FreeBSD/Linux Kernel Cross Reference
sys/vm/vnode_pager.c

     /*
      * (MPSAFE)
      *
      * Copyright (c) 1990 University of Utah.
      * Copyright (c) 1991 The Regents of the University of California.
      * All rights reserved.
      * Copyright (c) 1993, 1994 John S. Dyson
      * Copyright (c) 1995, David Greenman
      *
     * This code is derived from software contributed to Berkeley by
     * the Systems Programming Group of the University of Utah Computer
     * Science Department.
     *
     * 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, 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.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
     *
     *      from: @(#)vnode_pager.c 7.5 (Berkeley) 4/20/91
     * $FreeBSD: src/sys/vm/vnode_pager.c,v 1.116.2.7 2002/12/31 09:34:51 dillon Exp $
     */
    
    /*
     * Page to/from files (vnodes).
     */
    
    /*
     * TODO:
     *      Implement VOP_GETPAGES/PUTPAGES interface for filesystems. Will
     *      greatly re-simplify the vnode_pager.
     */
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/proc.h>
    #include <sys/vnode.h>
    #include <sys/mount.h>
    #include <sys/buf.h>
    #include <sys/vmmeter.h>
    #include <sys/conf.h>
    
    #include <cpu/lwbuf.h>
    
    #include <vm/vm.h>
    #include <vm/vm_object.h>
    #include <vm/vm_page.h>
    #include <vm/vm_pager.h>
    #include <vm/vm_map.h>
    #include <vm/vnode_pager.h>
    #include <vm/swap_pager.h>
    #include <vm/vm_extern.h>
    
    #include <sys/thread2.h>
    #include <vm/vm_page2.h>
    
    static void vnode_pager_dealloc (vm_object_t);
    static int vnode_pager_getpage (vm_object_t, vm_page_t *, int);
    static void vnode_pager_putpages (vm_object_t, vm_page_t *, int, boolean_t, int *);
    static boolean_t vnode_pager_haspage (vm_object_t, vm_pindex_t);
    
    struct pagerops vnodepagerops = {
            vnode_pager_dealloc,
            vnode_pager_getpage,
            vnode_pager_putpages,
            vnode_pager_haspage
    };
    
    static struct krate vbadrate = { 1 };
    static struct krate vresrate = { 1 };
    
    long vnode_pbuf_freecnt = -1;   /* start out unlimited */
    
    /*
     * Allocate a VM object for a vnode, typically a regular file vnode.
     *
     * Some additional information is required to generate a properly sized
     * object which covers the entire buffer cache buffer straddling the file
     * EOF.  Userland does not see the extra pages as the VM fault code tests
     * against v_filesize.
    */
   vm_object_t
   vnode_pager_alloc(void *handle, off_t length, vm_prot_t prot, off_t offset,
                     int blksize, int boff)
   {
           vm_object_t object;
           struct vnode *vp;
           off_t loffset;
           vm_pindex_t lsize;
   
           /*
            * Pageout to vnode, no can do yet.
            */
           if (handle == NULL)
                   return (NULL);
   
           /*
            * XXX hack - This initialization should be put somewhere else.
            */
           if (vnode_pbuf_freecnt < 0) {
               vnode_pbuf_freecnt = nswbuf / 2 + 1;
           }
   
           /*
            * Serialize potential vnode/object teardowns and interlocks
            */
           vp = (struct vnode *)handle;
           lwkt_gettoken(&vp->v_token);
   
           /*
            * If the object is being terminated, wait for it to
            * go away.
            */
           object = vp->v_object;
           if (object) {
                   vm_object_hold(object);
                   KKASSERT((object->flags & OBJ_DEAD) == 0);
           }
   
           if (VREFCNT(vp) <= 0)
                   panic("vnode_pager_alloc: no vnode reference");
   
