OpenZFS 8604 - Simplify snapshots unmounting code

Authored by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Andy Stormont <[email protected]>
Approved by: Robert Mustacchi <[email protected]>
Ported-by: Don Brady <[email protected]>

Every time we want to unmount a snapshot (happens during snapshot
deletion or renaming) we unnecessarily iterate through all the
mountpoints in the VFS layer (see zfs_get_vfs).

The current patch completely gets rid of that code and changes
the approach while keeping the behavior of that code path the
same. Specifically, it puts a hold on the dataset/snapshot and
gets its vfs resource reference directly, instead of linearly
searching for it. If that reference exists we attempt to amount
it.

With the above change, it became obvious that the nvlist
manipulations that we do (add_boolean and add_nvlist) take a
significant amount of time ensuring uniqueness of every new
element even though they don't have too. Thus, we updated the
patch so those nvlists are not trying to enforce the uniqueness
of their elements.

A more complete analysis of the problem solved by this patch
can be found below:
https://sdimitro.github.io/post/snap-unmount-perf/

OpenZFS-issue: https://www.illumos.org/issues/8604
OpenZFS-commit: openzfs/openzfs@126118fb

include/sys/zfs_ioctl.h

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
  * Copyright 2016 RackTop Systems.
  * Copyright (c) 2017, Intel Corporation.
  */
@@ -484,10 +484,11 @@ typedef struct zfs_creat {
 extern int zfs_secpolicy_snapshot_perms(const char *, cred_t *);
 extern int zfs_secpolicy_rename_perms(const char *, const char *, cred_t *);
 extern int zfs_secpolicy_destroy_perms(const char *, cred_t *);
-extern int zfs_unmount_snap(const char *);
+extern void zfs_unmount_snap(const char *);
 extern void zfs_destroy_unmount_origin(const char *);
 extern boolean_t dataset_name_hidden(const char *);
 extern int getzfsvfs_impl(struct objset *, struct zfsvfs **);
+extern int getzfsvfs(const char *, struct zfsvfs **);
 
 enum zfsdev_state_type {
 	ZST_ONEXIT,

module/zfs/dsl_destroy.c

@@ -496,23 +496,29 @@ dsl_destroy_snapshots_nvl(nvlist_t *snaps, boolean_t defer,
 	if (nvlist_next_nvpair(snaps, NULL) == NULL)
 		return (0);
 
-	nvlist_t *arg = fnvlist_alloc();
-	nvlist_t *snaps_normalized = fnvlist_alloc();
 	/*
 	 * lzc_destroy_snaps() is documented to take an nvlist whose
-	 * values "don't matter".  We need to convert that nvlist to one
-	 * that we know can be converted to LUA.
+	 * values "don't matter".  We need to convert that nvlist to
+	 * one that we know can be converted to LUA. We also don't
+	 * care about any duplicate entries because the nvlist will
+	 * be converted to a LUA table which should take care of this.
 	 */
+	nvlist_t *snaps_normalized;
+	VERIFY0(nvlist_alloc(&snaps_normalized, 0, KM_SLEEP));
 	for (nvpair_t *pair = nvlist_next_nvpair(snaps, NULL);
 	    pair != NULL; pair = nvlist_next_nvpair(snaps, pair)) {
 		fnvlist_add_boolean_value(snaps_normalized,
 		    nvpair_name(pair), B_TRUE);
 	}
+
+	nvlist_t *arg;
+	VERIFY0(nvlist_alloc(&arg, 0, KM_SLEEP));
 	fnvlist_add_nvlist(arg, "snaps", snaps_normalized);
 	fnvlist_free(snaps_normalized);
 	fnvlist_add_boolean_value(arg, "defer", defer);
 
-	nvlist_t *wrapper = fnvlist_alloc();
+	nvlist_t *wrapper;
+	VERIFY0(nvlist_alloc(&wrapper, 0, KM_SLEEP));
 	fnvlist_add_nvlist(wrapper, ZCP_ARG_ARGLIST, arg);
 	fnvlist_free(arg);
 
