Currently, the vm.panic_on_oom sysctl is a boolean which controls the
behavior of the VM system when it encounters an out-of-memory situation.
If set to 0, the VM system kills the largest process. If set to any other
value, the VM system will initiate a panic.
This change makes the sysctl a count of events. If set to 0, the VM system
kills the largest process. If set to any other value, the VM system will
kill the largest process until it has seen the specified number of
out-of-memory events. Once it reaches the specified number of events, it
will initiate a panic.
This change is helpful in capturing cores when the system is in a perpetual
cycle of out-of-memory events (as opposed to just hitting one or two
sporadic out-of-memory events).
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D23601
r355004 removed return statement from this loop with intention to also
call uma_reclaim_wakeup(). But in case of vm.lowmem_period=0 it causes
infinite loop.
Reviewed by: markj
Sponsored by: iXsystems, Inc.
The page daemon loops may move pages back to the active queue if
references are detected. In this case we must take care to clear
existing queue operation flags. In particular, PGA_REQUEUE_HEAD may be
set, and that flag is only valid if the page belongs to the inactive
queue.
Also fix a bug in the active queue scan where we were updating "old"
instead of "new". This would only have been hit in rare cases where the
page moved out of the active queue after the beginning of the scan.
Reported by: Bob Prohaska, Idwer Vollering
Tested by: Idwer Vollering
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D23001
With the previous reviews, the page lock is no longer required in order
to perform queue operations on a page. It is also no longer needed in
the page queue scans. This change effectively eliminates remaining uses
of the page lock and also the false sharing caused by multiple pages
sharing a page lock.
Reviewed by: jeff
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22885
Some recent work aims to remove the use of the page lock for
synchronizing updates to page queue state. This change adds a mechanism
to preserve the existing behaviour of lazily dequeuing wired pages,
which was previously synchronized using the page lock.
Handle this by setting PGA_DEQUEUE when a managed page's wire count
transitions from 0 to 1. When the page daemon encounters a page with a
flag in PGA_QUEUE_OP_MASK set, it creates a batch queue entry for that
page, but in so doing it does not modify the page itself and thus racing
with a concurrent free of the page is harmless. The flag is advisory;
the page daemon still checks for wirings after acquiring the object and
page xbusy locks.
vm_page_unwire_managed() now clears PGA_DEQUEUE on a 1->0 transition.
It must do this before dropping the reference to avoid a use-after-free
but also handles races with concurrent wirings to ensure that
PGA_DEQUEUE is not left unset on a wired page.
Reviewed by: jeff
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22882
This is in preparation for eliminating the use of the vm_page lock for
protecting queue state operations.
Introduce the vm_page_pqstate_commit_*() functions. These functions act
as helpers around vm_page_astate_fcmpset() and are specialized for
specific types of operations. vm_page_pqstate_commit() wraps these
functions.
Convert a number of routines to use these new helpers. Use
vm_page_release_toq() in vm_page_unwire() and vm_page_release() to
atomically release a wiring reference and release the page into a queue.
This has the side effect that vm_page_unwire() will leave the page in
the active queue if it is already present there.
Convert the page queue scans to use the new helpers. Simplify
vm_pageout_reinsert_inactive(), which requeues pages that were found to
be busy during an inactive queue scan, to avoid duplicating the work of
vm_pqbatch_process_page(). In particular, if PGA_REQUEUE or
PGA_REQUEUE_HEAD is set, let that be handled during batch processing.
Reviewed by: jeff
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22770
Differential Revision: https://reviews.freebsd.org/D22771
Differential Revision: https://reviews.freebsd.org/D22772
Differential Revision: https://reviews.freebsd.org/D22773
Differential Revision: https://reviews.freebsd.org/D22776
an exclusive object lock.
Previously swap space was freed on a best effort basis when a page that
had valid swap was dirtied, thus invalidating the swap copy. This may be
done inconsistently and requires the object lock which is not always
convenient.
Instead, track when swap space is present. The first dirty is responsible
for deleting space or setting PGA_SWAP_FREE which will trigger background
scans to free the swap space.
Simplify the locking in vm_fault_dirty() now that we can reliably identify
the first dirty.
Discussed with: alc, kib, markj
Differential Revision: https://reviews.freebsd.org/D22654
This is a 32-bit structure embedded in each vm_page, consisting mostly
of page queue state. The use of a structure makes it easy to store a
snapshot of a page's queue state in a stack variable and use cmpset
loops to update that state without requiring the page lock.
