text: /*-
   * 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. All advertising materials mentioning features or use of this software
   *    must display the following acknowledgement:
   *	This product includes software developed by the University of
   *	California, Berkeley and its contributors.
   * 4. 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
   */
  
  /*
   * Page to/from files (vnodes).
   */
  
  /*
   * TODO:
   *	Implement VOP_GETPAGES/PUTPAGES interface for filesystems. Will
   *	greatly re-simplify the vnode_pager.
   */
  
  #include <sys/cdefs.h>
  __FBSDID("$FreeBSD$");
  
  #include "opt_vm.h"
  
  #include <sys/param.h>
  #include <sys/systm.h>
  #include <sys/proc.h>
  #include <sys/vnode.h>
  #include <sys/mount.h>
  #include <sys/bio.h>
  #include <sys/buf.h>
  #include <sys/vmmeter.h>
  #include <sys/limits.h>
  #include <sys/conf.h>
  #include <sys/rwlock.h>
  #include <sys/sf_buf.h>
  
  #include <machine/atomic.h>
  
  #include <vm/vm.h>
  #include <vm/vm_param.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/vm_extern.h>
  
  static int vnode_pager_addr(struct vnode *vp, vm_ooffset_t address,
      daddr_t *rtaddress, int *run);
  static int vnode_pager_input_smlfs(vm_object_t object, vm_page_t m);
  static int vnode_pager_input_old(vm_object_t object, vm_page_t m);
  static void vnode_pager_dealloc(vm_object_t);
  static int vnode_pager_getpages(vm_object_t, vm_page_t *, int, int *, int *);
  static int vnode_pager_getpages_async(vm_object_t, vm_page_t *, int, int *,
      int *, vop_getpages_iodone_t, void *);
  static void vnode_pager_putpages(vm_object_t, vm_page_t *, int, int, int *);
  static boolean_t vnode_pager_haspage(vm_object_t, vm_pindex_t, int *, int *);
  static vm_object_t vnode_pager_alloc(void *, vm_ooffset_t, vm_prot_t,
      vm_ooffset_t, struct ucred *cred);
  static int vnode_pager_generic_getpages_done(struct buf *);
  static void vnode_pager_generic_getpages_done_async(struct buf *);
  
  struct pagerops vnodepagerops = {
  	.pgo_alloc =	vnode_pager_alloc,
  	.pgo_dealloc =	vnode_pager_dealloc,
  	.pgo_getpages =	vnode_pager_getpages,
  	.pgo_getpages_async = vnode_pager_getpages_async,
  	.pgo_putpages =	vnode_pager_putpages,
  	.pgo_haspage =	vnode_pager_haspage,
  };
  
  int vnode_pbuf_freecnt;
  int vnode_async_pbuf_freecnt;
  
  /* Create the VM system backing object for this vnode */
  int
  vnode_create_vobject(struct vnode *vp, off_t isize, struct thread *td)
  {
  	vm_object_t object;
  	vm_ooffset_t size = isize;
  	struct vattr va;
  
  	if (!vn_isdisk(vp, NULL) && vn_canvmio(vp) == FALSE)
  		return (0);
  
  	while ((object = vp->v_object) != NULL) {
  		VM_OBJECT_WLOCK(object);
  		if (!(object->flags & OBJ_DEAD)) {
  			VM_OBJECT_WUNLOCK(object);
  			return (0);
  		}
  		VOP_UNLOCK(vp, 0);
  		vm_object_set_flag(object, OBJ_DISCONNECTWNT);
  		VM_OBJECT_SLEEP(object, object, PDROP | PVM, "vodead", 0);
  		vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
  	}
  
  	if (size == 0) {
  		if (vn_isdisk(vp, NULL)) {
  			size = IDX_TO_OFF(INT_MAX);
  		} else {
  			if (VOP_GETATTR(vp, &va, td->td_ucred))
  				return (0);
  			size = va.va_size;
  		}
  	}
  
  	object = vnode_pager_alloc(vp, size, 0, 0, td->td_ucred);
  	/*
  	 * Dereference the reference we just created.  This assumes
  	 * that the object is associated with the vp.
  	 */
  	VM_OBJECT_WLOCK(object);
  	object->ref_count--;
  	VM_OBJECT_WUNLOCK(object);
  	vrele(vp);
  
  	KASSERT(vp->v_object != NULL, ("vnode_create_vobject: NULL object"));
  
  	return (0);
  }
  
  void
  vnode_destroy_vobject(struct vnode *vp)
  {
  	struct vm_object *obj;
  
  	obj = vp->v_object;
  	if (obj == NULL)
  		return;
  	ASSERT_VOP_ELOCKED(vp, "vnode_destroy_vobject");
  	VM_OBJECT_WLOCK(obj);
  	umtx_shm_object_terminated(obj);
  	if (obj->ref_count == 0) {
  		/*
  		 * don't double-terminate the object
  		 */
  		if ((obj->flags & OBJ_DEAD) == 0) {
  			vm_object_terminate(obj);
  		} else {
  			/*
  			 * Waiters were already handled during object
  			 * termination.  The exclusive vnode lock hopefully
  			 * prevented new waiters from referencing the dying
  			 * object.
  			 */
  			KASSERT((obj->flags & OBJ_DISCONNECTWNT) == 0,
  			    ("OBJ_DISCONNECTWNT set obj %p flags %x",
  			    obj, obj->flags));
  			vp->v_object = NULL;
  			VM_OBJECT_WUNLOCK(obj);
  		}
  	} else {
  		/*
  		 * Woe to the process that tries to page now :-).
  		 */
  		vm_pager_deallocate(obj);
  		VM_OBJECT_WUNLOCK(obj);
  	}
  	KASSERT(vp->v_object == NULL, ("vp %p obj %p", vp, vp->v_object));
  }
  
