2 * Copyright (c) 2004, 2005,
3 * Bosko Milekic <bmilekic@FreeBSD.org>. All rights reserved.
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
8 * 1. Redistributions of source code must retain the above copyright
9 * notice unmodified, this list of conditions and the following
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 #include <sys/cdefs.h>
29 __FBSDID("$FreeBSD$");
31 #include "opt_param.h"
33 #include <sys/param.h>
34 #include <sys/malloc.h>
35 #include <sys/types.h>
36 #include <sys/systm.h>
38 #include <sys/domain.h>
39 #include <sys/eventhandler.h>
40 #include <sys/kernel.h>
42 #include <sys/mutex.h>
43 #include <sys/protosw.h>
45 #include <sys/sysctl.h>
47 #include <security/mac/mac_framework.h>
50 #include <vm/vm_extern.h>
51 #include <vm/vm_kern.h>
52 #include <vm/vm_page.h>
53 #include <vm/vm_map.h>
55 #include <vm/uma_int.h>
56 #include <vm/uma_dbg.h>
59 * In FreeBSD, Mbufs and Mbuf Clusters are allocated from UMA
62 * Mbuf Clusters (2K, contiguous) are allocated from the Cluster
63 * Zone. The Zone can be capped at kern.ipc.nmbclusters, if the
64 * administrator so desires.
66 * Mbufs are allocated from a UMA Master Zone called the Mbuf
69 * Additionally, FreeBSD provides a Packet Zone, which it
70 * configures as a Secondary Zone to the Mbuf Master Zone,
71 * thus sharing backend Slab kegs with the Mbuf Master Zone.
73 * Thus common-case allocations and locking are simplified:
77 * | .------------>[(Packet Cache)] m_get(), m_gethdr()
79 * [(Cluster Cache)] [ Secondary ] [ (Mbuf Cache) ]
80 * [ Cluster Zone ] [ Zone ] [ Mbuf Master Zone ]
86 * \____________(VM)_________________/
89 * Whenever an object is allocated with uma_zalloc() out of
90 * one of the Zones its _ctor_ function is executed. The same
91 * for any deallocation through uma_zfree() the _dtor_ function
94 * Caches are per-CPU and are filled from the Master Zone.
96 * Whenever an object is allocated from the underlying global
97 * memory pool it gets pre-initialized with the _zinit_ functions.
98 * When the Keg's are overfull objects get decomissioned with
99 * _zfini_ functions and free'd back to the global memory pool.
103 int nmbufs; /* limits number of mbufs */
104 int nmbclusters; /* limits number of mbuf clusters */
105 int nmbjumbop; /* limits number of page size jumbo clusters */
106 int nmbjumbo9; /* limits number of 9k jumbo clusters */
107 int nmbjumbo16; /* limits number of 16k jumbo clusters */
109 static quad_t maxmbufmem; /* overall real memory limit for all mbufs */
111 SYSCTL_QUAD(_kern_ipc, OID_AUTO, maxmbufmem, CTLFLAG_RDTUN | CTLFLAG_NOFETCH, &maxmbufmem, 0,
112 "Maximum real memory allocatable to various mbuf types");
115 * tunable_mbinit() has to be run before any mbuf allocations are done.
118 tunable_mbinit(void *dummy)
123 * The default limit for all mbuf related memory is 1/2 of all
124 * available kernel memory (physical or kmem).
125 * At most it can be 3/4 of available kernel memory.