This change merely adds the structure and updates references to atomic
state fields. No functional change intended.
Reviewed by: alc, jeff, kib
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22650
Stop subtracting 1024/200 from vmd_page_count/200. I cannot see how
such precise accounting can make a difference on modern systems.
Add some explanation of what the page daemon does and how it handles
memory shortages.
Reviewed by: dougm
Discussed with: jeff, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22396
Use the UMA reclaim thread to asynchronously drain all caches if
there is a severe shortage in a domain. Otherwise we only trigger UMA
reclamation every 10s even when the system has completely run out of
memory.
Stop entirely draining the caches when one domain falls below its min
threshold. In some workloads it is normal for one NUMA domain to end
up being nearly depleted by kernel memory allocations, for example for
the ZFS ARC. The domainset iterators skip domains below the
vmd_min_free theshold on the first iteration, so we should allow that
mechanism to limit further depletion of the domain's free pages before
taking the extreme step of calling uma_reclaim(UMA_RECLAIM_DRAIN_CPU).
Discussed with: jeff
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22395
Atomics are used for page busy and valid state when the shared busy is
held. The details of the locking protocol and valid and dirty
synchronization are in the updated vm_page.h comments.
Reviewed by: kib, markj
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D21594
This is the first in a series of patches that promotes the page busy field
to a first class lock that no longer requires the object lock for
consistency.
Reviewed by: kib, markj
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D21548
In case the implementation ever changes from using a chain of next pointers,
then changing the macro definition will be necessary, but changing all the
files that iterate over vm_map entries will not.
Drop a counter in vm_object.c that would have an effect only if the
vm_map entry count was wrong.
Discussed with: alc
Reviewed by: markj
Tested by: pho (earlier version)
Differential Revision: https://reviews.freebsd.org/D21882
- Remove a dead variable from the amd64 pmap_extract_and_hold().
- Fix grammar in the vm_page_wire man page.
Reported by: alc
Reviewed by: alc, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21639
There are several mechanisms by which a vm_page reference is held,
preventing the page from being freed back to the page allocator. In
particular, holding the page's object lock is sufficient to prevent the
page from being freed; holding the busy lock or a wiring is sufficent as
well. These references are protected by the page lock, which must
therefore be acquired for many per-page operations. This results in
false sharing since the page locks are external to the vm_page
structures themselves and each lock protects multiple structures.
Transition to using an atomically updated per-page reference counter.
The object's reference is counted using a flag bit in the counter. A
second flag bit is used to atomically block new references via
pmap_extract_and_hold() while removing managed mappings of a page.
Thus, the reference count of a page is guaranteed not to increase if the
page is unbusied, unmapped, and the object's write lock is held. As
a consequence of this, the page lock no longer protects a page's
identity; operations which move pages between objects are now
synchronized solely by the objects' locks.
The vm_page_wire() and vm_page_unwire() KPIs are changed. The former
requires that either the object lock or the busy lock is held. The
latter no longer has a return value and may free the page if it releases
the last reference to that page. vm_page_unwire_noq() behaves the same
as before; the caller is responsible for checking its return value and
freeing or enqueuing the page as appropriate. vm_page_wire_mapped() is
introduced for use in pmap_extract_and_hold(). It fails if the page is
concurrently being unmapped, typically triggering a fallback to the
fault handler. vm_page_wire() no longer requires the page lock and
vm_page_unwire() now internally acquires the page lock when releasing
the last wiring of a page (since the page lock still protects a page's
queue state). In particular, synchronization details are no longer
leaked into the caller.
The change excises the page lock from several frequently executed code
paths. In particular, vm_object_terminate() no longer bounces between
page locks as it releases an object's pages, and direct I/O and
sendfile(SF_NOCACHE) completions no longer require the page lock. In
these latter cases we now get linear scalability in the common scenario
where different threads are operating on different files.
__FreeBSD_version is bumped. The DRM ports have been updated to
accomodate the KPI changes.
Reviewed by: jeff (earlier version)
Tested by: gallatin (earlier version), pho
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20486
Queue operations on a page use the page lock when updating the page to
reflect the desired queue state, and the page queue lock when physically
enqueuing or dequeuing a page. Multiple pages share a given page lock,
but queue state is per-page; this false sharing results in heavy lock
contention.