  
  /*
   * Allocate (or lookup) pager for a vnode.
   * Handle is a vnode pointer.
   *
   * MPSAFE
   */
  vm_object_t
  vnode_pager_alloc(void *handle, vm_ooffset_t size, vm_prot_t prot,
      vm_ooffset_t offset, struct ucred *cred)
  {
  	vm_object_t object;
  	struct vnode *vp;
  
  	/*
  	 * Pageout to vnode, no can do yet.
  	 */
  	if (handle == NULL)
  		return (NULL);
  
  	vp = (struct vnode *) handle;
  
  	/*
  	 * If the object is being terminated, wait for it to
  	 * go away.
  	 */
  retry:
  	while ((object = vp->v_object) != NULL) {
  		VM_OBJECT_WLOCK(object);
  		if ((object->flags & OBJ_DEAD) == 0)
  			break;
  		vm_object_set_flag(object, OBJ_DISCONNECTWNT);
  		VM_OBJECT_SLEEP(object, object, PDROP | PVM, "vadead", 0);
  	}
  
  	KASSERT(vp->v_usecount != 0, ("vnode_pager_alloc: no vnode reference"));
  
  	if (object == NULL) {
  		/*
  		 * Add an object of the appropriate size
  		 */
  		object = vm_object_allocate(OBJT_VNODE, OFF_TO_IDX(round_page(size)));
  
  		object->un_pager.vnp.vnp_size = size;
  		object->un_pager.vnp.writemappings = 0;
  
  		object->handle = handle;
  		VI_LOCK(vp);
  		if (vp->v_object != NULL) {
  			/*
  			 * Object has been created while we were sleeping
  			 */
  			VI_UNLOCK(vp);
  			VM_OBJECT_WLOCK(object);
  			KASSERT(object->ref_count == 1,
  			    ("leaked ref %p %d", object, object->ref_count));
  			object->type = OBJT_DEAD;
  			object->ref_count = 0;
  			VM_OBJECT_WUNLOCK(object);
  			vm_object_destroy(object);
  			goto retry;
  		}
  		vp->v_object = object;
  		VI_UNLOCK(vp);
  	} else {
  		object->ref_count++;
  #if VM_NRESERVLEVEL > 0
  		vm_object_color(object, 0);
  #endif
  		VM_OBJECT_WUNLOCK(object);
  	}
  	vref(vp);
  	return (object);
  }
  
  /*
   *	The object must be locked.
   */
  static void
  vnode_pager_dealloc(vm_object_t object)
  {
  	struct vnode *vp;
  	int refs;
  
  	vp = object->handle;
  	if (vp == NULL)
  		panic("vnode_pager_dealloc: pager already dealloced");
  
  	VM_OBJECT_ASSERT_WLOCKED(object);
  	vm_object_pip_wait(object, "vnpdea");
  	refs = object->ref_count;
  
  	object->handle = NULL;
  	object->type = OBJT_DEAD;
  	if (object->flags & OBJ_DISCONNECTWNT) {
  		vm_object_clear_flag(object, OBJ_DISCONNECTWNT);
  		wakeup(object);
  	}
  	ASSERT_VOP_ELOCKED(vp, "vnode_pager_dealloc");
  	if (object->un_pager.vnp.writemappings > 0) {
  		object->un_pager.vnp.writemappings = 0;
  		VOP_ADD_WRITECOUNT(vp, -1);
  		CTR3(KTR_VFS, "%s: vp %p v_writecount decreased to %d",
  		    __func__, vp, vp->v_writecount);
  	}
  	vp->v_object = NULL;
  	VOP_UNSET_TEXT(vp);
  	VM_OBJECT_WUNLOCK(object);
  	while (refs-- > 0)
  		vunref(vp);
  	VM_OBJECT_WLOCK(object);
  }
  
  static boolean_t
  vnode_pager_haspage(vm_object_t object, vm_pindex_t pindex, int *before,
      int *after)
  {
  	struct vnode *vp = object->handle;
  	daddr_t bn;
  	int err;
  	daddr_t reqblock;
  	int poff;
  	int bsize;
  	int pagesperblock, blocksperpage;
  
  	VM_OBJECT_ASSERT_WLOCKED(object);
  	/*
  	 * If no vp or vp is doomed or marked transparent to VM, we do not
  	 * have the page.
  	 */
  	if (vp == NULL || vp->v_iflag & VI_DOOMED)
  		return FALSE;
  	/*
  	 * If the offset is beyond end of file we do
  	 * not have the page.
  	 */
  	if (IDX_TO_OFF(pindex) >= object->un_pager.vnp.vnp_size)
  		return FALSE;
  
