode *ip);
+bool xchk_iscan_want_live_update(struct xchk_iscan *iscan, xfs_ino_t ino);
+
+#endif /* __XFS_SCRUB_ISCAN_H__ */