           /*
            * Round up to the *next* block, then destroy the buffers in question.
            * Since we are only removing some of the buffers we must rely on the
            * scan count to determine whether a loop is necessary.
            *
            * Destroy any pages beyond the last buffer.
            */
           if (boff < 0)
                   boff = (int)(length % blksize);
           if (boff)
                   loffset = length + (blksize - boff);
           else
                   loffset = length;
           lsize = OFF_TO_IDX(round_page64(loffset));
   
           if (object == NULL) {
                   /*
                    * And an object of the appropriate size
                    */
                   object = vm_object_allocate_hold(OBJT_VNODE, lsize);
                   object->handle = handle;
                   vp->v_object = object;
                   vp->v_filesize = length;
                   if (vp->v_mount && (vp->v_mount->mnt_kern_flag & MNTK_NOMSYNC))
                           vm_object_set_flag(object, OBJ_NOMSYNC);
                   vref(vp);
           } else {
                   vm_object_reference_quick(object);      /* also vref's */
                   if (object->size != lsize) {
                           kprintf("vnode_pager_alloc: Warning, objsize "
                                   "mismatch %jd/%jd vp=%p obj=%p\n",
                                   (intmax_t)object->size,
                                   (intmax_t)lsize,
                                   vp, object);
                   }
                   if (vp->v_filesize != length) {
                           kprintf("vnode_pager_alloc: Warning, filesize "
                                   "mismatch %jd/%jd vp=%p obj=%p\n",
                                   (intmax_t)vp->v_filesize,
                                   (intmax_t)length,
                                   vp, object);
                   }
           }
           vm_object_drop(object);
           lwkt_reltoken(&vp->v_token);
   
           return (object);
   }
   
   /*
    * Add a ref to a vnode's existing VM object, return the object or
    * NULL if the vnode did not have one.  This does not create the
    * object (we can't since we don't know what the proper blocksize/boff
    * is to match the VFS's use of the buffer cache).
    *
    * The vnode must be referenced and is typically open.  The object should
    * be stable in this situation.
    *
    * Returns the object with an additional reference but not locked.
    */
   vm_object_t
   vnode_pager_reference(struct vnode *vp)
   {
           vm_object_t object;
   
           if ((object = vp->v_object) != NULL)
                   vm_object_reference_quick(object); /* also vref's vnode */
           return (object);
   }
   
   static void
   vnode_pager_dealloc(vm_object_t object)
   {
           struct vnode *vp = object->handle;
   
           if (vp == NULL)
                   panic("vnode_pager_dealloc: pager already dealloced");
   
           vm_object_pip_wait(object, "vnpdea");
   
           object->handle = NULL;
           object->type = OBJT_DEAD;
           vp->v_object = NULL;
           vp->v_filesize = NOOFFSET;
           vclrflags(vp, VTEXT | VOBJBUF);
           swap_pager_freespace_all(object);
   }
   
   /*
    * Return whether the vnode pager has the requested page.  Return the
    * number of disk-contiguous pages before and after the requested page,
    * not including the requested page.
    */
   static boolean_t
   vnode_pager_haspage(vm_object_t object, vm_pindex_t pindex)
   {
           struct vnode *vp = object->handle;
           off_t loffset;
           off_t doffset;
           int voff;
           int bsize;
           int error;
   
           /*
            * If no vp or vp is doomed or marked transparent to VM, we do not
            * have the page.
            */
           if ((vp == NULL) || (vp->v_flag & VRECLAIMED))
                   return FALSE;
   
           /*
            * If filesystem no longer mounted or offset beyond end of file we do
            * not have the page.
            */
           loffset = IDX_TO_OFF(pindex);
   
           if (vp->v_mount == NULL || loffset >= vp->v_filesize)
                   return FALSE;
   
           bsize = vp->v_mount->mnt_stat.f_iosize;
           voff = loffset % bsize;
   