  	bsize = vp->v_mount->mnt_stat.f_iosize;
  	pagesperblock = bsize / PAGE_SIZE;
  	blocksperpage = 0;
  	if (pagesperblock > 0) {
  		reqblock = pindex / pagesperblock;
  	} else {
  		blocksperpage = (PAGE_SIZE / bsize);
  		reqblock = pindex * blocksperpage;
  	}
  	VM_OBJECT_WUNLOCK(object);
  	err = VOP_BMAP(vp, reqblock, NULL, &bn, after, before);
  	VM_OBJECT_WLOCK(object);
  	if (err)
  		return TRUE;
  	if (bn == -1)
  		return FALSE;
  	if (pagesperblock > 0) {
  		poff = pindex - (reqblock * pagesperblock);
  		if (before) {
  			*before *= pagesperblock;
  			*before += poff;
  		}
  		if (after) {
  			/*
  			 * The BMAP vop can report a partial block in the
  			 * 'after', but must not report blocks after EOF.
  			 * Assert the latter, and truncate 'after' in case
  			 * of the former.
  			 */
  			KASSERT((reqblock + *after) * pagesperblock <
  			    roundup2(object->size, pagesperblock),
  			    ("%s: reqblock %jd after %d size %ju", __func__,
  			    (intmax_t )reqblock, *after,
  			    (uintmax_t )object->size));
  			*after *= pagesperblock;
  			*after += pagesperblock - (poff + 1);
  			if (pindex + *after >= object->size)
  				*after = object->size - 1 - pindex;
  		}
  	} else {
  		if (before) {
  			*before /= blocksperpage;
  		}
  
  		if (after) {
  			*after /= blocksperpage;
  		}
  	}
  	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.
   */
  void
  vnode_pager_setsize(struct vnode *vp, vm_ooffset_t nsize)
  {
  	vm_object_t object;
  	vm_page_t m;
  	vm_pindex_t nobjsize;
  
  	if ((object = vp->v_object) == NULL)
  		return;
  /* 	ASSERT_VOP_ELOCKED(vp, "vnode_pager_setsize and not locked vnode"); */
  	VM_OBJECT_WLOCK(object);
  	if (object->type == OBJT_DEAD) {
  		VM_OBJECT_WUNLOCK(object);
  		return;
  	}
  	KASSERT(object->type == OBJT_VNODE,
  	    ("not vnode-backed object %p", object));
  	if (nsize == object->un_pager.vnp.vnp_size) {
  		/*
  		 * Hasn't changed size
  		 */
  		VM_OBJECT_WUNLOCK(object);
  		return;
  	}
  	nobjsize = OFF_TO_IDX(nsize + PAGE_MASK);
  	if (nsize < object->un_pager.vnp.vnp_size) {
  		/*
  		 * File has shrunk. Toss any cached pages beyond the new EOF.
  		 */
  		if (nobjsize < object->size)
  			vm_object_page_remove(object, nobjsize, object->size,
  			    0);
  		/*
  		 * this gets rid of garbage at the end of a page that is now
  		 * only partially backed by the vnode.
  		 *
  		 * XXX for some reason (I don't know yet), if we take a
  		 * completely invalid page and mark it partially valid
  		 * it can screw up NFS reads, so we don't allow the case.
  		 */
  		if ((nsize & PAGE_MASK) &&
  		    (m = vm_page_lookup(object, OFF_TO_IDX(nsize))) != NULL &&
  		    m->valid != 0) {
  			int base = (int)nsize & PAGE_MASK;
  			int size = PAGE_SIZE - base;
  
  			/*
  			 * Clear out partial-page garbage in case
  			 * the page has been mapped.
  			 */
  			pmap_zero_page_area(m, base, size);
  
  			/*
  			 * Update the valid bits to reflect the blocks that
  			 * have been zeroed.  Some of these valid bits may
  			 * have already been set.
  			 */
  			vm_page_set_valid_range(m, base, size);
  
  			/*
  			 * Round "base" to the next block boundary so that the
  			 * dirty bit for a partially zeroed block is not
  			 * cleared.
  			 */
  			base = roundup2(base, DEV_BSIZE);
  
  			/*
  			 * Clear out partial-page dirty bits.
  			 *
  			 * note that we do not clear out the valid
  			 * bits.  This would prevent bogus_page
  			 * replacement from working properly.
  			 */
  			vm_page_clear_dirty(m, base, PAGE_SIZE - base);
  		} else if ((nsize & PAGE_MASK) &&
  		    vm_page_is_cached(object, OFF_TO_IDX(nsize))) {
  			vm_page_cache_free(object, OFF_TO_IDX(nsize),
  			    nobjsize);
  		}
  	}
  	object->un_pager.vnp.vnp_size = nsize;
  	object->size = nobjsize;
  	VM_OBJECT_WUNLOCK(object);
  }
  
  /*
   * calculate the linear (byte) disk address of specified virtual
   * file address
   */
  static int
  vnode_pager_addr(struct vnode *vp, vm_ooffset_t address, daddr_t *rtaddress,
      int *run)
  {
  	int bsize;
  	int err;
  	daddr_t vblock;
  	daddr_t voffset;
  
  	if (address < 0)
  		return -1;
  
  	if (vp->v_iflag & VI_DOOMED)
  		return -1;
  
  	bsize = vp->v_mount->mnt_stat.f_iosize;
  	vblock = address / bsize;
  	voffset = address % bsize;
  
  	err = VOP_BMAP(vp, vblock, NULL, rtaddress, run, NULL);
  	if (err == 0) {
  		if (*rtaddress != -1)
  			*rtaddress += voffset / DEV_BSIZE;
  		if (run) {
  			*run += 1;
  			*run *= bsize/PAGE_SIZE;
  			*run -= voffset/PAGE_SIZE;
  		}
  	}
  
  	return (err);
  }
  
  /*
   * small block filesystem vnode pager input
   */
  static int
  vnode_pager_input_smlfs(vm_object_t object, vm_page_t m)
  {
  	struct vnode *vp;
  	struct bufobj *bo;
  	struct buf *bp;
  	struct sf_buf *sf;
  	daddr_t fileaddr;
  	vm_offset_t bsize;
  	vm_page_bits_t bits;
  	int error, i;
  
  	error = 0;
  	vp = object->handle;
  	if (vp->v_iflag & VI_DOOMED)
  		return VM_PAGER_BAD;
  
  	bsize = vp->v_mount->mnt_stat.f_iosize;
  