127 realmem = qmin((quad_t)physmem * PAGE_SIZE, vm_kmem_size);
128 maxmbufmem = realmem / 2;
129 TUNABLE_QUAD_FETCH("kern.ipc.maxmbufmem", &maxmbufmem);
130 if (maxmbufmem > realmem / 4 * 3)
131 maxmbufmem = realmem / 4 * 3;
133 TUNABLE_INT_FETCH("kern.ipc.nmbclusters", &nmbclusters);
134 if (nmbclusters == 0)
135 nmbclusters = maxmbufmem / MCLBYTES / 4;
137 TUNABLE_INT_FETCH("kern.ipc.nmbjumbop", &nmbjumbop);
139 nmbjumbop = maxmbufmem / MJUMPAGESIZE / 4;
141 TUNABLE_INT_FETCH("kern.ipc.nmbjumbo9", &nmbjumbo9);
143 nmbjumbo9 = maxmbufmem / MJUM9BYTES / 6;
145 TUNABLE_INT_FETCH("kern.ipc.nmbjumbo16", &nmbjumbo16);
147 nmbjumbo16 = maxmbufmem / MJUM16BYTES / 6;
150 * We need at least as many mbufs as we have clusters of
151 * the various types added together.
153 TUNABLE_INT_FETCH("kern.ipc.nmbufs", &nmbufs);
154 if (nmbufs < nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16)
155 nmbufs = lmax(maxmbufmem / MSIZE / 5,
156 nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16);
158 SYSINIT(tunable_mbinit, SI_SUB_KMEM, SI_ORDER_MIDDLE, tunable_mbinit, NULL);
161 sysctl_nmbclusters(SYSCTL_HANDLER_ARGS)
163 int error, newnmbclusters;
165 newnmbclusters = nmbclusters;
166 error = sysctl_handle_int(oidp, &newnmbclusters, 0, req);
167 if (error == 0 && req->newptr && newnmbclusters != nmbclusters) {
168 if (newnmbclusters > nmbclusters &&
169 nmbufs >= nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16) {
170 nmbclusters = newnmbclusters;
171 nmbclusters = uma_zone_set_max(zone_clust, nmbclusters);
172 EVENTHANDLER_INVOKE(nmbclusters_change);
178 SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbclusters, CTLTYPE_INT|CTLFLAG_RW,
179 &nmbclusters, 0, sysctl_nmbclusters, "IU",
180 "Maximum number of mbuf clusters allowed");
183 sysctl_nmbjumbop(SYSCTL_HANDLER_ARGS)
185 int error, newnmbjumbop;
187 newnmbjumbop = nmbjumbop;
188 error = sysctl_handle_int(oidp, &newnmbjumbop, 0, req);
189 if (error == 0 && req->newptr && newnmbjumbop != nmbjumbop) {
190 if (newnmbjumbop > nmbjumbop &&
191 nmbufs >= nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16) {
192 nmbjumbop = newnmbjumbop;
193 nmbjumbop = uma_zone_set_max(zone_jumbop, nmbjumbop);
199 SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbop, CTLTYPE_INT|CTLFLAG_RW,
200 &nmbjumbop, 0, sysctl_nmbjumbop, "IU",
201 "Maximum number of mbuf page size jumbo clusters allowed");
204 sysctl_nmbjumbo9(SYSCTL_HANDLER_ARGS)
206 int error, newnmbjumbo9;
208 newnmbjumbo9 = nmbjumbo9;
209 error = sysctl_handle_int(oidp, &newnmbjumbo9, 0, req);
210 if (error == 0 && req->newptr && newnmbjumbo9 != nmbjumbo9) {
211 if (newnmbjumbo9 > nmbjumbo9 &&
