Fixes for procfs files backed by linked lists

There are some issues with the way the seq_file interface is implemented
for kstats backed by linked lists (zfs_dbgmsgs and certain per-pool
debugging info):

* We don't account for the fact that seq_file sometimes visits a node
  multiple times, which results in missing messages when read through
  procfs.
* We don't keep separate state for each reader of a file, so concurrent
  readers will receive incorrect results.
* We don't account for the fact that entries may have been removed from
  the list between read syscalls, so reading from these files in procfs
  can cause the system to crash.

This change fixes these issues and adds procfs_list, a wrapper around a
linked list which abstracts away the details of implementing the
seq_file interface for a list and exposing the contents of the list
through procfs.

Reviewed by: Don Brady <[email protected]>
Reviewed-by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: John Gallagher <[email protected]>
External-issue: LX-1211
Closes #7819 

1 files changed, 215 insertions, 385 deletions

diff --git a/module/zfs/spa_stats.c b/module/zfs/spa_stats.c
index fa1cf9e98..c02ef86b5 100644
--- a/module/zfs/spa_stats.c
+++ b/module/zfs/spa_stats.c
@@ -55,7 +55,6 @@ int zfs_multihost_history = 0;
  * Read statistics - Information exported regarding each arc_read call
  */
 typedef struct spa_read_history {
-	uint64_t	uid;		/* unique identifier */
 	hrtime_t	start;		/* time read completed */
 	uint64_t	objset;		/* read from this objset */
 	uint64_t	object;		/* read of this object number */
@@ -65,13 +64,13 @@ typedef struct spa_read_history {
 	uint32_t	aflags;		/* ARC flags (cached, prefetch, etc.) */
 	pid_t		pid;		/* PID of task doing read */
 	char		comm[16];	/* process name of task doing read */
-	list_node_t	srh_link;
+	procfs_list_node_t	srh_node;
 } spa_read_history_t;
 
 static int
-spa_read_history_headers(char *buf, size_t size)
+spa_read_history_show_header(struct seq_file *f)
 {
-	(void) snprintf(buf, size, "%-8s %-16s %-8s %-8s %-8s %-8s %-8s "
+	seq_printf(f, "%-8s %-16s %-8s %-8s %-8s %-8s %-8s "
 	    "%-24s %-8s %-16s\n", "UID", "start", "objset", "object",
 	    "level", "blkid", "aflags", "origin", "pid", "process");
 
@@ -79,13 +78,13 @@ spa_read_history_headers(char *buf, size_t size)
 }
 
 static int
-spa_read_history_data(char *buf, size_t size, void *data)
+spa_read_history_show(struct seq_file *f, void *data)
 {
 	spa_read_history_t *srh = (spa_read_history_t *)data;
 
-	(void) snprintf(buf, size, "%-8llu %-16llu 0x%-6llx "
+	seq_printf(f, "%-8llu %-16llu 0x%-6llx "
 	    "%-8lli %-8lli %-8lli 0x%-6x %-24s %-8i %-16s\n",
-	    (u_longlong_t)srh->uid, srh->start,
+	    (u_longlong_t)srh->srh_node.pln_id, srh->start,
 	    (longlong_t)srh->objset, (longlong_t)srh->object,
 	    (longlong_t)srh->level, (longlong_t)srh->blkid,
 	    srh->aflags, srh->origin, srh->pid, srh->comm);
@@ -93,120 +92,73 @@ spa_read_history_data(char *buf, size_t size, void *data)
 	return (0);
 }
 
-/*
- * Calculate the address for the next spa_stats_history_t entry.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
-static void *
-spa_read_history_addr(kstat_t *ksp, loff_t n)
+/* Remove oldest elements from list until there are no more than 'size' left */
+static void
+spa_read_history_truncate(spa_history_list_t *shl, unsigned int size)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.read_history;
-
-	ASSERT(MUTEX_HELD(&ssh->lock));
-
-	if (n == 0)
-		ssh->private = list_tail(&ssh->list);
-	else if (ssh->private)
-		ssh->private = list_prev(&ssh->list, ssh->private);
+	spa_read_history_t *srh;
+	while (shl->size > size) {
+		srh = list_remove_head(&shl->procfs_list.pl_list);
+		ASSERT3P(srh, !=, NULL);
+		kmem_free(srh, sizeof (spa_read_history_t));
+		shl->size--;
+	}
 
-	return (ssh->private);
+	if (size == 0)
+		ASSERT(list_is_empty(&shl->procfs_list.pl_list));
 }
 
