xfs: introduce online scrub freeze

Introduce a new 'online scrub freeze' that we can use to lock out all
filesystem modifications and background activity so that we can perform
global scans in order to rebuild metadata.  This introduces a new IFLAG
to the scrub ioctl to indicate that userspace is willing to allow a
freeze.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

8 files changed, 159 insertions, 4 deletions

diff --git a/fs/xfs/libxfs/xfs_fs.h b/fs/xfs/libxfs/xfs_fs.h
index 66ac44e83a04..bb077766a690 100644
--- a/fs/xfs/libxfs/xfs_fs.h
+++ b/fs/xfs/libxfs/xfs_fs.h
@@ -717,7 +717,11 @@ struct xfs_scrub_metadata {
  */
 #define XFS_SCRUB_OFLAG_NO_REPAIR_NEEDED (1 << 7)
 
-#define XFS_SCRUB_FLAGS_IN	(XFS_SCRUB_IFLAG_REPAIR)
+/* i: Allow scrub to freeze the filesystem to perform global scans. */
+#define XFS_SCRUB_IFLAG_FREEZE_OK	(1 << 8)
+
+#define XFS_SCRUB_FLAGS_IN	(XFS_SCRUB_IFLAG_REPAIR | \
+				 XFS_SCRUB_IFLAG_FREEZE_OK)
 #define XFS_SCRUB_FLAGS_OUT	(XFS_SCRUB_OFLAG_CORRUPT | \
 				 XFS_SCRUB_OFLAG_PREEN | \
 				 XFS_SCRUB_OFLAG_XFAIL | \
diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
index 299567271e01..ec3f0d09b529 100644
--- a/fs/xfs/scrub/common.c
+++ b/fs/xfs/scrub/common.c
@@ -601,9 +601,13 @@ xchk_trans_alloc(
 	struct xfs_scrub	*sc,
 	uint			resblks)
 {
+	uint			flags = 0;
+
+	if (sc->flags & XCHK_FS_FROZEN)
+		flags |= XFS_TRANS_NO_WRITECOUNT;
 	if (sc->sm->sm_flags & XFS_SCRUB_IFLAG_REPAIR)
 		return xfs_trans_alloc(sc->mp, &M_RES(sc->mp)->tr_itruncate,
-				resblks, 0, 0, &sc->tp);
+				resblks, 0, flags, &sc->tp);
 
 	return xfs_trans_alloc_empty(sc->mp, &sc->tp);
 }
@@ -922,3 +926,86 @@ xchk_start_reaping(
 	xfs_start_block_reaping(sc->mp);
 	sc->flags &= ~XCHK_REAPING_DISABLED;
 }
+
+/*
+ * Exclusive Filesystem Access During Scrub and Repair
+ * ===================================================
+ *
+ * While most scrub activity can occur while the filesystem is live, there
+ * are certain scenarios where we cannot tolerate concurrent metadata updates.
+ * We therefore must freeze the filesystem against all other changes.
+ *
+ * The typical scenarios envisioned for scrub freezes are (a) to lock out all
+ * other filesystem changes in order to check the global summary counters,
+ * and anything else that requires unusual behavioral semantics.
+ *
+ * The typical scenarios envisioned for repair freezes are (a) to avoid ABBA
+ * deadlocks when need to take locks in an unusual order; or (b) to update
+ * global filesystem state.  For example, reconstruction of a damaged reverse
+ * mapping btree requires us to hold the AG header locks while scanning
+ * inodes, which goes against the usual inode -> AG header locking order.
+ *
+ * A note about inode reclaim: when we freeze the filesystem, users can't
+ * modify things and periodic background reclaim of speculative preallocations
+ * and copy-on-write staging extents is stopped.  However, the scrub/repair
+ * thread must be careful about evicting an inode from memory -- if the
+ * eviction would require a transaction, we must defer the iput until after
+ * the scrub freeze.  The reasons for this are twofold: first, scrub/repair
+ * already have a transaction and xfs can't nest transactions; and second, we
+ * froze the fs to prevent modifications that we can't control directly.
+ * This guarantee is made by freezing the inode inactivation worker while
+ * frozen.
+ *
+ * Userspace is prevented from freezing or thawing the filesystem during a
+ * repair freeze by the ->freeze_super and ->thaw_super superblock operations,
+ * which block any changes to the freeze state while a repair freeze is
+ * running through the use of the m_scrub_freeze mutex.  It only makes sense
+ * to run one scrub/repair freeze at a time, so the mutex is fine.
+ *
+ * Scrub/repair freezes cannot be initiated during a regular freeze because
+ * freeze_super does not allow nested freeze.  Repair activity that does not
+ * require a repair freeze is also prevented from running during a regular
+ * freeze because transaction allocation blocks on the regular freeze.  We
+ * assume that the only other users of XFS_TRANS_NO_WRITECOUNT transactions
+ * either aren't modifying space metadata in a way that would affect repair,
+ * or that we can inhibit any of the ones that do.
+ *
+ * Note that thaw_super and freeze_super can call deactivate_locked_super
+ * which can free the xfs_mount.  This can happen if someone freezes the block
+ * device, unmounts the filesystem, and thaws the block device.  Therefore, we
+ * must be careful about who gets to unlock the repair freeze mutex.  See the
+ * comments in xfs_fs_put_super.
+ */
+
+/* Start a scrub/repair freeze. */
+int
+xchk_fs_freeze(
+	struct xfs_scrub	*sc)
+{
+	int			error;
+
+	if (!(sc->sm->sm_flags & XFS_SCRUB_IFLAG_FREEZE_OK))
+		return -EUSERS;
+
+	mutex_lock(&sc->mp->m_scrub_freeze);
+	error = freeze_super(sc->mp->m_super);
+	if (error) {
+		mutex_unlock(&sc->mp->m_scrub_freeze);
+		return error;
+	}
+	sc->flags |= XCHK_FS_FROZEN;
+	return 0;
+}
+
+/* Release a scrub/repair freeze. */
+int
+xchk_fs_thaw(
+	struct xfs_scrub	*sc)
+{
+	int			error;
+
+	sc->flags &= ~XCHK_FS_FROZEN;
+	error = thaw_super(sc->mp->m_super);
+	mutex_unlock(&sc->mp->m_scrub_freeze);
+	return error;
+}
diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
index 475680576c1b..43856be95110 100644
--- a/fs/xfs/scrub/common.h
+++ b/fs/xfs/scrub/common.h
@@ -141,6 +141,8 @@ int xchk_metadata_inode_forks(struct xfs_scrub *sc);
 int xchk_ilock_inverted(struct xfs_inode *ip, uint lock_mode);
 void xchk_stop_reaping(struct xfs_scrub *sc);
 void xchk_start_reaping(struct xfs_scrub *sc);
+int xchk_fs_freeze(struct xfs_scrub *sc);
+int xchk_fs_thaw(struct xfs_scrub *sc);
 