212 nmbufs >= nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16) {
213 nmbjumbo9 = newnmbjumbo9;
214 nmbjumbo9 = uma_zone_set_max(zone_jumbo9, nmbjumbo9);
220 SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbo9, CTLTYPE_INT|CTLFLAG_RW,
221 &nmbjumbo9, 0, sysctl_nmbjumbo9, "IU",
222 "Maximum number of mbuf 9k jumbo clusters allowed");
225 sysctl_nmbjumbo16(SYSCTL_HANDLER_ARGS)
227 int error, newnmbjumbo16;
229 newnmbjumbo16 = nmbjumbo16;
230 error = sysctl_handle_int(oidp, &newnmbjumbo16, 0, req);
231 if (error == 0 && req->newptr && newnmbjumbo16 != nmbjumbo16) {
232 if (newnmbjumbo16 > nmbjumbo16 &&
233 nmbufs >= nmbclusters + nmbjumbop + nmbjumbo9 + nmbjumbo16) {
234 nmbjumbo16 = newnmbjumbo16;
235 nmbjumbo16 = uma_zone_set_max(zone_jumbo16, nmbjumbo16);
241 SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbo16, CTLTYPE_INT|CTLFLAG_RW,
242 &nmbjumbo16, 0, sysctl_nmbjumbo16, "IU",
243 "Maximum number of mbuf 16k jumbo clusters allowed");
246 sysctl_nmbufs(SYSCTL_HANDLER_ARGS)
248 int error, newnmbufs;
251 error = sysctl_handle_int(oidp, &newnmbufs, 0, req);
252 if (error == 0 && req->newptr && newnmbufs != nmbufs) {
253 if (newnmbufs > nmbufs) {
255 nmbufs = uma_zone_set_max(zone_mbuf, nmbufs);
256 EVENTHANDLER_INVOKE(nmbufs_change);
262 SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbufs, CTLTYPE_INT|CTLFLAG_RW,
263 &nmbufs, 0, sysctl_nmbufs, "IU",
264 "Maximum number of mbufs allowed");
267 * Zones from which we allocate.
269 uma_zone_t zone_mbuf;
270 uma_zone_t zone_clust;
271 uma_zone_t zone_pack;
272 uma_zone_t zone_jumbop;
273 uma_zone_t zone_jumbo9;
274 uma_zone_t zone_jumbo16;
275 uma_zone_t zone_ext_refcnt;
278 * Callout to assist us in freeing mbufs.
280 static struct callout mb_reclaim_callout;
281 static struct mtx mb_reclaim_callout_mtx;
286 static int mb_ctor_mbuf(void *, int, void *, int);
287 static int mb_ctor_clust(void *, int, void *, int);
288 static int mb_ctor_pack(void *, int, void *, int);
289 static void mb_dtor_mbuf(void *, int, void *);
290 static void mb_dtor_clust(void *, int, void *);
291 static void mb_dtor_pack(void *, int, void *);
292 static int mb_zinit_pack(void *, int, int);
293 static void mb_zfini_pack(void *, int);
295 static void mb_reclaim(void *);
296 static void *mbuf_jumbo_alloc(uma_zone_t, vm_size_t, uint8_t *, int);
297 static void mb_maxaction(uma_zone_t);
299 /* Ensure that MSIZE is a power of 2. */
300 CTASSERT((((MSIZE - 1) ^ MSIZE) + 1) >> 1 == MSIZE);
303 * Initialize FreeBSD Network buffer allocation.
306 mbuf_init(void *dummy)
310 * Configure UMA zones for Mbufs, Clusters, and Packets.
