atomic_cas_32 is implemented using atomic_fcmpset_32 on all platforms.
Ditto for atomic_cas_64 and atomic_fcmpset_64 on platforms that have it.
The only exception is sparc64 that provides MD atomic_cas_32 and
atomic_cas_64.
This is slightly inefficient as fcmpset reports whether the operation
updated the target and that information is not needed for cas.
Nevertheless, there is less code to maintain and to add for new platforms.
Also, the operations are done inline now as opposed to function calls before.
atomic_add_64_nv is implemented using atomic_fetchadd_64 on platforms
that provide it.
casptr, cas32, atomic_or_8, atomic_or_8_nv are completely removed as they
have no users.
atomic_mtx that is used to emulate 64-bit atomics on platforms that lack
them is defined only on those platforms.
As a result, platform specific opensolaris_atomic.S files have lost most of
their code. The only exception is i386 where the compat+contrib code
provides 64-bit atomics for userland use. That code assumes availability of
cmpxchg8b instruction. FreeBSD does not have that assumption for i386
userland and does not provide 64-bit atomics. Hopefully, this can and will
be fixed.
MFC after: 3 weeks
Some 32-bit platforms do not provide 64-bit atomic operations that ZFS
requires, either in userland or at all. We emulate those operations for
those platforms using a mutex. That is not entirely correct and it's
very efficient. Besides, the loads are plain loads, so torn values are
possible.
Nevertheless, the emulation seems to work for some definition of work.
This change adds atomic_swap_64, which is already used in ZFS code, and
atomic_load_64 that can be used to prevent torn reads.
MFC after: 1 week
New primitive is introduced to denote sections can operate locklessly
on aspects of struct mount, but which can also be disabled if necessary.
This provides an opportunity to start scaling common case modifications
while providing stable state of the struct when facing unmount, write
suspendion or other events.
mnt_ref is the first counter to start being managed in this manner with
the intent to make it per-cpu.
Reviewed by: kib, jeff
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21425
The page daemon periodically invokes uma_reclaim() to reclaim cached
items from each zone when the system is under memory pressure. This
is important since the size of these caches is unbounded by default.
However it also results in bursts of high latency when allocating from
heavily used zones as threads miss in the per-CPU caches and must
access the keg in order to allocate new items.
With r340405 we maintain an estimate of each zone's usage of its
(per-NUMA domain) cache of full buckets. Start making use of this
estimate to avoid reclaiming the entire cache when under memory
pressure. In particular, introduce TRIM, DRAIN and DRAIN_CPU
verbs for uma_reclaim() and uma_zone_reclaim(). When trimming, only
items in excess of the estimate are reclaimed. Draining a zone
reclaims all of the cached full buckets (the previous behaviour of
uma_reclaim()), and may further drain the per-CPU caches in extreme
cases.
Now, when under memory pressure, the page daemon will trim zones
rather than draining them. As a result, heavily used zones do not incur
bursts of bucket cache misses following reclamation, but large, unused
caches will be reclaimed as before.
Reviewed by: jeff
Tested by: pho (an earlier version)
MFC after: 2 months
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16667
This allows replacing "sys/eventfilter.h" includes with "sys/_eventfilter.h"
in other header files (e.g., sys/{bus,conf,cpu}.h) and reduces header
pollution substantially.
EVENTHANDLER_DECLARE and EVENTHANDLER_LIST_DECLAREs were moved out of .c
files into appropriate headers (e.g., sys/proc.h, powernv/opal.h).
As a side effect of reduced header pollution, many .c files and headers no
longer contain needed definitions. The remainder of the patch addresses
adding appropriate includes to fix those files.
LOCK_DEBUG and LOCK_FILE_LINE_ARG are moved to sys/_lock.h, as required by
sys/mutex.h since r326106 (but silently protected by header pollution prior
to this change).