-/*
- * When the kstat is written discard all spa_read_history_t entries.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
 static int
-spa_read_history_update(kstat_t *ksp, int rw)
+spa_read_history_clear(procfs_list_t *procfs_list)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.read_history;
-
-	if (rw == KSTAT_WRITE) {
-		spa_read_history_t *srh;
-
-		while ((srh = list_remove_head(&ssh->list))) {
-			ssh->size--;
-			kmem_free(srh, sizeof (spa_read_history_t));
-		}
-
-		ASSERT3U(ssh->size, ==, 0);
-	}
-
-	ksp->ks_ndata = ssh->size;
-	ksp->ks_data_size = ssh->size * sizeof (spa_read_history_t);
-
+	spa_history_list_t *shl = procfs_list->pl_private;
+	mutex_enter(&procfs_list->pl_lock);
+	spa_read_history_truncate(shl, 0);
+	mutex_exit(&procfs_list->pl_lock);
 	return (0);
 }
 
 static void
 spa_read_history_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.read_history;
-	char *name;
-	kstat_t *ksp;
+	spa_history_list_t *shl = &spa->spa_stats.read_history;
+	char *module;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
-	list_create(&ssh->list, sizeof (spa_read_history_t),
-	    offsetof(spa_read_history_t, srh_link));
+	shl->size = 0;
 
-	ssh->count = 0;
-	ssh->size = 0;
-	ssh->private = NULL;
+	module = kmem_asprintf("zfs/%s", spa_name(spa));
 
-	name = kmem_asprintf("zfs/%s", spa_name(spa));
+	shl->procfs_list.pl_private = shl;
+	procfs_list_install(module,
+	    "reads",
+	    &shl->procfs_list,
+	    spa_read_history_show,
+	    spa_read_history_show_header,
+	    spa_read_history_clear,
+	    offsetof(spa_read_history_t, srh_node));
 
-	ksp = kstat_create(name, 0, "reads", "misc",
-	    KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
-	ssh->kstat = ksp;
-
-	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
-		ksp->ks_data = NULL;
-		ksp->ks_private = spa;
-		ksp->ks_update = spa_read_history_update;
-		kstat_set_raw_ops(ksp, spa_read_history_headers,
-		    spa_read_history_data, spa_read_history_addr);
-		kstat_install(ksp);
-	}
-	strfree(name);
+	strfree(module);
 }
 
 static void
 spa_read_history_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.read_history;
-	spa_read_history_t *srh;
-	kstat_t *ksp;
-
-	ksp = ssh->kstat;
-	if (ksp)
-		kstat_delete(ksp);
-
-	mutex_enter(&ssh->lock);
-	while ((srh = list_remove_head(&ssh->list))) {
-		ssh->size--;
-		kmem_free(srh, sizeof (spa_read_history_t));
-	}
-
-	ASSERT3U(ssh->size, ==, 0);
-	list_destroy(&ssh->list);
-	mutex_exit(&ssh->lock);
-
-	mutex_destroy(&ssh->lock);
+	spa_history_list_t *shl = &spa->spa_stats.read_history;
+	procfs_list_uninstall(&shl->procfs_list);
+	spa_read_history_truncate(shl, 0);
+	procfs_list_destroy(&shl->procfs_list);
 }
 
 void
 spa_read_history_add(spa_t *spa, const zbookmark_phys_t *zb, uint32_t aflags)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.read_history;
-	spa_read_history_t *srh, *rm;
+	spa_history_list_t *shl = &spa->spa_stats.read_history;
+	spa_read_history_t *srh;
 
 	ASSERT3P(spa, !=, NULL);
 	ASSERT3P(zb,  !=, NULL);
 
-	if (zfs_read_history == 0 && ssh->size == 0)
+	if (zfs_read_history == 0 && shl->size == 0)
 		return;
 
 	if (zfs_read_history_hits == 0 && (aflags & ARC_FLAG_CACHED))
@@ -222,19 +174,14 @@ spa_read_history_add(spa_t *spa, const zbookmark_phys_t *zb, uint32_t aflags)
 	srh->aflags = aflags;
 	srh->pid    = getpid();
 
-	mutex_enter(&ssh->lock);
+	mutex_enter(&shl->procfs_list.pl_lock);
 
-	srh->uid = ssh->count++;
-	list_insert_head(&ssh->list, srh);
-	ssh->size++;
+	procfs_list_add(&shl->procfs_list, srh);
+	shl->size++;
 
-	while (ssh->size > zfs_read_history) {
-		ssh->size--;
-		rm = list_remove_tail(&ssh->list);
-		kmem_free(rm, sizeof (spa_read_history_t));
-	}
+	spa_read_history_truncate(shl, zfs_read_history);
 
-	mutex_exit(&ssh->lock);
+	mutex_exit(&shl->procfs_list.pl_lock);
 }
 
 /*
@@ -256,22 +203,21 @@ typedef struct spa_txg_history {
 	uint64_t	writes;		/* number of write operations */
 	uint64_t	ndirty;		/* number of dirty bytes */
 	hrtime_t	times[TXG_STATE_COMMITTED]; /* completion times */
-	list_node_t	sth_link;
+	procfs_list_node_t	sth_node;
 } spa_txg_history_t;
 
 static int
-spa_txg_history_headers(char *buf, size_t size)
+spa_txg_history_show_header(struct seq_file *f)
 {
-	(void) snprintf(buf, size, "%-8s %-16s %-5s %-12s %-12s %-12s "
+	seq_printf(f, "%-8s %-16s %-5s %-12s %-12s %-12s "
 	    "%-8s %-8s %-12s %-12s %-12s %-12s\n", "txg", "birth", "state",
 	    "ndirty", "nread", "nwritten", "reads", "writes",
 	    "otime", "qtime", "wtime", "stime");
-
 	return (0);
 }
 
 static int
-spa_txg_history_data(char *buf, size_t size, void *data)
+spa_txg_history_show(struct seq_file *f, void *data)
 {
 	spa_txg_history_t *sth = (spa_txg_history_t *)data;
 	uint64_t open = 0, quiesce = 0, wait = 0, sync = 0;
@@ -303,7 +249,7 @@ spa_txg_history_data(char *buf, size_t size, void *data)
 		sync = sth->times[TXG_STATE_SYNCED] -
 		    sth->times[TXG_STATE_WAIT_FOR_SYNC];
 
