FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_mbuf.c

     /*-
      * Copyright (c) 2004, 2005,
      *      Bosko Milekic <bmilekic@FreeBSD.org>.  All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
      *    notice unmodified, this list of conditions and the following
     *    disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_mac.h"
    #include "opt_param.h"
    
    #include <sys/param.h>
    #include <sys/malloc.h>
    #include <sys/systm.h>
    #include <sys/mbuf.h>
    #include <sys/domain.h>
    #include <sys/eventhandler.h>
    #include <sys/kernel.h>
    #include <sys/protosw.h>
    #include <sys/smp.h>
    #include <sys/sysctl.h>
    
    #include <security/mac/mac_framework.h>
    
    #include <vm/vm.h>
    #include <vm/vm_page.h>
    #include <vm/uma.h>
    #include <vm/uma_int.h>
    #include <vm/uma_dbg.h>
    
    /*
     * In FreeBSD, Mbufs and Mbuf Clusters are allocated from UMA
     * Zones.
     *
     * Mbuf Clusters (2K, contiguous) are allocated from the Cluster
     * Zone.  The Zone can be capped at kern.ipc.nmbclusters, if the
     * administrator so desires.
     *
     * Mbufs are allocated from a UMA Master Zone called the Mbuf
     * Zone.
     *
     * Additionally, FreeBSD provides a Packet Zone, which it
     * configures as a Secondary Zone to the Mbuf Master Zone,
     * thus sharing backend Slab kegs with the Mbuf Master Zone.
     *
     * Thus common-case allocations and locking are simplified:
     *
     *  m_clget()                m_getcl()
     *    |                         |
     *    |   .------------>[(Packet Cache)]    m_get(), m_gethdr()
     *    |   |             [     Packet   ]            |
     *  [(Cluster Cache)]   [    Secondary ]   [ (Mbuf Cache)     ]
     *  [ Cluster Zone  ]   [     Zone     ]   [ Mbuf Master Zone ]
     *        |                       \________         |
     *  [ Cluster Keg   ]                      \       /
     *        |                              [ Mbuf Keg   ]
     *  [ Cluster Slabs ]                         |
     *        |                              [ Mbuf Slabs ]
     *         \____________(VM)_________________/
     *
     *
     * Whenever an object is allocated with uma_zalloc() out of
     * one of the Zones its _ctor_ function is executed.  The same
     * for any deallocation through uma_zfree() the _dtor_ function
     * is executed.
     *
     * Caches are per-CPU and are filled from the Master Zone.
     *
     * Whenever an object is allocated from the underlying global
     * memory pool it gets pre-initialized with the _zinit_ functions.
     * When the Keg's are overfull objects get decomissioned with
     * _zfini_ functions and free'd back to the global memory pool.
     *
     */
    
    int nmbclusters;                /* limits number of mbuf clusters */
    int nmbjumbop;                  /* limits number of page size jumbo clusters */
   int nmbjumbo9;                  /* limits number of 9k jumbo clusters */
   int nmbjumbo16;                 /* limits number of 16k jumbo clusters */
   struct mbstat mbstat;
   
   static void
   tunable_mbinit(void *dummy)
   {
   
           /* This has to be done before VM init. */
           nmbclusters = 1024 + maxusers * 64;
           nmbjumbop = nmbclusters / 2;
           nmbjumbo9 = nmbjumbop / 2;
           nmbjumbo16 = nmbjumbo9 / 2;
           TUNABLE_INT_FETCH("kern.ipc.nmbclusters", &nmbclusters);
   }
   SYSINIT(tunable_mbinit, SI_SUB_TUNABLES, SI_ORDER_ANY, tunable_mbinit, NULL);
   