No functional change (intended). Of course, any out of tree modules that
relied on header pollution for sys/eventhandler.h, sys/lock.h, or
sys/mutex.h inclusion need to be fixed. __FreeBSD_version has been bumped.
the file associated with the given file descriptor.
Reviewed by: kib, asomers
Reviewed by: cem, jilles, brooks (they reviewed previous version)
Discussed with: pjd, and many others
Differential Revision: https://reviews.freebsd.org/D14567
Flags prevent open(2) and *at(2) vfs syscalls name lookup from
escaping the starting directory. Supposedly the interface is similar
to the same proposed Linux flags.
Reviewed by: jilles (code, previous version of manpages), 0mp (manpages)
Discussed with: allanjude, emaste, jonathan
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D17547
NB: disabled pending the addition of KSTAT_TYPE_RAW support to the
SPL
commit e0b0ca983d
Author: Brian Behlendorf <behlendorf1@llnl.gov>
Date: Wed Oct 2 17:11:19 2013 -0700
Add visibility in to cached dbufs
Currently there is no mechanism to inspect which dbufs are being
cached by the system. There are some coarse counters in arcstats
by they only give a rough idea of what's being cached. This patch
aims to improve the current situation by adding a new dbufs kstat.
When read this new kstat will walk all cached dbufs linked in to
the dbuf_hash. For each dbuf it will dump detailed information
about the buffer. It will also dump additional information about
the referenced arc buffer and its related dnode. This provides a
more complete view in to exactly what is being cached.
With this generic infrastructure in place utilities can be written
to post-process the data to understand exactly how the caching is
working. For example, the data could be processed to show a list
of all cached dnodes and how much space they're consuming. Or a
similar list could be generated based on dnode type. Many other
ways to interpret the data exist based on what kinds of questions
you're trying to answer.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
d4a72f2386
During scans (scrubs or resilvers), it sorts the blocks in each transaction
group by block offset; the result can be a significant improvement. (On my
test system just now, which I put some effort to introduce fragmentation into
the pool since I set it up yesterday, a scrub went from 1h2m to 33.5m with the
changes.) I've seen similar rations on production systems.
Approved by: Alexander Motin
Obtained from: ZFS On Linux
Relnotes: Yes (improved scrub performance, with tunables)
Differential Revision: https://reviews.freebsd.org/D15562
The 64-bit atomics defined for i386 are currently only available in
the kernel space.
Found by: cy@
MFC after: 1 week
Sponsored by: Mellanox Technologies
While at it add missing _acq_ and _rel_ variants for 64-bit atomic
operations under i386.
Reviewed by: kib @
MFC after: 1 week
Sponsored by: Mellanox Technologies
- Add macros to allow preinitialization of cap_rights_t.
- Convert most commonly used code paths to use preinitialized cap_rights_t.
A 3.6% speedup in fstat was measured with this change.
Reported by: mjg
Reviewed by: oshogbo
Approved by: sbruno
MFC after: 1 month
It gets called by dmu_buf_init_user, which is inline but not static. So it
needs global linkage itself.
Reported by: GCC-6
MFC after: 17 days
X-MFC-With: 329722
illumos/illumos-gate@36a64e6284
To prevent kmem_cache reaping from blocking other system resources, turn
kmem_cache_reap_now() (which blocks) into kmem_cache_reap_soon(). Callers
to kmem_cache_reap_soon() should use kmem_cache_reap_active(), which
exploits #9017's new taskq_empty().
Reviewed by: Bryan Cantrill <bryan@joyent.com>
Reviewed by: Dan McDonald <danmcd@joyent.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Author: Tim Kordas <tim.kordas@joyent.com>
FreeBSD does not use taskqueue for kmem caches reaping, so this change
is less dramatic then it is on Illumos, just limiting reaping to 1 time
per second. It may possibly be improved later, if needed.
global to per-domain state. Protect reservations with the free lock
from the domain that they belong to. Refactor to make vm domains more
of a first class object.
Reviewed by: markj, kib, gallatin
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14000
rather than kmem arena size to determine available memory.