           /*
            * XXX
            *
            * BMAP returns byte counts before and after, where after
            * is inclusive of the base page.  haspage must return page
            * counts before and after where after does not include the
            * base page.
            *
            * BMAP is allowed to return a *after of 0 for backwards
            * compatibility.  The base page is still considered valid if
            * no error is returned.
            */
           error = VOP_BMAP(vp, loffset - voff, &doffset, NULL, NULL, 0);
           if (error)
                   return TRUE;
           if (doffset == NOOFFSET)
                   return FALSE;
           return TRUE;
   }
   
   /*
    * Lets the VM system know about a change in size for a file.
    * We adjust our own internal size and flush any cached pages in
    * the associated object that are affected by the size change.
    *
    * NOTE: This routine may be invoked as a result of a pager put
    * operation (possibly at object termination time), so we must be careful.
    *
    * NOTE: vp->v_filesize is initialized to NOOFFSET (-1), be sure that
    * we do not blow up on the case.  nsize will always be >= 0, however.
    */
   void
   vnode_pager_setsize(struct vnode *vp, vm_ooffset_t nsize)
   {
           vm_pindex_t nobjsize;
           vm_pindex_t oobjsize;
           vm_object_t object;
   
           object = vp->v_object;
           if (object == NULL)
                   return;
           vm_object_hold(object);
           KKASSERT(vp->v_object == object);
   
           /*
            * Hasn't changed size
            */
           if (nsize == vp->v_filesize) {
                   vm_object_drop(object);
                   return;
           }
   
           /*
            * Has changed size.  Adjust the VM object's size and v_filesize
            * before we start scanning pages to prevent new pages from being
            * allocated during the scan.
            */
           nobjsize = OFF_TO_IDX(nsize + PAGE_MASK);
           oobjsize = object->size;
           object->size = nobjsize;
   
           /*
            * File has shrunk. Toss any cached pages beyond the new EOF.
            */
           if (nsize < vp->v_filesize) {
                   vp->v_filesize = nsize;
                   if (nobjsize < oobjsize) {
                           vm_object_page_remove(object, nobjsize, oobjsize,
                                                 FALSE);
                   }
                   /*
                    * This gets rid of garbage at the end of a page that is now
                    * only partially backed by the vnode.  Since we are setting
                    * the entire page valid & clean after we are done we have
                    * to be sure that the portion of the page within the file
                    * bounds is already valid.  If it isn't then making it
                    * valid would create a corrupt block.
                    */
                   if (nsize & PAGE_MASK) {
                           vm_offset_t kva;
                           vm_page_t m;
   
                           m = vm_page_lookup_busy_wait(object, OFF_TO_IDX(nsize),
                                                        TRUE, "vsetsz");
   
                           if (m && m->valid) {
                                   int base = (int)nsize & PAGE_MASK;
                                   int size = PAGE_SIZE - base;
                                   struct lwbuf *lwb;
                                   struct lwbuf lwb_cache;
   
                                   /*
                                    * Clear out partial-page garbage in case
                                    * the page has been mapped.
                                    *
                                    * This is byte aligned.
                                    */
                                   lwb = lwbuf_alloc(m, &lwb_cache);
                                   kva = lwbuf_kva(lwb);
                                   bzero((caddr_t)kva + base, size);
                                   lwbuf_free(lwb);
   
                                   /*
                                    * XXX work around SMP data integrity race
                                    * by unmapping the page from user processes.
                                    * The garbage we just cleared may be mapped
                                    * to a user process running on another cpu
                                    * and this code is not running through normal
                                    * I/O channels which handle SMP issues for
                                    * us, so unmap page to synchronize all cpus.
                                    *
                                    * XXX should vm_pager_unmap_page() have
                                    * dealt with this?
                                    */
                                   vm_page_protect(m, VM_PROT_NONE);
   