   /* XXX: These should be tuneables. Can't change UMA limits on the fly. */
   static int
   sysctl_nmbclusters(SYSCTL_HANDLER_ARGS)
   {
           int error, newnmbclusters;
   
           newnmbclusters = nmbclusters;
           error = sysctl_handle_int(oidp, &newnmbclusters, 0, req); 
           if (error == 0 && req->newptr) {
                   if (newnmbclusters > nmbclusters) {
                           nmbclusters = newnmbclusters;
                           uma_zone_set_max(zone_clust, nmbclusters);
                           EVENTHANDLER_INVOKE(nmbclusters_change);
                   } else
                           error = EINVAL;
           }
           return (error);
   }
   SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbclusters, CTLTYPE_INT|CTLFLAG_RW,
   &nmbclusters, 0, sysctl_nmbclusters, "IU",
       "Maximum number of mbuf clusters allowed");
   
   static int
   sysctl_nmbjumbop(SYSCTL_HANDLER_ARGS)
   {
           int error, newnmbjumbop;
   
           newnmbjumbop = nmbjumbop;
           error = sysctl_handle_int(oidp, &newnmbjumbop, 0, req); 
           if (error == 0 && req->newptr) {
                   if (newnmbjumbop> nmbjumbop) {
                           nmbjumbop = newnmbjumbop;
                           uma_zone_set_max(zone_jumbop, nmbjumbop);
                   } else
                           error = EINVAL;
           }
           return (error);
   }
   SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbop, CTLTYPE_INT|CTLFLAG_RW,
   &nmbjumbop, 0, sysctl_nmbjumbop, "IU",
            "Maximum number of mbuf page size jumbo clusters allowed");
   
   
   static int
   sysctl_nmbjumbo9(SYSCTL_HANDLER_ARGS)
   {
           int error, newnmbjumbo9;
   
           newnmbjumbo9 = nmbjumbo9;
           error = sysctl_handle_int(oidp, &newnmbjumbo9, 0, req); 
           if (error == 0 && req->newptr) {
                   if (newnmbjumbo9> nmbjumbo9) {
                           nmbjumbo9 = newnmbjumbo9;
                           uma_zone_set_max(zone_jumbo9, nmbjumbo9);
                   } else
                           error = EINVAL;
           }
           return (error);
   }
   SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbo9, CTLTYPE_INT|CTLFLAG_RW,
   &nmbjumbo9, 0, sysctl_nmbjumbo9, "IU",
           "Maximum number of mbuf 9k jumbo clusters allowed"); 
   
   static int
   sysctl_nmbjumbo16(SYSCTL_HANDLER_ARGS)
   {
           int error, newnmbjumbo16;
   
           newnmbjumbo16 = nmbjumbo16;
           error = sysctl_handle_int(oidp, &newnmbjumbo16, 0, req); 
           if (error == 0 && req->newptr) {
                   if (newnmbjumbo16> nmbjumbo16) {
                           nmbjumbo16 = newnmbjumbo16;
                           uma_zone_set_max(zone_jumbo16, nmbjumbo16);
                   } else
                           error = EINVAL;
           }
           return (error);
   }
   SYSCTL_PROC(_kern_ipc, OID_AUTO, nmbjumbo16, CTLTYPE_INT|CTLFLAG_RW,
   &nmbjumbo16, 0, sysctl_nmbjumbo16, "IU",
       "Maximum number of mbuf 16k jumbo clusters allowed");
   
   
   
   SYSCTL_STRUCT(_kern_ipc, OID_AUTO, mbstat, CTLFLAG_RD, &mbstat, mbstat,
       "Mbuf general information and statistics");
   
   /*
    * Zones from which we allocate.
    */
   uma_zone_t      zone_mbuf;
   uma_zone_t      zone_clust;
   uma_zone_t      zone_pack;
   uma_zone_t      zone_jumbop;
   uma_zone_t      zone_jumbo9;
   uma_zone_t      zone_jumbo16;
   uma_zone_t      zone_ext_refcnt;
   