312 zone_mbuf = uma_zcreate(MBUF_MEM_NAME, MSIZE,
313 mb_ctor_mbuf, mb_dtor_mbuf,
315 trash_init, trash_fini,
319 MSIZE - 1, UMA_ZONE_MAXBUCKET);
321 nmbufs = uma_zone_set_max(zone_mbuf, nmbufs);
322 uma_zone_set_warning(zone_mbuf, "kern.ipc.nmbufs limit reached");
323 uma_zone_set_maxaction(zone_mbuf, mb_maxaction);
325 zone_clust = uma_zcreate(MBUF_CLUSTER_MEM_NAME, MCLBYTES,
326 mb_ctor_clust, mb_dtor_clust,
328 trash_init, trash_fini,
332 UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
334 nmbclusters = uma_zone_set_max(zone_clust, nmbclusters);
335 uma_zone_set_warning(zone_clust, "kern.ipc.nmbclusters limit reached");
336 uma_zone_set_maxaction(zone_clust, mb_maxaction);
338 zone_pack = uma_zsecond_create(MBUF_PACKET_MEM_NAME, mb_ctor_pack,
339 mb_dtor_pack, mb_zinit_pack, mb_zfini_pack, zone_mbuf);
341 /* Make jumbo frame zone too. Page size, 9k and 16k. */
342 zone_jumbop = uma_zcreate(MBUF_JUMBOP_MEM_NAME, MJUMPAGESIZE,
343 mb_ctor_clust, mb_dtor_clust,
345 trash_init, trash_fini,
349 UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
351 nmbjumbop = uma_zone_set_max(zone_jumbop, nmbjumbop);
352 uma_zone_set_warning(zone_jumbop, "kern.ipc.nmbjumbop limit reached");
353 uma_zone_set_maxaction(zone_jumbop, mb_maxaction);
355 zone_jumbo9 = uma_zcreate(MBUF_JUMBO9_MEM_NAME, MJUM9BYTES,
356 mb_ctor_clust, mb_dtor_clust,
358 trash_init, trash_fini,
362 UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
363 uma_zone_set_allocf(zone_jumbo9, mbuf_jumbo_alloc);
365 nmbjumbo9 = uma_zone_set_max(zone_jumbo9, nmbjumbo9);
366 uma_zone_set_warning(zone_jumbo9, "kern.ipc.nmbjumbo9 limit reached");
367 uma_zone_set_maxaction(zone_jumbo9, mb_maxaction);
369 zone_jumbo16 = uma_zcreate(MBUF_JUMBO16_MEM_NAME, MJUM16BYTES,
370 mb_ctor_clust, mb_dtor_clust,
372 trash_init, trash_fini,
376 UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
377 uma_zone_set_allocf(zone_jumbo16, mbuf_jumbo_alloc);
379 nmbjumbo16 = uma_zone_set_max(zone_jumbo16, nmbjumbo16);
380 uma_zone_set_warning(zone_jumbo16, "kern.ipc.nmbjumbo16 limit reached");
381 uma_zone_set_maxaction(zone_jumbo16, mb_maxaction);
383 zone_ext_refcnt = uma_zcreate(MBUF_EXTREFCNT_MEM_NAME, sizeof(u_int),
386 UMA_ALIGN_PTR, UMA_ZONE_ZINIT);
388 /* uma_prealloc() goes here... */
390 /* Initialize the mb_reclaim() callout. */
391 mtx_init(&mb_reclaim_callout_mtx, "mb_reclaim_callout_mtx", NULL,
393 callout_init(&mb_reclaim_callout, 1);
396 * Hook event handler for low-memory situation, used to
397 * drain protocols and push data back to the caches (UMA
398 * later pushes it back to VM).
400 EVENTHANDLER_REGISTER(vm_lowmem, mb_reclaim, NULL,
401 EVENTHANDLER_PRI_FIRST);
403 SYSINIT(mbuf, SI_SUB_MBUF, SI_ORDER_FIRST, mbuf_init, NULL);
406 * UMA backend page allocator for the jumbo frame zones.
408 * Allocates kernel virtual memory that is backed by contiguous physical
412 mbuf_jumbo_alloc(uma_zone_t zone, vm_size_t bytes, uint8_t *flags, int wait)
415 /* Inform UMA that this allocator uses kernel_map/object. */
416 *flags = UMA_SLAB_KERNEL;
417 return ((void *)kmem_alloc_contig(kernel_arena, bytes, wait,
418 (vm_paddr_t)0, ~(vm_paddr_t)0, 1, 0, VM_MEMATTR_DEFAULT));
422 * Constructor for Mbuf master zone.
424 * The 'arg' pointer points to a mb_args structure which
425 * contains call-specific information required to support the
426 * mbuf allocation API. See mbuf.h.
429 mb_ctor_mbuf(void *mem, int size, void *arg, int how)
432 struct mb_args *args;
438 trash_ctor(mem, size, arg, how);
440 args = (struct mb_args *)arg;
444 * The mbuf is initialized later. The caller has the
445 * responsibility to set up any MAC labels too.