Initialize the UMA limit to LONG_MAX to avoid spurious wakeups on boot before
the real limit is set.
PR: 224330 (partial), 224080
Reviewed by: markj, avg
Sponsored by: Netflix / Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13494
uiocopy() is currently unused, its purpose is copy data from a uio
without modifying the uio. It was in use before the vn_io_fault support
was added to ZFS, at which point our code diverged from the illumos code
a little bit. Because ZFS is the only (potential) user of the function
we are free to modify it to better suit ZFS needs.
The intention behind this change is to remove the differences introduced
earlier in zfs_write().
While here, re-implement uioskip() using uiomove() with
uio_segflg == UIO_NOCOPY.
The story of uioskip is the same as with uiocopy.
Reviewed by: mav
MFC after: 1 week
In general, higher-level code will atomically verify that the process
is not exiting and hold the process. In one case, we were using uwrite()
to copy a probed instruction to a per-thread scratch space block, but
copyout() can be used for this purpose instead; this change effectively
reverts r227291.
MFC after: 1 week
Zero, <TYPE>_MIN and <TYPE>_MAX values can result from valid conversions.
They don't necessarily imply any error.
Since we do not have any reliable error signaling from libkern's strto*(),
it's better to always assume success rather than to report an error when
there is none.
Reviewed by: tsoome
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12565
7431 ZFS Channel Programs
illumos/illumos-gate@dfc115332cdfc115332chttps://www.illumos.org/issues/7431
ZFS channel programs (ZCP) adds support for performing compound ZFS
administrative actions via Lua scripts in a sandboxed environment (with time
and memory limits).
This initial commit includes both base support for running ZCP scripts, and a
small initial library of API calls which support getting properties and
listing, destroying, and promoting datasets.
Testing: in addition to the included unit tests, channel programs have been in
use at Delphix for several months for batch destroying filesystems. The
dsl_destroy_snaps_nvl() call has also been replaced with
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: Chris Williamson <chris.williamson@delphix.com>
8552 ZFS LUA code uses floating point math
illumos/illumos-gate@916c8d8811916c8d8811https://www.illumos.org/issues/8552
In the LUA interpreter used by "zfs program", the lua format() function
accidentally includes support for '%f' and friends, which can cause compilation
problems when building on platforms that don't support floating-point math in
the kernel (e.g. sparc). Support for '%f' friends (%f %e %E %g %G) should be
removed, since there's no way to supply a floating-point value anyway (all
numbers in ZFS LUA are int64_t's).
Reviewed by: Yuri Pankov <yuripv@gmx.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>
8590 memory leak in dsl_destroy_snapshots_nvl()
illumos/illumos-gate@e6ab4525d1e6ab4525d1https://www.illumos.org/issues/8590
In dsl_destroy_snapshots_nvl(), "snaps_normalized" is not freed after it is
added to "arg".
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>
FreeBSD notes:
- zfs-program.8 manual page is taken almost as is from the vendor repository,
no FreeBSD-ification done
- fixed multiple instances of NULL being used where an integer is expected
- replaced ETIME and ECHRNG with ETIMEDOUT and EDOM respectively
This commit adds a modified version of Lua 5.2.4 under
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/lua, mirroring the
upstream. See README.zfs in that directory for the description of Lua
customizations.
See zfs-program.8 on how to use the new feature.
MFC after: 5 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D12528
I managed to commit an older version of the change.
Plus, even the latest version was not ready for userland compilation.
Reported by: "O. Hartmann" <ohartmann@walstatt.org>,
cy
MFC after: 1 week
X-MFC with: r324011
As long as mnt_ref is not zero there can be a consumer that might try
to access mnt_vnodecovered. For this reason the covered vnode must not
be freed until mnt_ref goes to zero.
So, move the release of the covered vnode to vfs_mount_destroy.