                                   /*
                                    * Clear out partial-page dirty bits.  This
                                    * has the side effect of setting the valid
                                    * bits, but that is ok.  There are a bunch
                                    * of places in the VM system where we expected
                                    * m->dirty == VM_PAGE_BITS_ALL.  The file EOF
                                    * case is one of them.  If the page is still
                                    * partially dirty, make it fully dirty.
                                    *
                                    * NOTE: We do not clear out the valid
                                    * bits.  This would prevent bogus_page
                                    * replacement from working properly.
                                    *
                                    * NOTE: We do not want to clear the dirty
                                    * bit for a partial DEV_BSIZE'd truncation!
                                    * This is DEV_BSIZE aligned!
                                    */
                                   vm_page_clear_dirty_beg_nonincl(m, base, size);
                                   if (m->dirty != 0)
                                           m->dirty = VM_PAGE_BITS_ALL;
                                   vm_page_wakeup(m);
                           } else if (m) {
                                   vm_page_wakeup(m);
                           }
                   }
           } else {
                   vp->v_filesize = nsize;
           }
           vm_object_drop(object);
   }
   
   /*
    * Release a page busied for a getpages operation.  The page may have become
    * wired (typically due to being used by the buffer cache) or otherwise been
    * soft-busied and cannot be freed in that case.  A held page can still be
    * freed.
    */
   void
   vnode_pager_freepage(vm_page_t m)
   {
           if (m->busy || m->wire_count || (m->flags & PG_NEED_COMMIT)) {
                   vm_page_activate(m);
                   vm_page_wakeup(m);
           } else {
                   vm_page_free(m);
           }
   }
   
   /*
    * EOPNOTSUPP is no longer legal.  For local media VFS's that do not
    * implement their own VOP_GETPAGES, their VOP_GETPAGES should call to
    * vnode_pager_generic_getpages() to implement the previous behaviour.
    *
    * All other FS's should use the bypass to get to the local media
    * backing vp's VOP_GETPAGES.
    */
   static int
   vnode_pager_getpage(vm_object_t object, vm_page_t *mpp, int seqaccess)
   {
           int rtval;
           struct vnode *vp;
   
           vp = object->handle;
           rtval = VOP_GETPAGES(vp, mpp, PAGE_SIZE, 0, 0, seqaccess);
           if (rtval == EOPNOTSUPP)
                   panic("vnode_pager: vfs's must implement vop_getpages");
           return rtval;
   }
   
   /*
    * This is now called from local media FS's to operate against their
    * own vnodes if they fail to implement VOP_GETPAGES.
    *
    * With all the caching local media devices do these days there is really
    * very little point to attempting to restrict the I/O size to contiguous
    * blocks on-disk, especially if our caller thinks we need all the specified
    * pages.  Just construct and issue a READ.
    */
   int
   vnode_pager_generic_getpages(struct vnode *vp, vm_page_t *mpp, int bytecount,
                                int reqpage, int seqaccess)
   {
           struct iovec aiov;
           struct uio auio;
           off_t foff;
           int error;
           int count;
           int i;
           int ioflags;
   
           /*
            * Do not do anything if the vnode is bad.
            */
           if (vp->v_mount == NULL)
                   return VM_PAGER_BAD;
   
           /*
            * Calculate the number of pages.  Since we are paging in whole
            * pages, adjust bytecount to be an integral multiple of the page
            * size.  It will be clipped to the file EOF later on.
            */
           bytecount = round_page(bytecount);
           count = bytecount / PAGE_SIZE;
   
           /*
            * We could check m[reqpage]->valid here and shortcut the operation,
            * but doing so breaks read-ahead.  Instead assume that the VM
            * system has already done at least the check, don't worry about
            * any races, and issue the VOP_READ to allow read-ahead to function.
            *
            * This keeps the pipeline full for I/O bound sequentially scanned
            * mmap()'s
            */
           /* don't shortcut */
   