  	VOP_BMAP(vp, 0, &bo, 0, NULL, NULL);
  
  	sf = sf_buf_alloc(m, 0);
  
  	for (i = 0; i < PAGE_SIZE / bsize; i++) {
  		vm_ooffset_t address;
  
  		bits = vm_page_bits(i * bsize, bsize);
  		if (m->valid & bits)
  			continue;
  
  		address = IDX_TO_OFF(m->pindex) + i * bsize;
  		if (address >= object->un_pager.vnp.vnp_size) {
  			fileaddr = -1;
  		} else {
  			error = vnode_pager_addr(vp, address, &fileaddr, NULL);
  			if (error)
  				break;
  		}
  		if (fileaddr != -1) {
  			bp = getpbuf(&vnode_pbuf_freecnt);
  
  			/* build a minimal buffer header */
  			bp->b_iocmd = BIO_READ;
  			bp->b_iodone = bdone;
  			KASSERT(bp->b_rcred == NOCRED, ("leaking read ucred"));
  			KASSERT(bp->b_wcred == NOCRED, ("leaking write ucred"));
  			bp->b_rcred = crhold(curthread->td_ucred);
  			bp->b_wcred = crhold(curthread->td_ucred);
  			bp->b_data = (caddr_t)sf_buf_kva(sf) + i * bsize;
  			bp->b_blkno = fileaddr;
  			pbgetbo(bo, bp);
  			bp->b_vp = vp;
  			bp->b_bcount = bsize;
  			bp->b_bufsize = bsize;
  			bp->b_runningbufspace = bp->b_bufsize;
  			atomic_add_long(&runningbufspace, bp->b_runningbufspace);
  
  			/* do the input */
  			bp->b_iooffset = dbtob(bp->b_blkno);
  			bstrategy(bp);
  
  			bwait(bp, PVM, "vnsrd");
  
  			if ((bp->b_ioflags & BIO_ERROR) != 0)
  				error = EIO;
  
  			/*
  			 * free the buffer header back to the swap buffer pool
  			 */
  			bp->b_vp = NULL;
  			pbrelbo(bp);
  			relpbuf(bp, &vnode_pbuf_freecnt);
  			if (error)
  				break;
  		} else
  			bzero((caddr_t)sf_buf_kva(sf) + i * bsize, bsize);
  		KASSERT((m->dirty & bits) == 0,
  		    ("vnode_pager_input_smlfs: page %p is dirty", m));
  		VM_OBJECT_WLOCK(object);
  		m->valid |= bits;
  		VM_OBJECT_WUNLOCK(object);
  	}
  	sf_buf_free(sf);
  	if (error) {
  		return VM_PAGER_ERROR;
  	}
  	return VM_PAGER_OK;
  }
  
  /*
   * old style vnode pager input routine
   */
  static int
  vnode_pager_input_old(vm_object_t object, vm_page_t m)
  {
  	struct uio auio;
  	struct iovec aiov;
  	int error;
  	int size;
  	struct sf_buf *sf;
  	struct vnode *vp;
  
  	VM_OBJECT_ASSERT_WLOCKED(object);
  	error = 0;
  
  	/*
  	 * Return failure if beyond current EOF
  	 */
  	if (IDX_TO_OFF(m->pindex) >= object->un_pager.vnp.vnp_size) {
  		return VM_PAGER_BAD;
  	} else {
  		size = PAGE_SIZE;
  		if (IDX_TO_OFF(m->pindex) + size > object->un_pager.vnp.vnp_size)
  			size = object->un_pager.vnp.vnp_size - IDX_TO_OFF(m->pindex);
  		vp = object->handle;
  		VM_OBJECT_WUNLOCK(object);
  
  		/*
  		 * Allocate a kernel virtual address and initialize so that
  		 * we can use VOP_READ/WRITE routines.
  		 */
  		sf = sf_buf_alloc(m, 0);
  
  		aiov.iov_base = (caddr_t)sf_buf_kva(sf);
  		aiov.iov_len = size;
  		auio.uio_iov = &aiov;
  		auio.uio_iovcnt = 1;
  		auio.uio_offset = IDX_TO_OFF(m->pindex);
  		auio.uio_segflg = UIO_SYSSPACE;
  		auio.uio_rw = UIO_READ;
  		auio.uio_resid = size;
  		auio.uio_td = curthread;
  
  		error = VOP_READ(vp, &auio, 0, curthread->td_ucred);
  		if (!error) {
  			int count = size - auio.uio_resid;
  
  			if (count == 0)
  				error = EINVAL;
  			else if (count != PAGE_SIZE)
  				bzero((caddr_t)sf_buf_kva(sf) + count,
  				    PAGE_SIZE - count);
  		}
  		sf_buf_free(sf);
  
  		VM_OBJECT_WLOCK(object);
  	}
  	KASSERT(m->dirty == 0, ("vnode_pager_input_old: page %p is dirty", m));
  	if (!error)
  		m->valid = VM_PAGE_BITS_ALL;
  	return error ? VM_PAGER_ERROR : VM_PAGER_OK;
  }
  