Reviewed by: kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D12329
"zfs mount -o" passes a list of mount options directly to nmount(2) after
sanity checking them. In particular, zfs(8) will refuse to mount an already
existing file system unless "remount" is specified in the option list.
However, the "remount" option only exists in Illumos. FreeBSD's equivalent is
"update".
PR: 221985
Reviewed by: avg
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12233
o Replace __riscv64 with (__riscv && __riscv_xlen == 64)
This is required to support new GCC 7.1 compiler.
This is compatible with current GCC 6.1 compiler.
RISC-V is extensible ISA and the idea here is to have built-in define
per each extension, so together with __riscv we will have some subset
of these as well (depending on -march string passed to compiler):
__riscv_compressed
__riscv_atomic
__riscv_mul
__riscv_div
__riscv_muldiv
__riscv_fdiv
__riscv_fsqrt
__riscv_float_abi_soft
__riscv_float_abi_single
__riscv_float_abi_double
__riscv_cmodel_medlow
__riscv_cmodel_medany
__riscv_cmodel_pic
__riscv_xlen
Reviewed by: ngie
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11901
Extend the ino_t, dev_t, nlink_t types to 64-bit ints. Modify
struct dirent layout to add d_off, increase the size of d_fileno
to 64-bits, increase the size of d_namlen to 16-bits, and change
the required alignment. Increase struct statfs f_mntfromname[] and
f_mntonname[] array length MNAMELEN to 1024.
ABI breakage is mitigated by providing compatibility using versioned
symbols, ingenious use of the existing padding in structures, and
by employing other tricks. Unfortunately, not everything can be
fixed, especially outside the base system. For instance, third-party
APIs which pass struct stat around are broken in backward and
forward incompatible ways.
Kinfo sysctl MIBs ABI is changed in backward-compatible way, but
there is no general mechanism to handle other sysctl MIBS which
return structures where the layout has changed. It was considered
that the breakage is either in the management interfaces, where we
usually allow ABI slip, or is not important.
Struct xvnode changed layout, no compat shims are provided.
For struct xtty, dev_t tty device member was reduced to uint32_t.
It was decided that keeping ABI compat in this case is more useful
than reporting 64-bit dev_t, for the sake of pstat.
Update note: strictly follow the instructions in UPDATING. Build
and install the new kernel with COMPAT_FREEBSD11 option enabled,
then reboot, and only then install new world.
Credits: The 64-bit inode project, also known as ino64, started life
many years ago as a project by Gleb Kurtsou (gleb). Kirk McKusick
(mckusick) then picked up and updated the patch, and acted as a
flag-waver. Feedback, suggestions, and discussions were carried
by Ed Maste (emaste), John Baldwin (jhb), Jilles Tjoelker (jilles),
and Rick Macklem (rmacklem). Kris Moore (kris) performed an initial
ports investigation followed by an exp-run by Antoine Brodin (antoine).
Essential and all-embracing testing was done by Peter Holm (pho).
The heavy lifting of coordinating all these efforts and bringing the
project to completion were done by Konstantin Belousov (kib).
Sponsored by: The FreeBSD Foundation (emaste, kib)
Differential revision: https://reviews.freebsd.org/D10439
illumos/illumos-gate@8363e80ae7https://github.com/illumos/illumos-gate/commit/8363e80ae72609660f6090766ca8c2c18https://www.illumos.org/issues/7303
This change introduces a new weighting algorithm to improve metaslab selection.
The new weighting algorithm relies on the SPACEMAP_HISTOGRAM feature. As a result,
the metaslab weight now encodes the type of weighting algorithm used
(size-based vs segment-based).