   /*
    * Local prototypes.
    */
   static int      mb_ctor_mbuf(void *, int, void *, int);
   static int      mb_ctor_clust(void *, int, void *, int);
   static int      mb_ctor_pack(void *, int, void *, int);
   static void     mb_dtor_mbuf(void *, int, void *);
   static void     mb_dtor_clust(void *, int, void *);
   static void     mb_dtor_pack(void *, int, void *);
   static int      mb_zinit_pack(void *, int, int);
   static void     mb_zfini_pack(void *, int);
   
   static void     mb_reclaim(void *);
   static void     mbuf_init(void *);
   static void    *mbuf_jumbo_alloc(uma_zone_t, int, u_int8_t *, int);
   static void     mbuf_jumbo_free(void *, int, u_int8_t);
   
   static MALLOC_DEFINE(M_JUMBOFRAME, "jumboframes", "mbuf jumbo frame buffers");
   
   /* Ensure that MSIZE doesn't break dtom() - it must be a power of 2 */
   CTASSERT((((MSIZE - 1) ^ MSIZE) + 1) >> 1 == MSIZE);
   
   /*
    * Initialize FreeBSD Network buffer allocation.
    */
   SYSINIT(mbuf, SI_SUB_MBUF, SI_ORDER_FIRST, mbuf_init, NULL);
   static void
   mbuf_init(void *dummy)
   {
   
           /*
            * Configure UMA zones for Mbufs, Clusters, and Packets.
            */
           zone_mbuf = uma_zcreate(MBUF_MEM_NAME, MSIZE,
               mb_ctor_mbuf, mb_dtor_mbuf,
   #ifdef INVARIANTS
               trash_init, trash_fini,
   #else
               NULL, NULL,
   #endif
               MSIZE - 1, UMA_ZONE_MAXBUCKET);
   
           zone_clust = uma_zcreate(MBUF_CLUSTER_MEM_NAME, MCLBYTES,
               mb_ctor_clust, mb_dtor_clust,
   #ifdef INVARIANTS
               trash_init, trash_fini,
   #else
               NULL, NULL,
   #endif
               UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
           if (nmbclusters > 0)
                   uma_zone_set_max(zone_clust, nmbclusters);
   
           zone_pack = uma_zsecond_create(MBUF_PACKET_MEM_NAME, mb_ctor_pack,
               mb_dtor_pack, mb_zinit_pack, mb_zfini_pack, zone_mbuf);
   
           /* Make jumbo frame zone too. Page size, 9k and 16k. */
           zone_jumbop = uma_zcreate(MBUF_JUMBOP_MEM_NAME, MJUMPAGESIZE,
               mb_ctor_clust, mb_dtor_clust,
   #ifdef INVARIANTS
               trash_init, trash_fini,
   #else
               NULL, NULL,
   #endif
               UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
           if (nmbjumbop > 0)
                   uma_zone_set_max(zone_jumbop, nmbjumbop);
   
           zone_jumbo9 = uma_zcreate(MBUF_JUMBO9_MEM_NAME, MJUM9BYTES,
               mb_ctor_clust, mb_dtor_clust,
   #ifdef INVARIANTS
               trash_init, trash_fini,
   #else
               NULL, NULL,
   #endif
               UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
           if (nmbjumbo9 > 0)
                   uma_zone_set_max(zone_jumbo9, nmbjumbo9);
           uma_zone_set_allocf(zone_jumbo9, mbuf_jumbo_alloc);
           uma_zone_set_freef(zone_jumbo9, mbuf_jumbo_free);
   
           zone_jumbo16 = uma_zcreate(MBUF_JUMBO16_MEM_NAME, MJUM16BYTES,
               mb_ctor_clust, mb_dtor_clust,
   #ifdef INVARIANTS
               trash_init, trash_fini,
   #else
               NULL, NULL,
   #endif
               UMA_ALIGN_PTR, UMA_ZONE_REFCNT);
           if (nmbjumbo16 > 0)
                   uma_zone_set_max(zone_jumbo16, nmbjumbo16);
           uma_zone_set_allocf(zone_jumbo16, mbuf_jumbo_alloc);
           uma_zone_set_freef(zone_jumbo16, mbuf_jumbo_free);
   
           zone_ext_refcnt = uma_zcreate(MBUF_EXTREFCNT_MEM_NAME, sizeof(u_int),
               NULL, NULL,
               NULL, NULL,
               UMA_ALIGN_PTR, UMA_ZONE_ZINIT);
   