  /*
   * generic vnode pager input routine
   */
  
  /*
   * Local media VFS's that do not implement their own VOP_GETPAGES
   * should have their VOP_GETPAGES 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_getpages(vm_object_t object, vm_page_t *m, int count, int *rbehind,
      int *rahead)
  {
  	struct vnode *vp;
  	int rtval;
  
  	vp = object->handle;
  	VM_OBJECT_WUNLOCK(object);
  	rtval = VOP_GETPAGES(vp, m, count, rbehind, rahead);
  	KASSERT(rtval != EOPNOTSUPP,
  	    ("vnode_pager: FS getpages not implemented\n"));
  	VM_OBJECT_WLOCK(object);
  	return rtval;
  }
  
  static int
  vnode_pager_getpages_async(vm_object_t object, vm_page_t *m, int count,
      int *rbehind, int *rahead, vop_getpages_iodone_t iodone, void *arg)
  {
  	struct vnode *vp;
  	int rtval;
  
  	vp = object->handle;
  	VM_OBJECT_WUNLOCK(object);
  	rtval = VOP_GETPAGES_ASYNC(vp, m, count, rbehind, rahead, iodone, arg);
  	KASSERT(rtval != EOPNOTSUPP,
  	    ("vnode_pager: FS getpages_async not implemented\n"));
  	VM_OBJECT_WLOCK(object);
  	return (rtval);
  }
  
  /*
   * The implementation of VOP_GETPAGES() and VOP_GETPAGES_ASYNC() for
   * local filesystems, where partially valid pages can only occur at
   * the end of file.
   */
  int
  vnode_pager_local_getpages(struct vop_getpages_args *ap)
  {
  
  	return (vnode_pager_generic_getpages(ap->a_vp, ap->a_m, ap->a_count,
  	    ap->a_rbehind, ap->a_rahead, NULL, NULL));
  }
  
  int
  vnode_pager_local_getpages_async(struct vop_getpages_async_args *ap)
  {
  
  	return (vnode_pager_generic_getpages(ap->a_vp, ap->a_m, ap->a_count,
  	    ap->a_rbehind, ap->a_rahead, ap->a_iodone, ap->a_arg));
  }
  
  /*
   * This is now called from local media FS's to operate against their
   * own vnodes if they fail to implement VOP_GETPAGES.
   */
  int
  vnode_pager_generic_getpages(struct vnode *vp, vm_page_t *m, int count,
      int *a_rbehind, int *a_rahead, vop_getpages_iodone_t iodone, void *arg)
  {
  	vm_object_t object;
  	struct bufobj *bo;
  	struct buf *bp;
  	off_t foff;
  	int bsize, pagesperblock, *freecnt;
  	int error, before, after, rbehind, rahead, poff, i;
  	int bytecount, secmask;
  
  	KASSERT(vp->v_type != VCHR && vp->v_type != VBLK,
  	    ("%s does not support devices", __func__));
  
  	if (vp->v_iflag & VI_DOOMED)
  		return (VM_PAGER_BAD);
  
  	object = vp->v_object;
  	foff = IDX_TO_OFF(m[0]->pindex);
  	bsize = vp->v_mount->mnt_stat.f_iosize;
  	pagesperblock = bsize / PAGE_SIZE;
  
  	KASSERT(foff < object->un_pager.vnp.vnp_size,
  	    ("%s: page %p offset beyond vp %p size", __func__, m[0], vp));
  	KASSERT(count <= sizeof(bp->b_pages),
  	    ("%s: requested %d pages", __func__, count));
  
  	/*
  	 * The last page has valid blocks.  Invalid part can only
  	 * exist at the end of file, and the page is made fully valid
  	 * by zeroing in vm_pager_get_pages().
  	 */
  	if (m[count - 1]->valid != 0 && --count == 0) {
  		if (iodone != NULL)
  			iodone(arg, m, 1, 0);
  		return (VM_PAGER_OK);
  	}
  
  	/*
  	 * Synchronous and asynchronous paging operations use different
  	 * free pbuf counters.  This is done to avoid asynchronous requests
  	 * to consume all pbufs.
  	 * Allocate the pbuf at the very beginning of the function, so that
  	 * if we are low on certain kind of pbufs don't even proceed to BMAP,
  	 * but sleep.
  	 */
  	freecnt = iodone != NULL ?
  	    &vnode_async_pbuf_freecnt : &vnode_pbuf_freecnt;
  	bp = getpbuf(freecnt);
  
  	/*
  	 * Get the underlying device blocks for the file with VOP_BMAP().
  	 * If the file system doesn't support VOP_BMAP, use old way of
  	 * getting pages via VOP_READ.
  	 */
  	error = VOP_BMAP(vp, foff / bsize, &bo, &bp->b_blkno, &after, &before);
  	if (error == EOPNOTSUPP) {
  		relpbuf(bp, freecnt);
  		VM_OBJECT_WLOCK(object);
  		for (i = 0; i < count; i++) {
  			PCPU_INC(cnt.v_vnodein);
  			PCPU_INC(cnt.v_vnodepgsin);
  			error = vnode_pager_input_old(object, m[i]);
  			if (error)
  				break;
  		}
  		VM_OBJECT_WUNLOCK(object);
  		return (error);
  	} else if (error != 0) {
  		relpbuf(bp, freecnt);
  		return (VM_PAGER_ERROR);
  	}
  
  	/*
  	 * If the file system supports BMAP, but blocksize is smaller
  	 * than a page size, then use special small filesystem code.
  	 */
  	if (pagesperblock == 0) {
  		relpbuf(bp, freecnt);
  		for (i = 0; i < count; i++) {
  			PCPU_INC(cnt.v_vnodein);
  			PCPU_INC(cnt.v_vnodepgsin);
  			error = vnode_pager_input_smlfs(object, m[i]);
  			if (error)
  				break;
  		}
  		return (error);
  	}
  