-	(void) snprintf(buf, size, "%-8llu %-16llu %-5c %-12llu "
+	seq_printf(f, "%-8llu %-16llu %-5c %-12llu "
 	    "%-12llu %-12llu %-8llu %-8llu %-12llu %-12llu %-12llu %-12llu\n",
 	    (longlong_t)sth->txg, sth->times[TXG_STATE_BIRTH], state,
 	    (u_longlong_t)sth->ndirty,
@@ -315,110 +261,62 @@ spa_txg_history_data(char *buf, size_t size, void *data)
 	return (0);
 }
 
-/*
- * Calculate the address for the next spa_stats_history_t entry.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
-static void *
-spa_txg_history_addr(kstat_t *ksp, loff_t n)
+/* Remove oldest elements from list until there are no more than 'size' left */
+static void
+spa_txg_history_truncate(spa_history_list_t *shl, unsigned int size)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
-
-	ASSERT(MUTEX_HELD(&ssh->lock));
+	spa_txg_history_t *sth;
+	while (shl->size > size) {
+		sth = list_remove_head(&shl->procfs_list.pl_list);
+		ASSERT3P(sth, !=, NULL);
+		kmem_free(sth, sizeof (spa_txg_history_t));
+		shl->size--;
+	}
 
-	if (n == 0)
-		ssh->private = list_tail(&ssh->list);
-	else if (ssh->private)
-		ssh->private = list_prev(&ssh->list, ssh->private);
+	if (size == 0)
+		ASSERT(list_is_empty(&shl->procfs_list.pl_list));
 
-	return (ssh->private);
 }
 
-/*
- * When the kstat is written discard all spa_txg_history_t entries.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
 static int
-spa_txg_history_update(kstat_t *ksp, int rw)
+spa_txg_history_clear(procfs_list_t *procfs_list)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
-
-	ASSERT(MUTEX_HELD(&ssh->lock));
-
-	if (rw == KSTAT_WRITE) {
-		spa_txg_history_t *sth;
-
-		while ((sth = list_remove_head(&ssh->list))) {
-			ssh->size--;
-			kmem_free(sth, sizeof (spa_txg_history_t));
-		}
-
-		ASSERT3U(ssh->size, ==, 0);
-	}
-
-	ksp->ks_ndata = ssh->size;
-	ksp->ks_data_size = ssh->size * sizeof (spa_txg_history_t);
-
+	spa_history_list_t *shl = procfs_list->pl_private;
+	mutex_enter(&procfs_list->pl_lock);
+	spa_txg_history_truncate(shl, 0);
+	mutex_exit(&procfs_list->pl_lock);
 	return (0);
 }
 
 static void
 spa_txg_history_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
-	char *name;
-	kstat_t *ksp;
+	spa_history_list_t *shl = &spa->spa_stats.txg_history;
+	char *module;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
-	list_create(&ssh->list, sizeof (spa_txg_history_t),
-	    offsetof(spa_txg_history_t, sth_link));
+	shl->size = 0;
 
-	ssh->count = 0;
-	ssh->size = 0;
-	ssh->private = NULL;
+	module = kmem_asprintf("zfs/%s", spa_name(spa));
 
-	name = kmem_asprintf("zfs/%s", spa_name(spa));
+	shl->procfs_list.pl_private = shl;
+	procfs_list_install(module,
+	    "txgs",
+	    &shl->procfs_list,
+	    spa_txg_history_show,
+	    spa_txg_history_show_header,
+	    spa_txg_history_clear,
+	    offsetof(spa_txg_history_t, sth_node));
 
-	ksp = kstat_create(name, 0, "txgs", "misc",
-	    KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
-	ssh->kstat = ksp;
-
-	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
-		ksp->ks_data = NULL;
-		ksp->ks_private = spa;
-		ksp->ks_update = spa_txg_history_update;
-		kstat_set_raw_ops(ksp, spa_txg_history_headers,
-		    spa_txg_history_data, spa_txg_history_addr);
-		kstat_install(ksp);
-	}
-	strfree(name);
+	strfree(module);
 }
 
 static void
 spa_txg_history_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
-	spa_txg_history_t *sth;
-	kstat_t *ksp;
-
-	ksp = ssh->kstat;
-	if (ksp)
-		kstat_delete(ksp);
-
-	mutex_enter(&ssh->lock);
-	while ((sth = list_remove_head(&ssh->list))) {
-		ssh->size--;
-		kmem_free(sth, sizeof (spa_txg_history_t));
-	}
-
-	ASSERT3U(ssh->size, ==, 0);
-	list_destroy(&ssh->list);
-	mutex_exit(&ssh->lock);
-
-	mutex_destroy(&ssh->lock);
+	spa_history_list_t *shl = &spa->spa_stats.txg_history;
+	procfs_list_uninstall(&shl->procfs_list);
+	spa_txg_history_truncate(shl, 0);
+	procfs_list_destroy(&shl->procfs_list);
 }
 