@@ -546,7 +552,7 @@ dsl_destroy_snapshots_nvl(nvlist_t *snaps, boolean_t defer,
 	    program,
 	    0,
 	    zfs_lua_max_memlimit,
-	    fnvlist_lookup_nvpair(wrapper, ZCP_ARG_ARGLIST), result);
+	    nvlist_next_nvpair(wrapper, NULL), result);
 	if (error != 0) {
 		char *errorstr = NULL;
 		(void) nvlist_lookup_string(result, ZCP_RET_ERROR, &errorstr);

module/zfs/zfs_ioctl.c

@@ -27,7 +27,7 @@
  * Copyright (c) 2014, 2016 Joyent, Inc. All rights reserved.
  * Copyright 2016 Nexenta Systems, Inc.  All rights reserved.
  * Copyright (c) 2014, Joyent, Inc. All rights reserved.
- * Copyright (c) 2011, 2016 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
  * Copyright (c) 2013 Steven Hartland. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
@@ -1437,7 +1437,7 @@ getzfsvfs_impl(objset_t *os, zfsvfs_t **zfvp)
 	return (error);
 }
 
-static int
+int
 getzfsvfs(const char *dsname, zfsvfs_t **zfvp)
 {
 	objset_t *os;
@@ -3502,26 +3502,21 @@ zfs_ioc_log_history(const char *unused, nvlist_t *innvl, nvlist_t *outnvl)
  * Returns 0 if the argument is not a snapshot, or it is not currently a
  * filesystem, or we were able to unmount it.  Returns error code otherwise.
  */
-int
+void
 zfs_unmount_snap(const char *snapname)
 {
-	int err;
-
 	if (strchr(snapname, '@') == NULL)
-		return (0);
-
-	err = zfsctl_snapshot_unmount((char *)snapname, MNT_FORCE);
-	if (err != 0 && err != ENOENT)
-		return (SET_ERROR(err));
+		return;
 
-	return (0);
+	(void) zfsctl_snapshot_unmount((char *)snapname, MNT_FORCE);
 }
 
 /* ARGSUSED */
 static int
 zfs_unmount_snap_cb(const char *snapname, void *arg)
 {
-	return (zfs_unmount_snap(snapname));
+	zfs_unmount_snap(snapname);
+	return (0);
 }
 
 /*
@@ -3544,7 +3539,7 @@ zfs_destroy_unmount_origin(const char *fsname)
 		char originname[ZFS_MAX_DATASET_NAME_LEN];
 		dsl_dataset_name(ds->ds_prev, originname);
 		dmu_objset_rele(os, FTAG);
-		(void) zfs_unmount_snap(originname);
+		zfs_unmount_snap(originname);
 	} else {
 		dmu_objset_rele(os, FTAG);
 	}
@@ -3572,7 +3567,7 @@ zfs_ioc_destroy_snaps(const char *poolname, nvlist_t *innvl, nvlist_t *outnvl)
 
 	for (pair = nvlist_next_nvpair(snaps, NULL); pair != NULL;
 	    pair = nvlist_next_nvpair(snaps, pair)) {
-		(void) zfs_unmount_snap(nvpair_name(pair));
+		zfs_unmount_snap(nvpair_name(pair));
 	}
 
 	return (dsl_destroy_snapshots_nvl(snaps, defer, outnvl));
@@ -3715,11 +3710,8 @@ zfs_ioc_destroy(zfs_cmd_t *zc)
 {
 	int err;
 
-	if (zc->zc_objset_type == DMU_OST_ZFS) {
-		err = zfs_unmount_snap(zc->zc_name);
-		if (err != 0)
-			return (err);
-	}
+	if (zc->zc_objset_type == DMU_OST_ZFS)
+		zfs_unmount_snap(zc->zc_name);
 
 	if (strchr(zc->zc_name, '@')) {
 		err = dsl_destroy_snapshot(zc->zc_name, zc->zc_defer_destroy);
@@ -3814,13 +3806,12 @@ recursive_unmount(const char *fsname, void *arg)
 {
 	const char *snapname = arg;
 	char *fullname;
-	int error;
 
 	fullname = kmem_asprintf("%s@%s", fsname, snapname);
-	error = zfs_unmount_snap(fullname);
+	zfs_unmount_snap(fullname);
 	strfree(fullname);
 
-	return (error);
+	return (0);
 }
 
 /*