  	/*
  	 * A sparse file can be encountered only for a single page request,
  	 * which may not be preceded by call to vm_pager_haspage().
  	 */
  	if (bp->b_blkno == -1) {
  		KASSERT(count == 1,
  		    ("%s: array[%d] request to a sparse file %p", __func__,
  		    count, vp));
  		relpbuf(bp, freecnt);
  		pmap_zero_page(m[0]);
  		KASSERT(m[0]->dirty == 0, ("%s: page %p is dirty",
  		    __func__, m[0]));
  		VM_OBJECT_WLOCK(object);
  		m[0]->valid = VM_PAGE_BITS_ALL;
  		VM_OBJECT_WUNLOCK(object);
  		return (VM_PAGER_OK);
  	}
  
  	bp->b_blkno += (foff % bsize) / DEV_BSIZE;
  
  	/* Recalculate blocks available after/before to pages. */
  	poff = (foff % bsize) / PAGE_SIZE;
  	before *= pagesperblock;
  	before += poff;
  	after *= pagesperblock;
  	after += pagesperblock - (poff + 1);
  	if (m[0]->pindex + after >= object->size)
  		after = object->size - 1 - m[0]->pindex;
  	KASSERT(count <= after + 1, ("%s: %d pages asked, can do only %d",
  	    __func__, count, after + 1));
  	after -= count - 1;
  
  	/* Trim requested rbehind/rahead to possible values. */   
  	rbehind = a_rbehind ? *a_rbehind : 0;
  	rahead = a_rahead ? *a_rahead : 0;
  	rbehind = min(rbehind, before);
  	rbehind = min(rbehind, m[0]->pindex);
  	rahead = min(rahead, after);
  	rahead = min(rahead, object->size - m[count - 1]->pindex);
  	KASSERT(rbehind + rahead + count <= sizeof(bp->b_pages),
  	    ("%s: behind %d ahead %d count %d", __func__,
  	    rbehind, rahead, count));
  
  	/*
  	 * Fill in the bp->b_pages[] array with requested and optional   
  	 * read behind or read ahead pages.  Read behind pages are looked
  	 * up in a backward direction, down to a first cached page.  Same
  	 * for read ahead pages, but there is no need to shift the array
  	 * in case of encountering a cached page.
  	 */
  	i = bp->b_npages = 0;
  	if (rbehind) {
  		vm_pindex_t startpindex, tpindex;
  		vm_page_t p;
  
  		VM_OBJECT_WLOCK(object);
  		startpindex = m[0]->pindex - rbehind;
  		if ((p = TAILQ_PREV(m[0], pglist, listq)) != NULL &&
  		    p->pindex >= startpindex)
  			startpindex = p->pindex + 1;
  
  		/* tpindex is unsigned; beware of numeric underflow. */
  		for (tpindex = m[0]->pindex - 1;
  		    tpindex >= startpindex && tpindex < m[0]->pindex;
  		    tpindex--, i++) {
  			p = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
  			    VM_ALLOC_IFNOTCACHED);
  			if (p == NULL) {
  				/* Shift the array. */
  				for (int j = 0; j < i; j++)
  					bp->b_pages[j] = bp->b_pages[j + 
  					    tpindex + 1 - startpindex]; 
  				break;
  			}
  			bp->b_pages[tpindex - startpindex] = p;
  		}
  
  		bp->b_pgbefore = i;
  		bp->b_npages += i;
  		bp->b_blkno -= IDX_TO_OFF(i) / DEV_BSIZE;
  	} else
  		bp->b_pgbefore = 0;
  
  	/* Requested pages. */
  	for (int j = 0; j < count; j++, i++)
  		bp->b_pages[i] = m[j];
  	bp->b_npages += count;
  
  	if (rahead) {
  		vm_pindex_t endpindex, tpindex;
  		vm_page_t p;
  
  		if (!VM_OBJECT_WOWNED(object))
  			VM_OBJECT_WLOCK(object);
  		endpindex = m[count - 1]->pindex + rahead + 1;
  		if ((p = TAILQ_NEXT(m[count - 1], listq)) != NULL &&
  		    p->pindex < endpindex)
  			endpindex = p->pindex;
  		if (endpindex > object->size)
  			endpindex = object->size;
  
  		for (tpindex = m[count - 1]->pindex + 1;
  		    tpindex < endpindex; i++, tpindex++) {
  			p = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
  			    VM_ALLOC_IFNOTCACHED);
  			if (p == NULL)
  				break;
  			bp->b_pages[i] = p;
  		}
  
  		bp->b_pgafter = i - bp->b_npages;
  		bp->b_npages = i;
  	} else
  		bp->b_pgafter = 0;
  
  	if (VM_OBJECT_WOWNED(object))
  		VM_OBJECT_WUNLOCK(object);
  
  	/* Report back actual behind/ahead read. */
  	if (a_rbehind)
  		*a_rbehind = bp->b_pgbefore;
  	if (a_rahead)
  		*a_rahead = bp->b_pgafter;
  
  	KASSERT(bp->b_npages <= sizeof(bp->b_pages),
  	    ("%s: buf %p overflowed", __func__, bp));
  