 /* Do we need to invoke the repair tool? */
 static inline bool xfs_scrub_needs_repair(struct xfs_scrub_metadata *sm)
diff --git a/fs/xfs/scrub/scrub.c b/fs/xfs/scrub/scrub.c
index 05dc3144d143..6ea0b961fef7 100644
--- a/fs/xfs/scrub/scrub.c
+++ b/fs/xfs/scrub/scrub.c
@@ -151,6 +151,8 @@ xchk_teardown(
 	struct xfs_inode	*ip_in,
 	int			error)
 {
+	int				err2;
+
 	xchk_ag_free(sc, &sc->sa);
 	if (sc->tp) {
 		if (error == 0 && (sc->sm->sm_flags & XFS_SCRUB_IFLAG_REPAIR))
@@ -167,6 +169,11 @@ xchk_teardown(
 			xfs_irele(sc->ip);
 		sc->ip = NULL;
 	}
+	if (sc->flags & XCHK_FS_FROZEN) {
+		err2 = xchk_fs_thaw(sc);
+		if (!error && err2)
+			error = err2;
+	}
 	if (sc->flags & XCHK_REAPING_DISABLED)
 		xchk_start_reaping(sc);
 	if (sc->flags & XCHK_HAS_QUOTAOFFLOCK) {
diff --git a/fs/xfs/scrub/scrub.h b/fs/xfs/scrub/scrub.h
index 99c4a3021284..f96fd11eceb1 100644
--- a/fs/xfs/scrub/scrub.h
+++ b/fs/xfs/scrub/scrub.h
@@ -89,6 +89,7 @@ struct xfs_scrub {
 #define XCHK_TRY_HARDER		(1 << 0)  /* can't get resources, try again */
 #define XCHK_HAS_QUOTAOFFLOCK	(1 << 1)  /* we hold the quotaoff lock */
 #define XCHK_REAPING_DISABLED	(1 << 2)  /* background block reaping paused */
+#define XCHK_FS_FROZEN		(1 << 3)  /* we froze the fs to do things */
 #define XREP_RESET_PERAG_RESV	(1 << 30) /* must reset AG space reservation */
 #define XREP_ALREADY_FIXED	(1 << 31) /* checking our repair work */
 
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 3464a13b44d0..35e83f3edeea 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -227,6 +227,13 @@ typedef struct xfs_mount {
 	 * inactivating all the inodes.
 	 */
 	struct wait_queue_head	m_inactive_wait;
+
+	/*
+	 * Only allow one thread to initiate a repair freeze at a time.  We
+	 * also use this to block userspace from changing the freeze state
+	 * while a repair freeze is in progress.
+	 */
+	struct mutex		m_scrub_freeze;
 } xfs_mount_t;
 
