MFV r323795: 8604 Avoid unnecessary work search in VFS when unmounting snapshots

illumos/illumos-gate@ed992b0aac
ed992b0aac

https://www.illumos.org/issues/8604
  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).
  Ideally we would just put a hold on the snapshot and access its respective VFS
  resource directly.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>

FreeBSD note: I added a FreeBSD specific function getzfsvfs_ref() which
is like getzfsvfs() but returns a filesystem referenced, not busied.
We want a busied filesystem in most cases, because we access its private
data and, thus, we need to prevent the filesystem from being unmounted
and its private data destroyed.  But in some cases we can either get
away with just a referenced filesystem or we must not busy the
filesystem.  Unmounting the filesystem is one of such cases.

MFC after:	5 weeks
X-MFC after:	r324163

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dsl_destroy.c

@@ -488,23 +488,29 @@ 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);
 
@@ -538,7 +544,7 @@ 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);

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h

@@ -21,7 +21,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011-2012 Pawel Jakub Dawidek. 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) 2014 Integros [integros.com]
  */
@@ -424,9 +424,10 @@ 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_busy(void);
-extern int zfs_unmount_snap(const char *);
+extern void zfs_unmount_snap(const char *);
 extern void zfs_destroy_unmount_origin(const char *);
 extern int getzfsvfs_impl(struct objset *, struct zfsvfs **);
+extern int getzfsvfs(const char *, struct zfsvfs **);
 
 /*
  * ZFS minor numbers can refer to either a control device instance or

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c

@@ -1460,7 +1460,8 @@ getzfsvfs_impl(objset_t *os, zfsvfs_t **zfvp)
 	return (error);
 }
 
-static int
+#ifdef illumos
+int
 getzfsvfs(const char *dsname, zfsvfs_t **zfvp)
 {
 	objset_t *os;
@@ -1472,16 +1473,44 @@ getzfsvfs(const char *dsname, zfsvfs_t **zfvp)
 
 	error = getzfsvfs_impl(os, zfvp);
 	dmu_objset_rele(os, FTAG);
-	if (error == 0) {
-		error = vfs_busy((*zfvp)->z_vfs, 0);
-		vfs_rel((*zfvp)->z_vfs);
-		if (error != 0) {
-			*zfvp = NULL;
-			error = SET_ERROR(ESRCH);
-		}
+	return (error);
+}
+
+#else
+
+static int
+getzfsvfs_ref(const char *dsname, zfsvfs_t **zfvp)
+{
+	objset_t *os;
+	int error;
+
+	error = dmu_objset_hold(dsname, FTAG, &os);
+	if (error != 0)
+		return (error);
+
+	error = getzfsvfs_impl(os, zfvp);
+	dmu_objset_rele(os, FTAG);
+	return (error);
+}
+
+int
+getzfsvfs(const char *dsname, zfsvfs_t **zfvp)
+{
+	objset_t *os;
+	int error;
+
+	error = getzfsvfs_ref(dsname, zfvp);
+	if (error != 0)
+		return (error);
+	error = vfs_busy((*zfvp)->z_vfs, 0);
+	vfs_rel((*zfvp)->z_vfs);
+	if (error != 0) {
+		*zfvp = NULL;
+		error = SET_ERROR(ESRCH);
 	}
 	return (error);
 }
+#endif
 
 /*
  * Find a zfsvfs_t for a mounted filesystem, or create our own, in which
@@ -3055,27 +3084,6 @@ zfs_ioc_get_fsacl(zfs_cmd_t *zc)
 	return (error);
 }
 
-/*
- * Search the vfs list for a specified resource.  Returns a pointer to it
- * or NULL if no suitable entry is found. The caller of this routine
- * is responsible for releasing the returned vfs pointer.
- */
-static vfs_t *
-zfs_get_vfs(const char *resource)
-{
-	vfs_t *vfsp;
-
-	mtx_lock(&mountlist_mtx);
-	TAILQ_FOREACH(vfsp, &mountlist, mnt_list) {
-		if (strcmp(refstr_value(vfsp->vfs_resource), resource) == 0) {
-			vfs_ref(vfsp);
-			break;
-		}
-	}
-	mtx_unlock(&mountlist_mtx);
-	return (vfsp);
-}
-
 /* ARGSUSED */
 static void
 zfs_create_cb(objset_t *os, void *arg, cred_t *cr, dmu_tx_t *tx)
@@ -3557,30 +3565,29 @@ zfs_ioc_nextboot(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)
 {
-	vfs_t *vfsp;
-	zfsvfs_t *zfsvfs;
-#ifdef illumos
-	int err;
-#endif
+	vfs_t *vfsp = NULL;
+	zfsvfs_t *zfsvfs = NULL;
 
 	if (strchr(snapname, '@') == NULL)
-		return (0);
+		return;
 
-	vfsp = zfs_get_vfs(snapname);
-	if (vfsp == NULL)
-		return (0);
+	int err = getzfsvfs_ref(snapname, &zfsvfs);
+	if (err != 0) {
+		ASSERT3P(zfsvfs, ==, NULL);
+		return;
+	}
+	vfsp = zfsvfs->z_vfs;
 
-	zfsvfs = vfsp->vfs_data;
 	ASSERT(!dsl_pool_config_held(dmu_objset_pool(zfsvfs->z_os)));
 
 #ifdef illumos
 	err = vn_vfswlock(vfsp->vfs_vnodecovered);
 	VFS_RELE(vfsp);
 	if (err != 0)
-		return (SET_ERROR(err));
+		return;
 #endif
 
 	/*
@@ -3591,14 +3598,14 @@ zfs_unmount_snap(const char *snapname)
 #else
 	(void) dounmount(vfsp, MS_FORCE, curthread);
 #endif
-	return (0);
 }
 
 /* ARGSUSED */
 static int
 zfs_unmount_snap_cb(const char *snapname, void *arg)
 {
-	return (zfs_unmount_snap(snapname));
+	zfs_unmount_snap(snapname);
+	return (0);
 }
 
 /*
@@ -3621,7 +3628,7 @@ 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);
 	}
@@ -3662,9 +3669,7 @@ zfs_ioc_destroy_snaps(const char *poolname, nvlist_t *innvl, nvlist_t *outnvl)
 		    (name[poollen] != '/' && name[poollen] != '@'))
 			return (SET_ERROR(EXDEV));
 
-		error = zfs_unmount_snap(name);
-		if (error != 0)
-			return (error);
+		zfs_unmount_snap(nvpair_name(pair));
 #if defined(__FreeBSD__)
 		zvol_remove_minors(name);
 #endif
@@ -3810,11 +3815,8 @@ 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);
@@ -3886,7 +3888,9 @@ recursive_unmount(const char *fsname, void *arg)
 	char fullname[ZFS_MAX_DATASET_NAME_LEN];
 
 	(void) snprintf(fullname, sizeof (fullname), "%s@%s", fsname, snapname);
-	return (zfs_unmount_snap(fullname));
+	zfs_unmount_snap(fullname);
+
+	return (0);
 }
 
 /*