  	/*
  	 * Recalculate first offset and bytecount with regards to read behind.
  	 * Truncate bytecount to vnode real size and round up physical size
  	 * for real devices.
  	 */
  	foff = IDX_TO_OFF(bp->b_pages[0]->pindex);
  	bytecount = bp->b_npages << PAGE_SHIFT;
  	if ((foff + bytecount) > object->un_pager.vnp.vnp_size)
  		bytecount = object->un_pager.vnp.vnp_size - foff;
  	secmask = bo->bo_bsize - 1;
  	KASSERT(secmask < PAGE_SIZE && secmask > 0,
  	    ("%s: sector size %d too large", __func__, secmask + 1));
  	bytecount = (bytecount + secmask) & ~secmask;
  
  	/*
  	 * And map the pages to be read into the kva, if the filesystem
  	 * requires mapped buffers.
  	 */
  	if ((vp->v_mount->mnt_kern_flag & MNTK_UNMAPPED_BUFS) != 0 &&
  	    unmapped_buf_allowed) {
  		bp->b_data = unmapped_buf;
  		bp->b_offset = 0;
  	} else {
  		bp->b_data = bp->b_kvabase;
  		pmap_qenter((vm_offset_t)bp->b_data, bp->b_pages, bp->b_npages);
  	}
  
  	/* Build a minimal buffer header. */
  	bp->b_iocmd = BIO_READ;
  	KASSERT(bp->b_rcred == NOCRED, ("leaking read ucred"));
  	KASSERT(bp->b_wcred == NOCRED, ("leaking write ucred"));
  	bp->b_rcred = crhold(curthread->td_ucred);
  	bp->b_wcred = crhold(curthread->td_ucred);
  	pbgetbo(bo, bp);
  	bp->b_vp = vp;
  	bp->b_bcount = bp->b_bufsize = bp->b_runningbufspace = bytecount;
  	bp->b_iooffset = dbtob(bp->b_blkno);
  
  	atomic_add_long(&runningbufspace, bp->b_runningbufspace);
  	PCPU_INC(cnt.v_vnodein);
  	PCPU_ADD(cnt.v_vnodepgsin, bp->b_npages);
  
  	if (iodone != NULL) { /* async */
  		bp->b_pgiodone = iodone;
  		bp->b_caller1 = arg;
  		bp->b_iodone = vnode_pager_generic_getpages_done_async;
  		bp->b_flags |= B_ASYNC;
  		BUF_KERNPROC(bp);
  		bstrategy(bp);
  		return (VM_PAGER_OK);
  	} else {
  		bp->b_iodone = bdone;
  		bstrategy(bp);
  		bwait(bp, PVM, "vnread");
  		error = vnode_pager_generic_getpages_done(bp);
  		for (i = 0; i < bp->b_npages; i++)
  			bp->b_pages[i] = NULL;
  		bp->b_vp = NULL;
  		pbrelbo(bp);
  		relpbuf(bp, &vnode_pbuf_freecnt);
  		return (error != 0 ? VM_PAGER_ERROR : VM_PAGER_OK);
  	}
  }
  
  static void
  vnode_pager_generic_getpages_done_async(struct buf *bp)
  {
  	int error;
  
  	error = vnode_pager_generic_getpages_done(bp);
  	/* Run the iodone upon the requested range. */
  	bp->b_pgiodone(bp->b_caller1, bp->b_pages + bp->b_pgbefore,
  	    bp->b_npages - bp->b_pgbefore - bp->b_pgafter, error);
  	for (int i = 0; i < bp->b_npages; i++)
  		bp->b_pages[i] = NULL;
  	bp->b_vp = NULL;
  	pbrelbo(bp);
  	relpbuf(bp, &vnode_async_pbuf_freecnt);
  }
  
  static int
  vnode_pager_generic_getpages_done(struct buf *bp)
  {
  	vm_object_t object;
  	off_t tfoff, nextoff;
  	int i, error;
  
  	error = (bp->b_ioflags & BIO_ERROR) != 0 ? EIO : 0;
  	object = bp->b_vp->v_object;
  
  	if (error == 0 && bp->b_bcount != bp->b_npages * PAGE_SIZE) {
  		if (!buf_mapped(bp)) {
  			bp->b_data = bp->b_kvabase;
  			pmap_qenter((vm_offset_t)bp->b_data, bp->b_pages,
  			    bp->b_npages);
  		}
  		bzero(bp->b_data + bp->b_bcount,
  		    PAGE_SIZE * bp->b_npages - bp->b_bcount);
  	}
  	if (buf_mapped(bp)) {
  		pmap_qremove((vm_offset_t)bp->b_data, bp->b_npages);
  		bp->b_data = unmapped_buf;
  	}
  
  	VM_OBJECT_WLOCK(object);
  	for (i = 0, tfoff = IDX_TO_OFF(bp->b_pages[0]->pindex);
  	    i < bp->b_npages; i++, tfoff = nextoff) {
  		vm_page_t mt;
  
  		nextoff = tfoff + PAGE_SIZE;
  		mt = bp->b_pages[i];
  
  		if (nextoff <= object->un_pager.vnp.vnp_size) {
  			/*
  			 * Read filled up entire page.
  			 */
  			mt->valid = VM_PAGE_BITS_ALL;
  			KASSERT(mt->dirty == 0,
  			    ("%s: page %p is dirty", __func__, mt));
  			KASSERT(!pmap_page_is_mapped(mt),
  			    ("%s: page %p is mapped", __func__, mt));
  		} else {
  			/*
  			 * Read did not fill up entire page.
  			 *
  			 * Currently we do not set the entire page valid,
  			 * we just try to clear the piece that we couldn't
  			 * read.
  			 */
  			vm_page_set_valid_range(mt, 0,
  			    object->un_pager.vnp.vnp_size - tfoff);
  			KASSERT((mt->dirty & vm_page_bits(0,
  			    object->un_pager.vnp.vnp_size - tfoff)) == 0,
  			    ("%s: page %p is dirty", __func__, mt));
  		}
  