 /*
@@ -427,10 +325,10 @@ spa_txg_history_destroy(spa_t *spa)
 void
 spa_txg_history_add(spa_t *spa, uint64_t txg, hrtime_t birth_time)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
-	spa_txg_history_t *sth, *rm;
+	spa_history_list_t *shl = &spa->spa_stats.txg_history;
+	spa_txg_history_t *sth;
 
-	if (zfs_txg_history == 0 && ssh->size == 0)
+	if (zfs_txg_history == 0 && shl->size == 0)
 		return;
 
 	sth = kmem_zalloc(sizeof (spa_txg_history_t), KM_SLEEP);
@@ -438,18 +336,11 @@ spa_txg_history_add(spa_t *spa, uint64_t txg, hrtime_t birth_time)
 	sth->state = TXG_STATE_OPEN;
 	sth->times[TXG_STATE_BIRTH] = birth_time;
 
-	mutex_enter(&ssh->lock);
-
-	list_insert_head(&ssh->list, sth);
-	ssh->size++;
-
-	while (ssh->size > zfs_txg_history) {
-		ssh->size--;
-		rm = list_remove_tail(&ssh->list);
-		kmem_free(rm, sizeof (spa_txg_history_t));
-	}
-
-	mutex_exit(&ssh->lock);
+	mutex_enter(&shl->procfs_list.pl_lock);
+	procfs_list_add(&shl->procfs_list, sth);
+	shl->size++;
+	spa_txg_history_truncate(shl, zfs_txg_history);
+	mutex_exit(&shl->procfs_list.pl_lock);
 }
 
 /*
@@ -459,16 +350,16 @@ int
 spa_txg_history_set(spa_t *spa, uint64_t txg, txg_state_t completed_state,
     hrtime_t completed_time)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
+	spa_history_list_t *shl = &spa->spa_stats.txg_history;
 	spa_txg_history_t *sth;
 	int error = ENOENT;
 
 	if (zfs_txg_history == 0)
 		return (0);
 
-	mutex_enter(&ssh->lock);
-	for (sth = list_head(&ssh->list); sth != NULL;
-	    sth = list_next(&ssh->list, sth)) {
+	mutex_enter(&shl->procfs_list.pl_lock);
+	for (sth = list_tail(&shl->procfs_list.pl_list); sth != NULL;
+	    sth = list_prev(&shl->procfs_list.pl_list, sth)) {
 		if (sth->txg == txg) {
 			sth->times[completed_state] = completed_time;
 			sth->state++;
@@ -476,7 +367,7 @@ spa_txg_history_set(spa_t *spa, uint64_t txg, txg_state_t completed_state,
 			break;
 		}
 	}
-	mutex_exit(&ssh->lock);
+	mutex_exit(&shl->procfs_list.pl_lock);
 
 	return (error);
 }
@@ -488,16 +379,16 @@ static int
 spa_txg_history_set_io(spa_t *spa, uint64_t txg, uint64_t nread,
     uint64_t nwritten, uint64_t reads, uint64_t writes, uint64_t ndirty)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
+	spa_history_list_t *shl = &spa->spa_stats.txg_history;
 	spa_txg_history_t *sth;
 	int error = ENOENT;
 
 	if (zfs_txg_history == 0)
 		return (0);
 
-	mutex_enter(&ssh->lock);
-	for (sth = list_head(&ssh->list); sth != NULL;
-	    sth = list_next(&ssh->list, sth)) {
+	mutex_enter(&shl->procfs_list.pl_lock);
+	for (sth = list_tail(&shl->procfs_list.pl_list); sth != NULL;
+	    sth = list_prev(&shl->procfs_list.pl_list, sth)) {
 		if (sth->txg == txg) {
 			sth->nread = nread;
 			sth->nwritten = nwritten;
@@ -508,7 +399,7 @@ spa_txg_history_set_io(spa_t *spa, uint64_t txg, uint64_t nread,
 			break;
 		}
 	}
-	mutex_exit(&ssh->lock);
+	mutex_exit(&shl->procfs_list.pl_lock);
 
 	return (error);
 }
@@ -580,16 +471,16 @@ static int
 spa_tx_assign_update(kstat_t *ksp, int rw)
 {
 	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
+	spa_history_kstat_t *shk = &spa->spa_stats.tx_assign_histogram;
 	int i;
 
 	if (rw == KSTAT_WRITE) {
-		for (i = 0; i < ssh->count; i++)
-			((kstat_named_t *)ssh->private)[i].value.ui64 = 0;
+		for (i = 0; i < shk->count; i++)
+			((kstat_named_t *)shk->private)[i].value.ui64 = 0;
 	}
 