This also introduce a new allocation tracing facility and two new dcmds to help
debug allocation problems. Each zio now contains a zio_alloc_list_t structure
that is populated as the zio goes through the allocations stage. Here's an
example of how to use the tracing facility:
> c5ec000::print zio_t io_alloc_list | ::walk list | ::metaslab_trace
MSID DVA ASIZE WEIGHT RESULT VDEV
- 0 400 0 NOT_ALLOCATABLE ztest.0a
- 0 400 0 NOT_ALLOCATABLE ztest.0a
- 0 400 0 ENOSPC ztest.0a
- 0 200 0 NOT_ALLOCATABLE ztest.0a
- 0 200 0 NOT_ALLOCATABLE ztest.0a
- 0 200 0 ENOSPC ztest.0a
1 0 400 1 x 8M 17b1a00 ztest.0a
> 1ff2400::print zio_t io_alloc_list | ::walk list | ::metaslab_trace
MSID DVA ASIZE WEIGHT RESULT VDEV
- 0 200 0 NOT_ALLOCATABLE mirror-2
- 0 200 0 NOT_ALLOCATABLE mirror-0
1 0 200 1 x 4M 112ae00 mirror-1
- 1 200 0 NOT_ALLOCATABLE mirror-2
- 1 200 0 NOT_ALLOCATABLE mirror-0
1 1 200 1 x 4M 112b000 mirror-1
- 2 200 0 NOT_ALLOCATABLE mirror-2
If the metaslab is using segment-based weighting then the WEIGHT column will
display the number of segments available in the bucket where the allocation
attempt was made.
Author: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Chris Siden <christopher.siden@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Don Brady <don.brady@intel.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
The current code is written on top of GFS, a library with the generic
support for writing filesystems, which was ported from illumos.
Because of significant differences between illumos VFS and FreeBSD
VFS models, both the GFS and zfsctl code were heavily modified to
work on FreeBSD. Nonetheless, they still contain quite a few ugly
hacks and bugs.
This is a reimplementation of the zfsctl code where the VFS-specific
bits are written from scratch and only the code that interacts with
the rest of ZFS is reused.
Some highlights.
We use two types of nodes, static and on-demand. The static nodes
are used for permanent directories like .zfs, .zfs/snapshot, etc. The
on-demand nodes are used for ephemeral directories that act as snapshot
mount points.
Initially only static nodes are created. Their vnodes are instantiated
when they are looked up. The on-demand nodes and vnodes are instantiated
as needed and the nodes are destroyed as soon as the corresponding
vnodes are reclaimed.
We also try very hard to ensure that uncovered snapshot vnodes do not
linger. They are supposed to become inactive as soon as they are
uncovered and we try to recycle them immediately.
When a filesystem is unmounted all snapshots under .zfs are unmounted
first, then all vnodes are flushed and finally the static .zfs nodes
are destroyed.
There are some changes outside of zfsctl code too.
z_ctldir is never used directly (as it is an opaque pointer),
zfsctl_root() has to be used instead. The function returns a locked
vnode now, so it accepts a lock flags parameter. The function can
also fail now, e.g. during force unmounting, whereas previously it
was infallible.
zfsctl_root_lookup() is retired, instead of it VOP_LOOKUP() on the .zfs
vnode (obtained with zfsctl_root) is used.
Some ideas are picked from an independent work by will.
Reviewed by: asomers, smh
MFC after: 1 month
Relnotes: maybe
Differential Revision: https://reviews.freebsd.org/D7421
It was a temporary change to ease an import of native atomic_cas primitives.
Instead, atomic_fcmpset was devised with different semantics. See r311168.
longer used. More precisely, they are always zero because the code that
decremented and incremented them no longer exists.
Bump __FreeBSD_version to mark this change.
Reviewed by: kib, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8583
not remove user-space visible fields from vm_cnt or all of the references to
cached pages from comments. Those changes will come later.)
Reviewed by: kib, markj
Tested by: pho
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8497
Our shim for Solaris random_get_bytes() uses read_random(), that looks
reasonable, since it guaranties reliably seeded random data. On the other
side Solaris random_get_pseudo_bytes() does not provide this guarantie,
and its original Solaris implementation is equivalent to our arc4rand(),
using software crypto without stressing slower hardware RNG.