           /*
            * Discard pages past the file EOF.  If the requested page is past
            * the file EOF we just leave its valid bits set to 0, the caller
            * expects to maintain ownership of the requested page.  If the
            * entire range is past file EOF discard everything and generate
            * a pagein error.
            */
           foff = IDX_TO_OFF(mpp[0]->pindex);
           if (foff >= vp->v_filesize) {
                   for (i = 0; i < count; i++) {
                           if (i != reqpage)
                                   vnode_pager_freepage(mpp[i]);
                   }
                   return VM_PAGER_ERROR;
           }
   
           if (foff + bytecount > vp->v_filesize) {
                   bytecount = vp->v_filesize - foff;
                   i = round_page(bytecount) / PAGE_SIZE;
                   while (count > i) {
                           --count;
                           if (count != reqpage)
                                   vnode_pager_freepage(mpp[count]);
                   }
           }
   
           /*
            * The size of the transfer is bytecount.  bytecount will be an
            * integral multiple of the page size unless it has been clipped
            * to the file EOF.  The transfer cannot exceed the file EOF.
            *
            * When dealing with real devices we must round-up to the device
            * sector size.
            */
           if (vp->v_type == VBLK || vp->v_type == VCHR) {
                   int secmask = vp->v_rdev->si_bsize_phys - 1;
                   KASSERT(secmask < PAGE_SIZE, ("vnode_pager_generic_getpages: sector size %d too large", secmask + 1));
                   bytecount = (bytecount + secmask) & ~secmask;
           }
   
           /*
            * Severe hack to avoid deadlocks with the buffer cache
            */
           for (i = 0; i < count; ++i) {
                   vm_page_t mt = mpp[i];
   
                   vm_page_io_start(mt);
                   vm_page_wakeup(mt);
           }
   
           /*
            * Issue the I/O with some read-ahead if bytecount > PAGE_SIZE
            */
           ioflags = IO_VMIO;
           if (seqaccess)
                   ioflags |= IO_SEQMAX << IO_SEQSHIFT;
   
           aiov.iov_base = NULL;
           aiov.iov_len = bytecount;
           auio.uio_iov = &aiov;
           auio.uio_iovcnt = 1;
           auio.uio_offset = foff;
           auio.uio_segflg = UIO_NOCOPY;
           auio.uio_rw = UIO_READ;
           auio.uio_resid = bytecount;
           auio.uio_td = NULL;
           mycpu->gd_cnt.v_vnodein++;
           mycpu->gd_cnt.v_vnodepgsin += count;
   
           error = VOP_READ(vp, &auio, ioflags, proc0.p_ucred);
   
           /*
            * Severe hack to avoid deadlocks with the buffer cache
            */
           for (i = 0; i < count; ++i) {
                   vm_page_busy_wait(mpp[i], FALSE, "getpgs");
                   vm_page_io_finish(mpp[i]);
           }
   
           /*
            * Calculate the actual number of bytes read and clean up the
            * page list.  
            */
           bytecount -= auio.uio_resid;
   
           for (i = 0; i < count; ++i) {
                   vm_page_t mt = mpp[i];
   
                   if (i != reqpage) {
                           if (error == 0 && mt->valid) {
                                   if (mt->flags & PG_REFERENCED)
                                           vm_page_activate(mt);
                                   else
                                           vm_page_deactivate(mt);
                                   vm_page_wakeup(mt);
                           } else {
                                   vnode_pager_freepage(mt);
                           }
                   } else if (mt->valid == 0) {
                           if (error == 0) {
                                   kprintf("page failed but no I/O error page "
                                           "%p object %p pindex %d\n",
                                           mt, mt->object, (int) mt->pindex);
                                   /* whoops, something happened */
                                   error = EINVAL;
                           }
                   } else if (mt->valid != VM_PAGE_BITS_ALL) {
                           /*
                            * Zero-extend the requested page if necessary (if
                            * the filesystem is using a small block size).
                            */
                           vm_page_zero_invalid(mt, TRUE);
                   }
           }
           if (error) {
                   kprintf("vnode_pager_getpage: I/O read error\n");
           }
           return (error ? VM_PAGER_ERROR : VM_PAGER_OK);
   }
   