-	for (i = ssh->count; i > 0; i--)
-		if (((kstat_named_t *)ssh->private)[i-1].value.ui64 != 0)
+	for (i = shk->count; i > 0; i--)
+		if (((kstat_named_t *)shk->private)[i-1].value.ui64 != 0)
 			break;
 
 	ksp->ks_ndata = i;
@@ -601,22 +492,22 @@ spa_tx_assign_update(kstat_t *ksp, int rw)
 static void
 spa_tx_assign_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
+	spa_history_kstat_t *shk = &spa->spa_stats.tx_assign_histogram;
 	char *name;
 	kstat_named_t *ks;
 	kstat_t *ksp;
 	int i;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&shk->lock, NULL, MUTEX_DEFAULT, NULL);
 
-	ssh->count = 42; /* power of two buckets for 1ns to 2,199s */
-	ssh->size = ssh->count * sizeof (kstat_named_t);
-	ssh->private = kmem_alloc(ssh->size, KM_SLEEP);
+	shk->count = 42; /* power of two buckets for 1ns to 2,199s */
+	shk->size = shk->count * sizeof (kstat_named_t);
+	shk->private = kmem_alloc(shk->size, KM_SLEEP);
 
 	name = kmem_asprintf("zfs/%s", spa_name(spa));
 
-	for (i = 0; i < ssh->count; i++) {
-		ks = &((kstat_named_t *)ssh->private)[i];
+	for (i = 0; i < shk->count; i++) {
+		ks = &((kstat_named_t *)shk->private)[i];
 		ks->data_type = KSTAT_DATA_UINT64;
 		ks->value.ui64 = 0;
 		(void) snprintf(ks->name, KSTAT_STRLEN, "%llu ns",
@@ -625,13 +516,13 @@ spa_tx_assign_init(spa_t *spa)
 
 	ksp = kstat_create(name, 0, "dmu_tx_assign", "misc",
 	    KSTAT_TYPE_NAMED, 0, KSTAT_FLAG_VIRTUAL);
-	ssh->kstat = ksp;
+	shk->kstat = ksp;
 
 	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
-		ksp->ks_data = ssh->private;
-		ksp->ks_ndata = ssh->count;
-		ksp->ks_data_size = ssh->size;
+		ksp->ks_lock = &shk->lock;
+		ksp->ks_data = shk->private;
+		ksp->ks_ndata = shk->count;
+		ksp->ks_data_size = shk->size;
 		ksp->ks_private = spa;
 		ksp->ks_update = spa_tx_assign_update;
 		kstat_install(ksp);
@@ -642,27 +533,27 @@ spa_tx_assign_init(spa_t *spa)
 static void
 spa_tx_assign_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
+	spa_history_kstat_t *shk = &spa->spa_stats.tx_assign_histogram;
 	kstat_t *ksp;
 
-	ksp = ssh->kstat;
+	ksp = shk->kstat;
 	if (ksp)
 		kstat_delete(ksp);
 
-	kmem_free(ssh->private, ssh->size);
-	mutex_destroy(&ssh->lock);
+	kmem_free(shk->private, shk->size);
+	mutex_destroy(&shk->lock);
 }
 
 void
 spa_tx_assign_add_nsecs(spa_t *spa, uint64_t nsecs)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.tx_assign_histogram;
+	spa_history_kstat_t *shk = &spa->spa_stats.tx_assign_histogram;
 	uint64_t idx = 0;
 
-	while (((1ULL << idx) < nsecs) && (idx < ssh->size - 1))
+	while (((1ULL << idx) < nsecs) && (idx < shk->size - 1))
 		idx++;
 
-	atomic_inc_64(&((kstat_named_t *)ssh->private)[idx].value.ui64);
+	atomic_inc_64(&((kstat_named_t *)shk->private)[idx].value.ui64);
 }
 
 /*
@@ -682,19 +573,19 @@ spa_io_history_update(kstat_t *ksp, int rw)
 static void
 spa_io_history_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.io_history;
+	spa_history_kstat_t *shk = &spa->spa_stats.io_history;
 	char *name;
 	kstat_t *ksp;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&shk->lock, NULL, MUTEX_DEFAULT, NULL);
 
 	name = kmem_asprintf("zfs/%s", spa_name(spa));
 
 	ksp = kstat_create(name, 0, "io", "disk", KSTAT_TYPE_IO, 1, 0);
-	ssh->kstat = ksp;
+	shk->kstat = ksp;
 
 	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
+		ksp->ks_lock = &shk->lock;
 		ksp->ks_private = spa;
 		ksp->ks_update = spa_io_history_update;
 		kstat_install(ksp);
@@ -705,12 +596,12 @@ spa_io_history_init(spa_t *spa)
 static void
 spa_io_history_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.io_history;
+	spa_history_kstat_t *shk = &spa->spa_stats.io_history;
 
-	if (ssh->kstat)
-		kstat_delete(ssh->kstat);
+	if (shk->kstat)
+		kstat_delete(shk->kstat);
 