           /* uma_prealloc() goes here... */
   
           /*
            * Hook event handler for low-memory situation, used to
            * drain protocols and push data back to the caches (UMA
            * later pushes it back to VM).
            */
           EVENTHANDLER_REGISTER(vm_lowmem, mb_reclaim, NULL,
               EVENTHANDLER_PRI_FIRST);
   
           /*
            * [Re]set counters and local statistics knobs.
            * XXX Some of these should go and be replaced, but UMA stat
            * gathering needs to be revised.
            */
           mbstat.m_mbufs = 0;
           mbstat.m_mclusts = 0;
           mbstat.m_drain = 0;
           mbstat.m_msize = MSIZE;
           mbstat.m_mclbytes = MCLBYTES;
           mbstat.m_minclsize = MINCLSIZE;
           mbstat.m_mlen = MLEN;
           mbstat.m_mhlen = MHLEN;
           mbstat.m_numtypes = MT_NTYPES;
   
           mbstat.m_mcfail = mbstat.m_mpfail = 0;
           mbstat.sf_iocnt = 0;
           mbstat.sf_allocwait = mbstat.sf_allocfail = 0;
   }
   
   /*
    * UMA backend page allocator for the jumbo frame zones.
    *
    * Allocates kernel virtual memory that is backed by contiguous physical
    * pages.
    */
   static void *
   mbuf_jumbo_alloc(uma_zone_t zone, int bytes, u_int8_t *flags, int wait)
   {
   
           /* Inform UMA that this allocator uses kernel_map/object. */
           *flags = UMA_SLAB_KERNEL;
           return (contigmalloc(bytes, M_JUMBOFRAME, wait, (vm_paddr_t)0,
               ~(vm_paddr_t)0, 1, 0));
   }
   
   /*
    * UMA backend page deallocator for the jumbo frame zones.
    */
   static void
   mbuf_jumbo_free(void *mem, int size, u_int8_t flags)
   {
   
           contigfree(mem, size, M_JUMBOFRAME);
   }
   
   /*
    * Constructor for Mbuf master zone.
    *
    * The 'arg' pointer points to a mb_args structure which
    * contains call-specific information required to support the
    * mbuf allocation API.  See mbuf.h.
    */
   static int
   mb_ctor_mbuf(void *mem, int size, void *arg, int how)
   {
           struct mbuf *m;
           struct mb_args *args;
   #ifdef MAC
           int error;
   #endif
           int flags;
           short type;
   
   #ifdef INVARIANTS
           trash_ctor(mem, size, arg, how);
   #endif
           m = (struct mbuf *)mem;
           args = (struct mb_args *)arg;
           flags = args->flags;
           type = args->type;
   
           /*
            * The mbuf is initialized later.  The caller has the
            * responsibility to set up any MAC labels too.
            */
           if (type == MT_NOINIT)
                   return (0);
   
           m->m_next = NULL;
           m->m_nextpkt = NULL;
           m->m_len = 0;
           m->m_flags = flags;
           m->m_type = type;
           if (flags & M_PKTHDR) {
                   m->m_data = m->m_pktdat;
                   m->m_pkthdr.rcvif = NULL;
                   m->m_pkthdr.len = 0;
                   m->m_pkthdr.header = NULL;
                   m->m_pkthdr.csum_flags = 0;
                   m->m_pkthdr.csum_data = 0;
                   m->m_pkthdr.tso_segsz = 0;
                   m->m_pkthdr.ether_vtag = 0;
                   SLIST_INIT(&m->m_pkthdr.tags);
   #ifdef MAC
                   /* If the label init fails, fail the alloc */
                   error = mac_init_mbuf(m, how);
                   if (error)
                           return (error);
   #endif
           } else
                   m->m_data = m->m_dat;
           return (0);
   }
   