 #define M_IGEO(mp)		(&(mp)->m_ino_geo)
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index bba942a91f6c..7081bd2df75d 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1460,13 +1460,41 @@ xfs_fs_unfreeze(
 /*
  * Before we get to stage 1 of a freeze, force all the inactivation work so
  * that there's less work to do if we crash during the freeze.
+ *
+ * Don't let userspace freeze while scrub has the filesystem frozen.  Note
+ * that freeze_super can free the xfs_mount, so we must be careful to recheck
+ * XFS_M before trying to access anything in the xfs_mount afterwards.
  */
 STATIC int
 xfs_fs_freeze_super(
 	struct super_block	*sb)
 {
+	int			error;
+
 	xfs_inactive_force(XFS_M(sb));
-	return freeze_super(sb);
+	mutex_lock(&XFS_M(sb)->m_scrub_freeze);
+	error = freeze_super(sb);
+	if (XFS_M(sb))
+		mutex_unlock(&XFS_M(sb)->m_scrub_freeze);
+	return error;
+}
+
+/*
+ * Don't let userspace thaw while scrub has the filesystem frozen.  Note that
+ * thaw_super can free the xfs_mount, so we must be careful to recheck XFS_M
+ * before trying to access anything in the xfs_mount afterwards.
+ */
+STATIC int
+xfs_fs_thaw_super(
+	struct super_block	*sb)
+{
+	int			error;
+
+	mutex_lock(&XFS_M(sb)->m_scrub_freeze);
+	error = thaw_super(sb);
+	if (XFS_M(sb))
+		mutex_unlock(&XFS_M(sb)->m_scrub_freeze);
+	return error;
 }
 
 STATIC int
@@ -1637,6 +1665,7 @@ xfs_mount_alloc(
 	INIT_RADIX_TREE(&mp->m_perag_tree, GFP_ATOMIC);
 	spin_lock_init(&mp->m_perag_lock);
 	mutex_init(&mp->m_growlock);
+	mutex_init(&mp->m_scrub_freeze);
 	atomic_set(&mp->m_active_trans, 0);
 	INIT_WORK(&mp->m_flush_inodes_work, xfs_flush_inodes_worker);
 	INIT_DELAYED_WORK(&mp->m_reclaim_work, xfs_reclaim_worker);
@@ -1843,6 +1872,7 @@ xfs_fs_fill_super(
  out_free_fsname:
 	sb->s_fs_info = NULL;
 	xfs_free_fsname(mp);
+	mutex_destroy(&mp->m_scrub_freeze);
 	kfree(mp);
  out:
 	return error;
@@ -1875,6 +1905,19 @@ xfs_fs_put_super(
 
 	sb->s_fs_info = NULL;
 	xfs_free_fsname(mp);
+	/*
+	 * fs freeze takes an active reference to the filesystem and fs thaw
+	 * drops it.  If a filesystem on a frozen (dm) block device is
+	 * unmounted before the block device is thawed, we can end up tearing
+	 * down the super from within thaw_super when the device is thawed.
+	 * xfs_fs_thaw_super grabbed the scrub repair mutex before calling
+	 * thaw_super, so we must avoid freeing a locked mutex.  At this point
+	 * we know we're the only user of the filesystem, so we can safely
+	 * unlock the scrub/repair mutex if it's locked.
+	 */
+	if (mutex_is_locked(&mp->m_scrub_freeze))
+		mutex_unlock(&mp->m_scrub_freeze);
+	mutex_destroy(&mp->m_scrub_freeze);
 	kfree(mp);
 }
 
@@ -1922,6 +1965,7 @@ static const struct super_operations xfs_super_operations = {
 	.nr_cached_objects	= xfs_fs_nr_cached_objects,
 	.free_cached_objects	= xfs_fs_free_cached_objects,
 	.freeze_super		= xfs_fs_freeze_super,
+	.thaw_super		= xfs_fs_thaw_super,
 };
 
 static struct file_system_type xfs_fs_type = {
diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index d0d27c7972a7..ef4b071d2daf 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -323,9 +323,12 @@ xfs_trans_alloc(
 
 	/*
 	 * Zero-reservation ("empty") transactions can't modify anything, so
-	 * they're allowed to run while we're frozen.
+	 * they're allowed to run while we're frozen.  Scrub is allowed to
+	 * freeze the filesystem in order to obtain exclusive access to the
+	 * filesystem.
 	 */
 	WARN_ON(resp->tr_logres > 0 &&
+	        !mutex_is_locked(&mp->m_scrub_freeze) &&
 		mp->m_super->s_writers.frozen == SB_FREEZE_COMPLETE);
 	atomic_inc(&mp->m_active_trans);