-	mutex_destroy(&ssh->lock);
+	mutex_destroy(&shk->lock);
 }
 
 /*
@@ -733,7 +624,7 @@ spa_io_history_destroy(spa_t *spa)
  */
 
 typedef struct spa_mmp_history {
-	uint64_t	mmp_kstat_id;	/* unique # for updates */
+	uint64_t	mmp_node_id;	/* unique # for updates */
 	uint64_t	txg;		/* txg of last sync */
 	uint64_t	timestamp;	/* UTC time MMP write issued */
 	uint64_t	mmp_delay;	/* mmp_thread.mmp_delay at timestamp */
@@ -743,20 +634,20 @@ typedef struct spa_mmp_history {
 	int		io_error;	/* error status of MMP write */
 	hrtime_t	error_start;	/* hrtime of start of error period */
 	hrtime_t	duration;	/* time from submission to completion */
-	list_node_t	smh_link;
+	procfs_list_node_t	smh_node;
 } spa_mmp_history_t;
 
 static int
-spa_mmp_history_headers(char *buf, size_t size)
+spa_mmp_history_show_header(struct seq_file *f)
 {
-	(void) snprintf(buf, size, "%-10s %-10s %-10s %-6s %-10s %-12s %-24s "
+	seq_printf(f, "%-10s %-10s %-10s %-6s %-10s %-12s %-24s "
 	    "%-10s %s\n", "id", "txg", "timestamp", "error", "duration",
 	    "mmp_delay", "vdev_guid", "vdev_label", "vdev_path");
 	return (0);
 }
 
 static int
-spa_mmp_history_data(char *buf, size_t size, void *data)
+spa_mmp_history_show(struct seq_file *f, void *data)
 {
 	spa_mmp_history_t *smh = (spa_mmp_history_t *)data;
 	char skip_fmt[] = "%-10llu %-10llu %10llu %#6llx %10lld %12llu %-24llu "
@@ -764,8 +655,8 @@ spa_mmp_history_data(char *buf, size_t size, void *data)
 	char write_fmt[] = "%-10llu %-10llu %10llu %6lld %10lld %12llu %-24llu "
 	    "%-10lld %s\n";
 
-	(void) snprintf(buf, size, (smh->error_start ? skip_fmt : write_fmt),
-	    (u_longlong_t)smh->mmp_kstat_id, (u_longlong_t)smh->txg,
+	seq_printf(f, (smh->error_start ? skip_fmt : write_fmt),
+	    (u_longlong_t)smh->mmp_node_id, (u_longlong_t)smh->txg,
 	    (u_longlong_t)smh->timestamp, (longlong_t)smh->io_error,
 	    (longlong_t)smh->duration, (u_longlong_t)smh->mmp_delay,
 	    (u_longlong_t)smh->vdev_guid, (u_longlong_t)smh->vdev_label,
@@ -774,137 +665,86 @@ spa_mmp_history_data(char *buf, size_t size, void *data)
 	return (0);
 }
 
-/*
- * Calculate the address for the next spa_stats_history_t entry.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
-static void *
-spa_mmp_history_addr(kstat_t *ksp, loff_t n)
+/* Remove oldest elements from list until there are no more than 'size' left */
+static void
+spa_mmp_history_truncate(spa_history_list_t *shl, unsigned int size)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
-
-	ASSERT(MUTEX_HELD(&ssh->lock));
+	spa_mmp_history_t *smh;
+	while (shl->size > size) {
+		smh = list_remove_head(&shl->procfs_list.pl_list);
+		if (smh->vdev_path)
+			strfree(smh->vdev_path);
+		kmem_free(smh, sizeof (spa_mmp_history_t));
+		shl->size--;
+	}
 
-	if (n == 0)
-		ssh->private = list_tail(&ssh->list);
-	else if (ssh->private)
-		ssh->private = list_prev(&ssh->list, ssh->private);
+	if (size == 0)
+		ASSERT(list_is_empty(&shl->procfs_list.pl_list));
 
-	return (ssh->private);
 }
 
-/*
- * When the kstat is written discard all spa_mmp_history_t entries.  The
- * ssh->lock will be held until ksp->ks_ndata entries are processed.
- */
 static int
-spa_mmp_history_update(kstat_t *ksp, int rw)
+spa_mmp_history_clear(procfs_list_t *procfs_list)
 {
-	spa_t *spa = ksp->ks_private;
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
-
-	ASSERT(MUTEX_HELD(&ssh->lock));
-
-	if (rw == KSTAT_WRITE) {
-		spa_mmp_history_t *smh;
-
-		while ((smh = list_remove_head(&ssh->list))) {
-			ssh->size--;
-			if (smh->vdev_path)
-				strfree(smh->vdev_path);
-			kmem_free(smh, sizeof (spa_mmp_history_t));
-		}
-
-		ASSERT3U(ssh->size, ==, 0);
-	}
-
-	ksp->ks_ndata = ssh->size;
-	ksp->ks_data_size = ssh->size * sizeof (spa_mmp_history_t);
-
+	spa_history_list_t *shl = procfs_list->pl_private;
+	mutex_enter(&procfs_list->pl_lock);
+	spa_mmp_history_truncate(shl, 0);
+	mutex_exit(&procfs_list->pl_lock);
 	return (0);
 }
 
 static void
 spa_mmp_history_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
-	char *name;
-	kstat_t *ksp;
+	spa_history_list_t *shl = &spa->spa_stats.mmp_history;
+	char *module;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
-	list_create(&ssh->list, sizeof (spa_mmp_history_t),
-	    offsetof(spa_mmp_history_t, smh_link));
+	shl->size = 0;
 