   /*
    * The Mbuf master zone destructor.
    */
   static void
   mb_dtor_mbuf(void *mem, int size, void *arg)
   {
           struct mbuf *m;
           unsigned long flags; 
   
           m = (struct mbuf *)mem;
           flags = (unsigned long)arg;
           
           if ((flags & MB_NOTAGS) == 0 && (m->m_flags & M_PKTHDR) != 0)
                   m_tag_delete_chain(m, NULL);
           KASSERT((m->m_flags & M_EXT) == 0, ("%s: M_EXT set", __func__));
           KASSERT((m->m_flags & M_NOFREE) == 0, ("%s: M_NOFREE set", __func__));  
   #ifdef INVARIANTS
           trash_dtor(mem, size, arg);
   #endif
   }
   
   /*
    * The Mbuf Packet zone destructor.
    */
   static void
   mb_dtor_pack(void *mem, int size, void *arg)
   {
           struct mbuf *m;
   
           m = (struct mbuf *)mem;
           if ((m->m_flags & M_PKTHDR) != 0)
                   m_tag_delete_chain(m, NULL);
   
           /* Make sure we've got a clean cluster back. */
           KASSERT((m->m_flags & M_EXT) == M_EXT, ("%s: M_EXT not set", __func__));
           KASSERT(m->m_ext.ext_buf != NULL, ("%s: ext_buf == NULL", __func__));
           KASSERT(m->m_ext.ext_free == NULL, ("%s: ext_free != NULL", __func__));
           KASSERT(m->m_ext.ext_args == NULL, ("%s: ext_args != NULL", __func__));
           KASSERT(m->m_ext.ext_size == MCLBYTES, ("%s: ext_size != MCLBYTES", __func__));
           KASSERT(m->m_ext.ext_type == EXT_PACKET, ("%s: ext_type != EXT_PACKET", __func__));
           KASSERT(*m->m_ext.ref_cnt == 1, ("%s: ref_cnt != 1", __func__));
   #ifdef INVARIANTS
           trash_dtor(m->m_ext.ext_buf, MCLBYTES, arg);
   #endif
           /*
            * If there are processes blocked on zone_clust, waiting for pages to be freed up,
            * cause them to be woken up by draining the packet zone. We are exposed to a race here 
            * (in the check for the UMA_ZFLAG_FULL) where we might miss the flag set, but that is 
            * deliberate. We don't want to acquire the zone lock for every mbuf free.
            */
           if (uma_zone_exhausted_nolock(zone_clust))
                   zone_drain(zone_pack);
   }
   
   /*
    * The Cluster and Jumbo[PAGESIZE|9|16] zone constructor.
    *
    * Here the 'arg' pointer points to the Mbuf which we
    * are configuring cluster storage for.  If 'arg' is
    * empty we allocate just the cluster without setting
    * the mbuf to it.  See mbuf.h.
    */
   static int
   mb_ctor_clust(void *mem, int size, void *arg, int how)
   {
           struct mbuf *m;
           u_int *refcnt;
           int type;
           uma_zone_t zone;
           
   #ifdef INVARIANTS
           trash_ctor(mem, size, arg, how);
   #endif
           switch (size) {
           case MCLBYTES:
                   type = EXT_CLUSTER;
                   zone = zone_clust;
                   break;
   #if MJUMPAGESIZE != MCLBYTES
           case MJUMPAGESIZE:
                   type = EXT_JUMBOP;
                   zone = zone_jumbop;
                   break;
   #endif
           case MJUM9BYTES:
                   type = EXT_JUMBO9;
                   zone = zone_jumbo9;
                   break;
           case MJUM16BYTES:
                   type = EXT_JUMBO16;
                   zone = zone_jumbo16;
                   break;
           default:
                   panic("unknown cluster size");
                   break;
           }
   
           m = (struct mbuf *)arg;
           refcnt = uma_find_refcnt(zone, mem);
           *refcnt = 1;                    
           if (m != NULL) {
                   m->m_ext.ext_buf = (caddr_t)mem;
                   m->m_data = m->m_ext.ext_buf;
                   m->m_flags |= M_EXT;
                   m->m_ext.ext_free = NULL;
                   m->m_ext.ext_args = NULL;
                   m->m_ext.ext_size = size;
                   m->m_ext.ext_type = type;
                   m->m_ext.ref_cnt = refcnt;
           }
   