   /*
    * EOPNOTSUPP is no longer legal.  For local media VFS's that do not
    * implement their own VOP_PUTPAGES, their VOP_PUTPAGES should call to
    * vnode_pager_generic_putpages() to implement the previous behaviour.
    *
    * Caller has already cleared the pmap modified bits, if any.
    *
    * All other FS's should use the bypass to get to the local media
    * backing vp's VOP_PUTPAGES.
    */
   static void
   vnode_pager_putpages(vm_object_t object, vm_page_t *m, int count,
                        boolean_t sync, int *rtvals)
   {
           int rtval;
           struct vnode *vp;
           int bytes = count * PAGE_SIZE;
   
           /*
            * Force synchronous operation if we are extremely low on memory
            * to prevent a low-memory deadlock.  VOP operations often need to
            * allocate more memory to initiate the I/O ( i.e. do a BMAP 
            * operation ).  The swapper handles the case by limiting the amount
            * of asynchronous I/O, but that sort of solution doesn't scale well
            * for the vnode pager without a lot of work.
            *
            * Also, the backing vnode's iodone routine may not wake the pageout
            * daemon up.  This should be probably be addressed XXX.
            */
   
           if ((vmstats.v_free_count + vmstats.v_cache_count) <
               vmstats.v_pageout_free_min) {
                   sync |= OBJPC_SYNC;
           }
   
           /*
            * Call device-specific putpages function
            */
           vp = object->handle;
           rtval = VOP_PUTPAGES(vp, m, bytes, sync, rtvals, 0);
           if (rtval == EOPNOTSUPP) {
               kprintf("vnode_pager: *** WARNING *** stale FS putpages\n");
               rtval = vnode_pager_generic_putpages( vp, m, bytes, sync, rtvals);
           }
   }
   
   
   /*
    * This is now called from local media FS's to operate against their
    * own vnodes if they fail to implement VOP_PUTPAGES.
    *
    * This is typically called indirectly via the pageout daemon and
    * clustering has already typically occured, so in general we ask the
    * underlying filesystem to write the data out asynchronously rather
    * then delayed.
    */
   int
   vnode_pager_generic_putpages(struct vnode *vp, vm_page_t *m, int bytecount,
                                int flags, int *rtvals)
   {
           int i;
           int maxsize, ncount, count;
           vm_ooffset_t poffset;
           struct uio auio;
           struct iovec aiov;
           int error;
           int ioflags;
   
           count = bytecount / PAGE_SIZE;
   
           for (i = 0; i < count; i++)
                   rtvals[i] = VM_PAGER_AGAIN;
   
           if ((int) m[0]->pindex < 0) {
                   kprintf("vnode_pager_putpages: attempt to write meta-data!!! -- 0x%lx(%x)\n",
                           (long)m[0]->pindex, m[0]->dirty);
                   rtvals[0] = VM_PAGER_BAD;
                   return VM_PAGER_BAD;
           }
   
           maxsize = count * PAGE_SIZE;
           ncount = count;
   
           poffset = IDX_TO_OFF(m[0]->pindex);
   
           /*
            * If the page-aligned write is larger then the actual file we
            * have to invalidate pages occuring beyond the file EOF.
            *
            * If the file EOF resides in the middle of a page we still clear
            * all of that page's dirty bits later on.  If we didn't it would
            * endlessly re-write.
            *
            * We do not under any circumstances truncate the valid bits, as
            * this will screw up bogus page replacement.
            *
            * The caller has already read-protected the pages.  The VFS must
            * use the buffer cache to wrap the pages.  The pages might not
            * be immediately flushed by the buffer cache but once under its
            * control the pages themselves can wind up being marked clean
            * and their covering buffer cache buffer can be marked dirty.
            */
           if (poffset + maxsize > vp->v_filesize) {
                   if (poffset < vp->v_filesize) {
                           maxsize = vp->v_filesize - poffset;
                           ncount = btoc(maxsize);
                   } else {
                           maxsize = 0;
                           ncount = 0;
                   }
                   if (ncount < count) {
                           for (i = ncount; i < count; i++) {
                                   rtvals[i] = VM_PAGER_BAD;
                           }
                   }
           }
   