Take a small step towards the use of atomic_cmpset to synchronize
updates to per-page queue state by introducing vm_page_pqstate_cmpset()
and using it in the page daemon. In the longer term the plan is to stop
using the page lock to protect page identity and rely only on the object
and page busy locks. However, since the page daemon avoids acquiring
the object lock except when necessary, some synchronization with a
concurrent free of the page is required. vm_page_pqstate_cmpset() can
be used to ensure that queue state updates are successful only if the
page is not scheduled for a dequeue, which is sufficient for the page
daemon.
Add vm_page_swapqueue(), which moves a page from one queue to another
using vm_page_pqstate_cmpset(). Use it in the active queue scan, which
does not use the object lock. Modify vm_page_dequeue_deferred() to
use vm_page_pqstate_cmpset() as well.
Reviewed by: kib
Discussed with: jeff
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21257
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
In addition to pagedaemon initiating OOM, also do it from the
vm_fault() internals. Namely, if the thread waits for a free page to
satisfy page fault some preconfigured amount of time, trigger OOM.
These triggers are rate-limited, due to a usual case of several
threads of the same multi-threaded process to enter fault handler
simultaneously. The faults from pagedaemon threads participate in the
calculation of OOM rate, but are not under the limit.
Reviewed by: markj (previous version)
Tested by: pho
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D13671
The hold_count and wire_count fields of struct vm_page are separate
reference counters with similar semantics. The remaining essential
differences are that holds are not counted as a reference with respect
to LRU, and holds have an implicit free-on-last unhold semantic whereas
vm_page_unwire() callers must explicitly determine whether to free the
page once the last reference to the page is released.
This change removes the KPIs which directly manipulate hold_count.
Functions such as vm_fault_quick_hold_pages() now return wired pages
instead. Since r328977 the overhead of maintaining LRU for wired pages
is lower, and in many cases vm_fault_quick_hold_pages() callers would
swap holds for wirings on the returned pages anyway, so with this change
we remove a number of page lock acquisitions.
No functional change is intended. __FreeBSD_version is bumped.
Reviewed by: alc, kib
Discussed with: jeff
Discussed with: jhb, np (cxgbe)
Tested by: pho (previous version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19247
When QUEUE_MACRO_DEBUG_TRASH is configured, removing a queue element
invalidates its queue linkage pointers. vm_pageout_collect_batch()
was relying on these pointers remaining valid after a removal, so
modify it to fetch the next queued page before dequeuing the current
page.
Submitted by: Don Morris <dgmorris@earthlink.net>
Reviewed by: cem, vangyzen
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20842
Use it instead of accessing the wire_count field directly. No
functional change intended.
Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20485
Historically we have not distinguished between kernel wirings and user
wirings for accounting purposes. User wirings (via mlock(2)) were
subject to a global limit on the number of wired pages, so if large
swaths of physical memory were wired by the kernel, as happens with
the ZFS ARC among other things, the limit could be exceeded, causing
user wirings to fail.
The change adds a new counter, v_user_wire_count, which counts the
number of virtual pages wired by user processes via mlock(2) and
mlockall(2). Only user-wired pages are subject to the system-wide
limit which helps provide some safety against deadlocks. In
particular, while sources of kernel wirings typically support some
backpressure mechanism, there is no way to reclaim user-wired pages
shorting of killing the wiring process. The limit is exported as
vm.max_user_wired, renamed from vm.max_wired, and changed from u_int
to u_long.
The choice to count virtual user-wired pages rather than physical
pages was done for simplicity. There are mechanisms that can cause
user-wired mappings to be destroyed while maintaining a wiring of
the backing physical page; these make it difficult to accurately
track user wirings at the physical page layer.
The change also closes some holes which allowed user wirings to succeed
even when they would cause the system limit to be exceeded. For
instance, mmap() may now fail with ENOMEM in a process that has called
mlockall(MCL_FUTURE) if the new mapping would cause the user wiring
limit to be exceeded.
Note that bhyve -S is subject to the user wiring limit, which defaults
to 1/3 of physical RAM. Users that wish to exceed the limit must tune
vm.max_user_wired.
Reviewed by: kib, ngie (mlock() test changes)
Tested by: pho (earlier version)
MFC after: 45 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19908
linear search can, so use it to avoid a linear search in isqrt.