-	ssh->count = 0;
-	ssh->size = 0;
-	ssh->private = NULL;
-
-	name = kmem_asprintf("zfs/%s", spa_name(spa));
+	module = kmem_asprintf("zfs/%s", spa_name(spa));
 
-	ksp = kstat_create(name, 0, "multihost", "misc",
-	    KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
-	ssh->kstat = ksp;
+	shl->procfs_list.pl_private = shl;
+	procfs_list_install(module,
+	    "multihost",
+	    &shl->procfs_list,
+	    spa_mmp_history_show,
+	    spa_mmp_history_show_header,
+	    spa_mmp_history_clear,
+	    offsetof(spa_mmp_history_t, smh_node));
 
-	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
-		ksp->ks_data = NULL;
-		ksp->ks_private = spa;
-		ksp->ks_update = spa_mmp_history_update;
-		kstat_set_raw_ops(ksp, spa_mmp_history_headers,
-		    spa_mmp_history_data, spa_mmp_history_addr);
-		kstat_install(ksp);
-	}
-	strfree(name);
+	strfree(module);
 }
 
 static void
 spa_mmp_history_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
-	spa_mmp_history_t *smh;
-	kstat_t *ksp;
-
-	ksp = ssh->kstat;
-	if (ksp)
-		kstat_delete(ksp);
-
-	mutex_enter(&ssh->lock);
-	while ((smh = list_remove_head(&ssh->list))) {
-		ssh->size--;
-		if (smh->vdev_path)
-			strfree(smh->vdev_path);
-		kmem_free(smh, sizeof (spa_mmp_history_t));
-	}
-
-	ASSERT3U(ssh->size, ==, 0);
-	list_destroy(&ssh->list);
-	mutex_exit(&ssh->lock);
-
-	mutex_destroy(&ssh->lock);
+	spa_history_list_t *shl = &spa->spa_stats.mmp_history;
+	procfs_list_uninstall(&shl->procfs_list);
+	spa_mmp_history_truncate(shl, 0);
+	procfs_list_destroy(&shl->procfs_list);
 }
 
 /*
  * Set duration in existing "skip" record to how long we have waited for a leaf
  * vdev to become available.
  *
- * Important that we start search at the head of the list where new
+ * Important that we start search at the tail of the list where new
  * records are inserted, so this is normally an O(1) operation.
  */
 int
-spa_mmp_history_set_skip(spa_t *spa, uint64_t mmp_kstat_id)
+spa_mmp_history_set_skip(spa_t *spa, uint64_t mmp_node_id)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+	spa_history_list_t *shl = &spa->spa_stats.mmp_history;
 	spa_mmp_history_t *smh;
 	int error = ENOENT;
 
-	if (zfs_multihost_history == 0 && ssh->size == 0)
+	if (zfs_multihost_history == 0 && shl->size == 0)
 		return (0);
 
-	mutex_enter(&ssh->lock);
-	for (smh = list_head(&ssh->list); smh != NULL;
-	    smh = list_next(&ssh->list, smh)) {
-		if (smh->mmp_kstat_id == mmp_kstat_id) {
+	mutex_enter(&shl->procfs_list.pl_lock);
+	for (smh = list_tail(&shl->procfs_list.pl_list); smh != NULL;
+	    smh = list_prev(&shl->procfs_list.pl_list, smh)) {
+		if (smh->mmp_node_id == mmp_node_id) {
 			ASSERT3U(smh->io_error, !=, 0);
 			smh->duration = gethrtime() - smh->error_start;
 			smh->vdev_guid++;
@@ -912,7 +752,7 @@ spa_mmp_history_set_skip(spa_t *spa, uint64_t mmp_kstat_id)
 			break;
 		}
 	}
-	mutex_exit(&ssh->lock);
+	mutex_exit(&shl->procfs_list.pl_lock);
 
 	return (error);
 }
@@ -922,20 +762,20 @@ spa_mmp_history_set_skip(spa_t *spa, uint64_t mmp_kstat_id)
  * See comment re: search order above spa_mmp_history_set_skip().
  */
 int
-spa_mmp_history_set(spa_t *spa, uint64_t mmp_kstat_id, int io_error,
+spa_mmp_history_set(spa_t *spa, uint64_t mmp_node_id, int io_error,
     hrtime_t duration)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
+	spa_history_list_t *shl = &spa->spa_stats.mmp_history;
 	spa_mmp_history_t *smh;
 	int error = ENOENT;
 
-	if (zfs_multihost_history == 0 && ssh->size == 0)
+	if (zfs_multihost_history == 0 && shl->size == 0)
 		return (0);
 