           return (0);
   }
   
   /*
    * The Mbuf Cluster zone destructor.
    */
   static void
   mb_dtor_clust(void *mem, int size, void *arg)
   {
   #ifdef INVARIANTS
           uma_zone_t zone;
   
           zone = m_getzone(size);
           KASSERT(*(uma_find_refcnt(zone, mem)) <= 1,
                   ("%s: refcnt incorrect %u", __func__,
                    *(uma_find_refcnt(zone, mem))) );
   
           trash_dtor(mem, size, arg);
   #endif
   }
   
   /*
    * The Packet secondary zone's init routine, executed on the
    * object's transition from mbuf keg slab to zone cache.
    */
   static int
   mb_zinit_pack(void *mem, int size, int how)
   {
           struct mbuf *m;
   
           m = (struct mbuf *)mem;         /* m is virgin. */
           if (uma_zalloc_arg(zone_clust, m, how) == NULL ||
               m->m_ext.ext_buf == NULL)
                   return (ENOMEM);
           m->m_ext.ext_type = EXT_PACKET; /* Override. */
   #ifdef INVARIANTS
           trash_init(m->m_ext.ext_buf, MCLBYTES, how);
   #endif
           return (0);
   }
   
   /*
    * The Packet secondary zone's fini routine, executed on the
    * object's transition from zone cache to keg slab.
    */
   static void
   mb_zfini_pack(void *mem, int size)
   {
           struct mbuf *m;
   
           m = (struct mbuf *)mem;
   #ifdef INVARIANTS
           trash_fini(m->m_ext.ext_buf, MCLBYTES);
   #endif
           uma_zfree_arg(zone_clust, m->m_ext.ext_buf, NULL);
   #ifdef INVARIANTS
           trash_dtor(mem, size, NULL);
   #endif
   }
   
   /*
    * The "packet" keg constructor.
    */
   static int
   mb_ctor_pack(void *mem, int size, void *arg, int how)
   {
           struct mbuf *m;
           struct mb_args *args;
   #ifdef MAC
           int error;
   #endif
           int flags;
           short type;
   
           m = (struct mbuf *)mem;
           args = (struct mb_args *)arg;
           flags = args->flags;
           type = args->type;
   
   #ifdef INVARIANTS
           trash_ctor(m->m_ext.ext_buf, MCLBYTES, arg, how);
   #endif
           m->m_next = NULL;
           m->m_nextpkt = NULL;
           m->m_data = m->m_ext.ext_buf;
           m->m_len = 0;
           m->m_flags = (flags | M_EXT);
           m->m_type = type;
               
           if (flags & M_PKTHDR) {
                   m->m_pkthdr.rcvif = NULL;
                   m->m_pkthdr.len = 0;
                   m->m_pkthdr.header = NULL;
                   m->m_pkthdr.csum_flags = 0;
                   m->m_pkthdr.csum_data = 0;
                   m->m_pkthdr.tso_segsz = 0;
                   m->m_pkthdr.ether_vtag = 0;
                   SLIST_INIT(&m->m_pkthdr.tags);
   #ifdef MAC
                   /* If the label init fails, fail the alloc */
                   error = mac_init_mbuf(m, how);
                   if (error)
                           return (error);
   #endif
           }
           /* m_ext is already initialized. */
   
           return (0);
   }
   
   /*
    * This is the protocol drain routine.
    *
    * No locks should be held when this is called.  The drain routines have to
    * presently acquire some locks which raises the possibility of lock order
    * reversal.
    */
   static void
   mb_reclaim(void *junk)
   {
           struct domain *dp;
           struct protosw *pr;
   
           WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK | WARN_PANIC, NULL,
               "mb_reclaim()");
   
           for (dp = domains; dp != NULL; dp = dp->dom_next)
                   for (pr = dp->dom_protosw; pr < dp->dom_protoswNPROTOSW; pr++)
                           if (pr->pr_drain != NULL)
                                   (*pr->pr_drain)();
   }

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