Approved by: kib (mentor), markj (mentor)
Differential Revision: https://reviews.freebsd.org/D20102
Use the object pointer itself to determine whether the object is locked.
No functional change intended.
Reviewed by: kib
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19215
In practice it is always initialized because nfreed must be positive
in order to trigger background laundering, but this isn't obvious.
CID: 1387997
MFC after: 1 week
- In uma_prealloc(), we need to check for an empty domain before the
first allocation attempt, not after. Fix this by switching
uma_prealloc() to use a vm_domainset iterator, which addresses the
secondary issue of using a signed domain identifier in round-robin
iteration.
- Don't automatically create a page daemon for domain 0.
- In domainset_empty_vm(), recompute ds_cnt and ds_order after
excluding empty domains; otherwise we may frequently specify an empty
domain when calling in to the page allocator, wasting CPU time.
Convert DOMAINSET_PREF() policies for empty domains to round-robin.
- When freeing bootstrap pages, don't count them towards the per-domain
total page counts for now: some vm_phys segments are created before
the SRAT is parsed and are thus always identified as being in domain 0
even when they are not. Then, when bootstrap pages are freed, they
are added to a domain that we had previously thought was empty. Until
this is corrected, we simply exclude them from the per-domain page
count.
Reported and tested by: Rajesh Kumar <rajfbsd@gmail.com>
Reviewed by: gallatin
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17704
The AMD Threadripper 2990WX is basically a slightly crippled Epyc.
Rather than having 4 memory controllers, one per NUMA domain, it has
only 2 memory controllers enabled. This means that only 2 of the
4 NUMA domains can be populated with physical memory, and the
others are empty.
Add support to FreeBSD for empty NUMA domains by:
- creating empty memory domains when parsing the SRAT table,
rather than failing to parse the table
- not running the pageout deamon threads in empty domains
- adding defensive code to UMA to avoid allocating from empty domains
- adding defensive code to cpuset to avoid binding to an empty domain
Thanks to Jeff for suggesting this strategy.
Reviewed by: alc, markj
Approved by: re (gjb@)
Differential Revision: https://reviews.freebsd.org/D1683
Expose these counters under the vm.domain sysctl node. The existing
vm.stats.vm.v_pdpages sysctl is preserved.
Reviewed by: alc (previous version)
Differential Revision: https://reviews.freebsd.org/D14666
Before r329882 the target would be computed after lowmem handlers run
and free pages. On some systems a significant amount of page
reclamation happens this way. However, with r329882 the target is
computed first, which can lead to unnecessary reclamation from the
page cache, and this in turn may result in excessive swapping.
Instead, adjust the target after running lowmem handlers. Don't
invoke the lowmem handlers before the PID controller, though, since
that would hide the true rate of page allocation.
Reviewed by: alc, kib (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16606
so that a reference from a concurrently destroyed mapping is observed
during the current scan.
Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16277
It serves little purpose after r308474 and r329882. As a side
effect, the removal fixes a bug in r329882 which caused the
page daemon to periodically invoke lowmem handlers even in the
absence of memory pressure.
Reviewed by: jeff
Differential Revision: https://reviews.freebsd.org/D15491
The scans are largely independent, so this helps make the code
marginally neater, and makes it easier to incorporate feedback from the
active queue scan into the page daemon control loop.
Improve some comments while here. No functional change intended.
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D15490
Such pages are dequeued as they're encountered during the inactive queue
scan, so by the time we get to the active queue scan, they should have
already been subtracted from the inactive queue length.
Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D15479
vm_page_queue(), added in r333256, generalizes vm_pageout_page_queued(),
so use it instead. No functional change intended.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D15402
With r332974, when performing a synchronized access of a page's "queue"
field, one must first check whether the page is logically dequeued. If
so, then the page lock does not prevent the page from being removed
from its page queue. Intoduce vm_page_queue(), which returns the page's
logical queue index. In some cases, direct access to the "queue" field
is still required, but such accesses should be confined to sys/vm.
Reported and tested by: pho
Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15280
Currently both the page lock and a page queue lock must be held in
order to enqueue, dequeue or requeue a page in a given page queue.
The queue locks are a scalability bottleneck in many workloads. This
change reduces page queue lock contention by batching queue operations.