-	mutex_enter(&ssh->lock);
-	for (smh = list_head(&ssh->list); smh != NULL;
-	    smh = list_next(&ssh->list, smh)) {
-		if (smh->mmp_kstat_id == mmp_kstat_id) {
+	mutex_enter(&shl->procfs_list.pl_lock);
+	for (smh = list_tail(&shl->procfs_list.pl_list); smh != NULL;
+	    smh = list_prev(&shl->procfs_list.pl_list, smh)) {
+		if (smh->mmp_node_id == mmp_node_id) {
 			ASSERT(smh->io_error == 0);
 			smh->io_error = io_error;
 			smh->duration = duration;
@@ -943,7 +783,7 @@ spa_mmp_history_set(spa_t *spa, uint64_t mmp_kstat_id, int io_error,
 			break;
 		}
 	}
-	mutex_exit(&ssh->lock);
+	mutex_exit(&shl->procfs_list.pl_lock);
 
 	return (error);
 }
@@ -953,16 +793,16 @@ spa_mmp_history_set(spa_t *spa, uint64_t mmp_kstat_id, int io_error,
  * error == 0 : a write was issued.
  * error != 0 : a write was not issued because no leaves were found.
  */
-void *
+void
 spa_mmp_history_add(spa_t *spa, uint64_t txg, uint64_t timestamp,
-    uint64_t mmp_delay, vdev_t *vd, int label, uint64_t mmp_kstat_id,
+    uint64_t mmp_delay, vdev_t *vd, int label, uint64_t mmp_node_id,
     int error)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.mmp_history;
-	spa_mmp_history_t *smh, *rm;
+	spa_history_list_t *shl = &spa->spa_stats.mmp_history;
+	spa_mmp_history_t *smh;
 
-	if (zfs_multihost_history == 0 && ssh->size == 0)
-		return (NULL);
+	if (zfs_multihost_history == 0 && shl->size == 0)
+		return;
 
 	smh = kmem_zalloc(sizeof (spa_mmp_history_t), KM_SLEEP);
 	smh->txg = txg;
@@ -974,7 +814,7 @@ spa_mmp_history_add(spa_t *spa, uint64_t txg, uint64_t timestamp,
 			smh->vdev_path = strdup(vd->vdev_path);
 	}
 	smh->vdev_label = label;
-	smh->mmp_kstat_id = mmp_kstat_id;
+	smh->mmp_node_id = mmp_node_id;
 
 	if (error) {
 		smh->io_error = error;
@@ -982,21 +822,11 @@ spa_mmp_history_add(spa_t *spa, uint64_t txg, uint64_t timestamp,
 		smh->vdev_guid = 1;
 	}
 
-	mutex_enter(&ssh->lock);
-
-	list_insert_head(&ssh->list, smh);
-	ssh->size++;
-
-	while (ssh->size > zfs_multihost_history) {
-		ssh->size--;
-		rm = list_remove_tail(&ssh->list);
-		if (rm->vdev_path)
-			strfree(rm->vdev_path);
-		kmem_free(rm, sizeof (spa_mmp_history_t));
-	}
-
-	mutex_exit(&ssh->lock);
-	return ((void *)smh);
+	mutex_enter(&shl->procfs_list.pl_lock);
+	procfs_list_add(&shl->procfs_list, smh);
+	shl->size++;
+	spa_mmp_history_truncate(shl, zfs_multihost_history);
+	mutex_exit(&shl->procfs_list.pl_lock);
 }
 
 static void *
@@ -1023,19 +853,19 @@ spa_state_data(char *buf, size_t size, void *data)
 static void
 spa_state_init(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.state;
+	spa_history_kstat_t *shk = &spa->spa_stats.state;
 	char *name;
 	kstat_t *ksp;
 
-	mutex_init(&ssh->lock, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&shk->lock, NULL, MUTEX_DEFAULT, NULL);
 
 	name = kmem_asprintf("zfs/%s", spa_name(spa));
 	ksp = kstat_create(name, 0, "state", "misc",
 	    KSTAT_TYPE_RAW, 0, KSTAT_FLAG_VIRTUAL);
 
-	ssh->kstat = ksp;
+	shk->kstat = ksp;
 	if (ksp) {
-		ksp->ks_lock = &ssh->lock;
+		ksp->ks_lock = &shk->lock;
 		ksp->ks_data = NULL;
 		ksp->ks_private = spa;
 		ksp->ks_flags |= KSTAT_FLAG_NO_HEADERS;
@@ -1049,12 +879,12 @@ spa_state_init(spa_t *spa)
 static void
 spa_health_destroy(spa_t *spa)
 {
-	spa_stats_history_t *ssh = &spa->spa_stats.state;
-	kstat_t *ksp = ssh->kstat;
+	spa_history_kstat_t *shk = &spa->spa_stats.state;
+	kstat_t *ksp = shk->kstat;
 	if (ksp)
 		kstat_delete(ksp);
 
-	mutex_destroy(&ssh->lock);
+	mutex_destroy(&shk->lock);
 }
 
 void