447 if (type == MT_NOINIT)
450 m = (struct mbuf *)mem;
453 error = m_init(m, NULL, size, how, type, flags);
459 * The Mbuf master zone destructor.
462 mb_dtor_mbuf(void *mem, int size, void *arg)
467 m = (struct mbuf *)mem;
468 flags = (unsigned long)arg;
470 KASSERT((m->m_flags & M_NOFREE) == 0, ("%s: M_NOFREE set", __func__));
471 if ((m->m_flags & M_PKTHDR) && !SLIST_EMPTY(&m->m_pkthdr.tags))
472 m_tag_delete_chain(m, NULL);
474 trash_dtor(mem, size, arg);
479 * The Mbuf Packet zone destructor.
482 mb_dtor_pack(void *mem, int size, void *arg)
486 m = (struct mbuf *)mem;
487 if ((m->m_flags & M_PKTHDR) != 0)
488 m_tag_delete_chain(m, NULL);
490 /* Make sure we've got a clean cluster back. */
491 KASSERT((m->m_flags & M_EXT) == M_EXT, ("%s: M_EXT not set", __func__));
492 KASSERT(m->m_ext.ext_buf != NULL, ("%s: ext_buf == NULL", __func__));
493 KASSERT(m->m_ext.ext_free == NULL, ("%s: ext_free != NULL", __func__));
494 KASSERT(m->m_ext.ext_arg1 == NULL, ("%s: ext_arg1 != NULL", __func__));
495 KASSERT(m->m_ext.ext_arg2 == NULL, ("%s: ext_arg2 != NULL", __func__));
496 KASSERT(m->m_ext.ext_size == MCLBYTES, ("%s: ext_size != MCLBYTES", __func__));
497 KASSERT(m->m_ext.ext_type == EXT_PACKET, ("%s: ext_type != EXT_PACKET", __func__));
498 KASSERT(*m->m_ext.ext_cnt == 1, ("%s: ext_cnt != 1", __func__));
500 trash_dtor(m->m_ext.ext_buf, MCLBYTES, arg);
503 * If there are processes blocked on zone_clust, waiting for pages
504 * to be freed up, * cause them to be woken up by draining the
505 * packet zone. We are exposed to a race here * (in the check for
506 * the UMA_ZFLAG_FULL) where we might miss the flag set, but that
507 * is deliberate. We don't want to acquire the zone lock for every
510 if (uma_zone_exhausted_nolock(zone_clust))
511 zone_drain(zone_pack);
515 * The Cluster and Jumbo[PAGESIZE|9|16] zone constructor.
517 * Here the 'arg' pointer points to the Mbuf which we
518 * are configuring cluster storage for. If 'arg' is
519 * empty we allocate just the cluster without setting
520 * the mbuf to it. See mbuf.h.
523 mb_ctor_clust(void *mem, int size, void *arg, int how)
531 trash_ctor(mem, size, arg, how);
538 #if MJUMPAGESIZE != MCLBYTES
553 panic("unknown cluster size");
557 m = (struct mbuf *)arg;
558 refcnt = uma_find_refcnt(zone, mem);
561 m->m_ext.ext_buf = (caddr_t)mem;
562 m->m_data = m->m_ext.ext_buf;
564 m->m_ext.ext_free = NULL;
565 m->m_ext.ext_arg1 = NULL;
566 m->m_ext.ext_arg2 = NULL;
567 m->m_ext.ext_size = size;
568 m->m_ext.ext_type = type;
569 m->m_ext.ext_flags = 0;
570 m->m_ext.ext_cnt = refcnt;
577 * The Mbuf Cluster zone destructor.