To detangle the page and page queue locks, per-CPU batch queues are
used to reference pages with pending queue operations. The requested
operation is encoded in the page's aflags field with the page lock
held, after which the page is enqueued for a deferred batch operation.
Page queue scans are similarly optimized to minimize the amount of
work performed with a page queue lock held.
Reviewed by: kib, jeff (previous versions)
Tested by: pho
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14893
They were previously initialized by the corresponding page daemon
threads, but for vmd_inacthead this may be too late if
vm_page_deactivate_noreuse() is called during boot.
Reported and tested by: cperciva
Reviewed by: alc, kib
MFC after: 1 week
The division added in r331732 meant that we wouldn't attempt a
background laundering until at least v_free_target - v_free_min clean
pages had been freed by the page daemon since the last laundering. If
the inactive queue is depleted but not completely empty (e.g., because
it contains busy pages), it can thus take a long time to meet this
threshold. Restore the pre-r331732 behaviour of using a non-zero
background laundering threshold if at least one inactive queue scan has
elapsed since the last attempt at background laundering.
Submitted by: tijl (original version)
Rather than using the number of inactive queue scans as a metric for
how many clean pages are being freed by the page daemon, have the
page daemon keep a running counter of the number of pages it has freed,
and have the laundry thread use that when computing the background
laundering threshold.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D14884
vmd_free_count manipulation. Reduce the scope of the free lock by
using a pageout lock to synchronize sleep and wakeup. Only trigger
the pageout daemon on transitions between states. Drive all wakeup
operations directly as side-effects from freeing memory rather than
requiring an additional function call.
Reviewed by: markj, kib
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14612
Page daemon threads for other domains show up in ps(1) output as
"pagedaemon/domN", so let that be the case for domain 0 as well.
Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14518
With r329882, in the absence of a free page shortage we would only take
len(PQ_INACTIVE)+len(PQ_LAUNDRY) into account when deciding whether to
aggressively scan PQ_ACTIVE. Previously we would also include the
number of free pages in this computation, ensuring that we wouldn't scan
PQ_ACTIVE with plenty of free memory available. The change in behaviour
was most noticeable immediately after booting, when PQ_INACTIVE and
PQ_LAUNDRY are nearly empty.
Reviewed by: jeff
use it to regulate page daemon output.
This provides much smoother and more responsive page daemon output, anticipating
demand and avoiding pageout stalls by increasing the number of pages to match
the workload. This is a reimplementation of work done by myself and mlaier at
Isilon.
Reviewed by: bsdimp
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14402
Make vm_wait() take the vm_object argument which specifies the domain
set to wait for the min condition pass. If there is no object
associated with the wait, use curthread' policy domainset. The
mechanics of the wait in vm_wait() and vm_wait_domain() is supplied by
the new helper vm_wait_doms(), which directly takes the bitmask of the
domains to wait for passing min condition.
Eliminate pagedaemon_wait(). vm_domain_clear() handles the same
operations.
Eliminate VM_WAIT and VM_WAITPFAULT macros, the direct functions calls
are enough.
Eliminate several control state variables from vm_domain, unneeded
after the vm_wait() conversion.
Scetched and reviewed by: jeff
Tested by: pho
Sponsored by: The FreeBSD Foundation, Mellanox Technologies
Differential revision: https://reviews.freebsd.org/D14384
Previously, wiring a page would cause it to be removed from its page
queue. In the common case, unwiring causes it to be enqueued at the tail
of that page queue. This change modifies vm_page_wire() to not dequeue
the page, thus avoiding the highly contended page queue locks. Instead,
vm_page_unwire() takes care of requeuing the page as a single operation,
and the page daemon dequeues wired pages as they are encountered during
a queue scan to avoid needlessly revisiting them later. For pages in
PQ_ACTIVE we do even better, since a requeue is unnecessary.
The change improves scalability for some common workloads. For instance,
threads wiring pages into the buffer cache no longer need to modify
global page queues, and unwiring is usually done by the bufspace thread,
so concurrency is not as much of an issue. As another example, many
sysctl handlers wire the output buffer to avoid faults on copyout, and
since the buffer is likely to be in PQ_ACTIVE, we now entirely avoid
modifying the page queue in this case.
The change also adds a block comment describing some properties of
struct vm_page's reference counters, and the busy lock.
Reviewed by: jeff
Discussed with: alc, kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D11943
