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: https://github.com/openzfs/openzfs/commit/126118fb

1 files changed, 13 insertions, 22 deletions

diff --git a/module/zfs/zfs_ioctl.c b/module/zfs/zfs_ioctl.c
index 8b0cbb40b..443005165 100644
--- a/module/zfs/zfs_ioctl.c
+++ b/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);
 }
 
 /*