ZFS POSIX Layer is originally written for Solaris VFS which is very
different from FreeBSD VFS. Most importantly many things that FreeBSD VFS
manages on behalf of all filesystems are implemented in ZPL in a different
way.
Thus, ZPL contains code that is redundant on FreeBSD or duplicates VFS
functionality or, in the worst cases, badly interacts / interferes
with VFS.
The most prominent problem is a deadlock caused by the lock order reversal
of vnode locks that may happen with concurrent zfs_rename() and lookup().
The deadlock is a result of zfs_rename() not observing the vnode locking
contract expected by VFS.
This commit removes all ZPL internal locking that protects parent-child
relationships of filesystem nodes. These relationships are protected
by vnode locks and the code is changed to take advantage of that fact
and to properly interact with VFS.
Removal of the internal locking allowed all ZPL dmu_tx_assign calls to
use TXG_WAIT mode.
Another victim, disputable perhaps, is ZFS support for filesystems with
mixed case sensitivity. That support is not provided by the OS anyway,
so in ZFS it was a buch of dead code.
To do:
- replace ZFS_ENTER mechanism with VFS managed / visible mechanism
- replace zfs_zget with zfs_vget[f] as much as possible
- get rid of not really useful now zfs_freebsd_* adapters
- more cleanups of unneeded / unused code
- fix / replace .zfs support
PR: 209158
Reported by: many
Tested by: many (thank you all!)
MFC after: 5 days
Sponsored by: HybridCluster / ClusterHQ
Differential Revision: https://reviews.freebsd.org/D6533
mp_maxid or CPU_FOREACH() as appropriate. This fixes a number of places in
the kernel that assumed CPU IDs are dense in [0, mp_ncpus) and would try,
for example, to run tasks on CPUs that did not exist or to allocate too
few buffers on systems with sparse CPU IDs in which there are holes in the
range and mp_maxid > mp_ncpus. Such circumstances generally occur on
systems with SMT, but on which SMT is disabled. This patch restores system
operation at least on POWER8 systems configured in this way.
There are a number of other places in the kernel with potential problems
in these situations, but where sparse CPU IDs are not currently known
to occur, mostly in the ARM machine-dependent code. These will be fixed
in a follow-up commit after the stable/11 branch.
PR: kern/210106
Reviewed by: jhb
Approved by: re (glebius)
This apparently puts ARC back under the limits after the vnode pressure
rework in r291244, in particular due to the kmem exhaustion.
Based on patch by: mckusick
Reviewed by: avg, mckusick
Tested by: allanjude, madpilot
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
This commit partially reverts r273641 which introduced the leak.
It did so to accomodate for some consumers of traverse() that expected
the starting vnode to stay as-is. But that introduced the leak in the
case when a mounted filesystem was found and its root vnode was
returned.
r299914 removed the troublesome consumers and now there is no reason to
keep the starting vnode. So, now the new rules are:
- if there is no mounted filesystem, then nothing is changed
- otherwise the starting vnode is always released
- the root vnode of the mounted filesystem is returned locked and
referenced in the case of success
MFC after: 5 weeks
X-MFC after: r299914
* Remove excessive references on a snapshot mountpoint vnode.
zfsctl_snapdir_lookup() called VN_HOLD() on a vnode returned from
zfsctl_snapshot_mknode() and the latter also had a call to VN_HOLD()
on the same vnode.
On top of that gfs_dir_create() already returns the vnode with the
use count of 1 (set in getnewvnode).
So there was 3 references on the vnode.
* mount_snapshot() should keep a reference to a covered vnode.
That reference is owned by the mountpoint (mounted snapshot filesystem).
* Remove cryptic manipulations of a covered vnode in zfs_umount().
FreeBSD dounmount() already does the right thing and releases the covered
vnode.
PR: 207464
Reported by: dustinwenz@ebureau.com
Tested by: Howard Powell <hpowell@lighthouseinstruments.com>
MFC after: 3 weeks