           /*
            * pageouts are already clustered, use IO_ASYNC to force a bawrite()
            * rather then a bdwrite() to prevent paging I/O from saturating
            * the buffer cache.  Dummy-up the sequential heuristic to cause
            * large ranges to cluster.  If neither IO_SYNC or IO_ASYNC is set,
            * the system decides how to cluster.
            */
           ioflags = IO_VMIO;
           if (flags & (VM_PAGER_PUT_SYNC | VM_PAGER_PUT_INVAL))
                   ioflags |= IO_SYNC;
           else if ((flags & VM_PAGER_CLUSTER_OK) == 0)
                   ioflags |= IO_ASYNC;
           ioflags |= (flags & VM_PAGER_PUT_INVAL) ? IO_INVAL: 0;
           ioflags |= IO_SEQMAX << IO_SEQSHIFT;
   
           aiov.iov_base = (caddr_t) 0;
           aiov.iov_len = maxsize;
           auio.uio_iov = &aiov;
           auio.uio_iovcnt = 1;
           auio.uio_offset = poffset;
           auio.uio_segflg = UIO_NOCOPY;
           auio.uio_rw = UIO_WRITE;
           auio.uio_resid = maxsize;
           auio.uio_td = NULL;
           error = VOP_WRITE(vp, &auio, ioflags, proc0.p_ucred);
           mycpu->gd_cnt.v_vnodeout++;
           mycpu->gd_cnt.v_vnodepgsout += ncount;
   
           if (error) {
                   krateprintf(&vbadrate,
                               "vnode_pager_putpages: I/O error %d\n", error);
           }
           if (auio.uio_resid) {
                   krateprintf(&vresrate,
                               "vnode_pager_putpages: residual I/O %zd at %lu\n",
                               auio.uio_resid, (u_long)m[0]->pindex);
           }
           if (error == 0) {
                   for (i = 0; i < ncount; i++) {
                           rtvals[i] = VM_PAGER_OK;
                           vm_page_undirty(m[i]);
                   }
           }
           return rtvals[0];
   }
   
   /*
    * Run the chain and if the bottom-most object is a vnode-type lock the
    * underlying vnode.  A locked vnode or NULL is returned.
    */
   struct vnode *
   vnode_pager_lock(vm_object_t object)
   {
           struct vnode *vp = NULL;
           vm_object_t lobject;
           vm_object_t tobject;
           int error;
   
           if (object == NULL)
                   return(NULL);
   
           ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
           lobject = object;
   
           while (lobject->type != OBJT_VNODE) {
                   if (lobject->flags & OBJ_DEAD)
                           break;
                   tobject = lobject->backing_object;
                   if (tobject == NULL)
                           break;
                   vm_object_hold_shared(tobject);
                   if (tobject == lobject->backing_object) {
                           if (lobject != object) {
                                   vm_object_lock_swap();
                                   vm_object_drop(lobject);
                           }
                           lobject = tobject;
                   } else {
                           vm_object_drop(tobject);
                   }
           }
           while (lobject->type == OBJT_VNODE &&
                  (lobject->flags & OBJ_DEAD) == 0) {
                   /*
                    * Extract the vp
                    */
                   vp = lobject->handle;
                   error = vget(vp, LK_SHARED | LK_RETRY | LK_CANRECURSE);
                   if (error == 0) {
                           if (lobject->handle == vp)
                                   break;
                           vput(vp);
                   } else {
                           kprintf("vnode_pager_lock: vp %p error %d "
                                   "lockstatus %d, retrying\n",
                                   vp, error,
                                   lockstatus(&vp->v_lock, curthread));
                           tsleep(object->handle, 0, "vnpgrl", hz);
                   }
                   vp = NULL;
           }
           if (lobject != object)
                   vm_object_drop(lobject);
           return (vp);
   }

Cache object: 4acea8685c41d319b2ec53bc79adf8e6