580 mb_dtor_clust(void *mem, int size, void *arg)
585 zone = m_getzone(size);
586 KASSERT(*(uma_find_refcnt(zone, mem)) <= 1,
587 ("%s: refcnt incorrect %u", __func__,
588 *(uma_find_refcnt(zone, mem))) );
590 trash_dtor(mem, size, arg);
595 * The Packet secondary zone's init routine, executed on the
596 * object's transition from mbuf keg slab to zone cache.
599 mb_zinit_pack(void *mem, int size, int how)
603 m = (struct mbuf *)mem; /* m is virgin. */
604 if (uma_zalloc_arg(zone_clust, m, how) == NULL ||
605 m->m_ext.ext_buf == NULL)
607 m->m_ext.ext_type = EXT_PACKET; /* Override. */
609 trash_init(m->m_ext.ext_buf, MCLBYTES, how);
615 * The Packet secondary zone's fini routine, executed on the
616 * object's transition from zone cache to keg slab.
619 mb_zfini_pack(void *mem, int size)
623 m = (struct mbuf *)mem;
625 trash_fini(m->m_ext.ext_buf, MCLBYTES);
627 uma_zfree_arg(zone_clust, m->m_ext.ext_buf, NULL);
629 trash_dtor(mem, size, NULL);
634 * The "packet" keg constructor.
637 mb_ctor_pack(void *mem, int size, void *arg, int how)
640 struct mb_args *args;
644 m = (struct mbuf *)mem;
645 args = (struct mb_args *)arg;
650 trash_ctor(m->m_ext.ext_buf, MCLBYTES, arg, how);
653 error = m_init(m, NULL, size, how, type, flags);
655 /* m_ext is already initialized. */
656 m->m_data = m->m_ext.ext_buf;
657 m->m_flags = (flags | M_EXT);
663 m_pkthdr_init(struct mbuf *m, int how)
668 m->m_data = m->m_pktdat;
669 bzero(&m->m_pkthdr, sizeof(m->m_pkthdr));
671 /* If the label init fails, fail the alloc */
672 error = mac_mbuf_init(m, how);
681 * This is the protocol drain routine.
683 * No locks should be held when this is called. The drain routines have to
684 * presently acquire some locks which raises the possibility of lock order
688 mb_reclaim(void *junk)
693 WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK | WARN_PANIC, NULL,
696 for (dp = domains; dp != NULL; dp = dp->dom_next)
697 for (pr = dp->dom_protosw; pr < dp->dom_protoswNPROTOSW; pr++)
698 if (pr->pr_drain != NULL)
703 * This is the function called by the mb_reclaim_callout, which is
704 * used when we hit the maximum for a zone.
706 * (See mb_maxaction() below.)
709 mb_reclaim_timer(void *junk __unused)
712 mtx_lock(&mb_reclaim_callout_mtx);
715 * Avoid running this function extra times by skipping this invocation
716 * if the callout has already been rescheduled.
718 if (callout_pending(&mb_reclaim_callout) ||
719 !callout_active(&mb_reclaim_callout)) {
720 mtx_unlock(&mb_reclaim_callout_mtx);
723 mtx_unlock(&mb_reclaim_callout_mtx);
727 mtx_lock(&mb_reclaim_callout_mtx);
728 callout_deactivate(&mb_reclaim_callout);
729 mtx_unlock(&mb_reclaim_callout_mtx);
733 * This function is called when we hit the maximum for a zone.
735 * At that point, we want to call the protocol drain routine to free up some
736 * mbufs. However, we will use the callout routines to schedule this to
737 * occur in another thread. (The thread calling this function holds the
741 mb_maxaction(uma_zone_t zone __unused)
745 * If we can't immediately obtain the lock, either the callout
746 * is currently running, or another thread is scheduling the
749 if (!mtx_trylock(&mb_reclaim_callout_mtx))
752 /* If not already scheduled/running, schedule the callout. */
753 if (!callout_active(&mb_reclaim_callout)) {
754 callout_reset(&mb_reclaim_callout, 1, mb_reclaim_timer, NULL);
757 mtx_unlock(&mb_reclaim_callout_mtx);