  		if (i < bp->b_pgbefore || i >= bp->b_npages - bp->b_pgafter)
  			vm_page_readahead_finish(mt);
  	}
  	VM_OBJECT_WUNLOCK(object);
  	if (error != 0)
  		printf("%s: I/O read error %d\n", __func__, error);
  
  	return (error);
  }
  
  /*
   * 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.
   *
   * 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,
      int flags, 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 (vm_cnt.v_free_count + vm_cnt.v_cache_count <
  	    vm_cnt.v_pageout_free_min)
  		flags |= VM_PAGER_PUT_SYNC;
  
  	/*
  	 * Call device-specific putpages function
  	 */
  	vp = object->handle;
  	VM_OBJECT_WUNLOCK(object);
  	rtval = VOP_PUTPAGES(vp, m, bytes, flags, rtvals);
  	KASSERT(rtval != EOPNOTSUPP, 
  	    ("vnode_pager: stale FS putpages\n"));
  	VM_OBJECT_WLOCK(object);
  }
  
  
  /*
   * 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 occurred, 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 *ma, int bytecount,
      int flags, int *rtvals)
  {
  	int i;
  	vm_object_t object;
  	vm_page_t m;
  	int count;
  
  	int maxsize, ncount;
  	vm_ooffset_t poffset;
  	struct uio auio;
  	struct iovec aiov;
  	int error;
  	int ioflags;
  	int ppscheck = 0;
  	static struct timeval lastfail;
  	static int curfail;
  
  	object = vp->v_object;
  	count = bytecount / PAGE_SIZE;
  
  	for (i = 0; i < count; i++)
  		rtvals[i] = VM_PAGER_ERROR;
  
  	if ((int64_t)ma[0]->pindex < 0) {
  		printf("vnode_pager_putpages: attempt to write meta-data!!! -- 0x%lx(%lx)\n",
  		    (long)ma[0]->pindex, (u_long)ma[0]->dirty);
  		rtvals[0] = VM_PAGER_BAD;
  		return VM_PAGER_BAD;
  	}
  
  	maxsize = count * PAGE_SIZE;
  	ncount = count;
  
  	poffset = IDX_TO_OFF(ma[0]->pindex);
  
  	/*
  	 * If the page-aligned write is larger then the actual file we
  	 * have to invalidate pages occurring beyond the file EOF.  However,
  	 * there is an edge case where a file may not be page-aligned where
  	 * the last page is partially invalid.  In this case the filesystem
  	 * may not properly clear the dirty bits for the entire page (which
  	 * could be VM_PAGE_BITS_ALL due to the page having been mmap()d).
  	 * With the page locked we are free to fix-up the dirty bits here.
  	 *
  	 * We do not under any circumstances truncate the valid bits, as
  	 * this will screw up bogus page replacement.
  	 */
  	VM_OBJECT_WLOCK(object);
  	if (maxsize + poffset > object->un_pager.vnp.vnp_size) {
  		if (object->un_pager.vnp.vnp_size > poffset) {
  			int pgoff;
  
  			maxsize = object->un_pager.vnp.vnp_size - poffset;
  			ncount = btoc(maxsize);
  			if ((pgoff = (int)maxsize & PAGE_MASK) != 0) {
  				/*
  				 * If the object is locked and the following
  				 * conditions hold, then the page's dirty
  				 * field cannot be concurrently changed by a
  				 * pmap operation.
  				 */
  				m = ma[ncount - 1];
  				vm_page_assert_sbusied(m);
  				KASSERT(!pmap_page_is_write_mapped(m),
  		("vnode_pager_generic_putpages: page %p is not read-only", m));
  				vm_page_clear_dirty(m, pgoff, PAGE_SIZE -
  				    pgoff);
  			}
  		} else {
  			maxsize = 0;
  			ncount = 0;
  		}
  		if (ncount < count) {
  			for (i = ncount; i < count; i++) {
  				rtvals[i] = VM_PAGER_BAD;
  			}
  		}
  	}
  	VM_OBJECT_WUNLOCK(object);
  
  	/*
  	 * 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 = (struct thread *) 0;
  	error = VOP_WRITE(vp, &auio, ioflags, curthread->td_ucred);
  	PCPU_INC(cnt.v_vnodeout);
  	PCPU_ADD(cnt.v_vnodepgsout, ncount);
  
  	if (error) {
  		if ((ppscheck = ppsratecheck(&lastfail, &curfail, 1)))
  			printf("vnode_pager_putpages: I/O error %d\n", error);
  	}
  	if (auio.uio_resid) {
  		if (ppscheck || ppsratecheck(&lastfail, &curfail, 1))
  			printf("vnode_pager_putpages: residual I/O %zd at %lu\n",
  			    auio.uio_resid, (u_long)ma[0]->pindex);
  	}
  	for (i = 0; i < ncount; i++) {
  		rtvals[i] = VM_PAGER_OK;
  	}
  	return rtvals[0];
  }
  
  void
  vnode_pager_undirty_pages(vm_page_t *ma, int *rtvals, int written)
  {
  	vm_object_t obj;
  	int i, pos;
  
  	if (written == 0)
  		return;
  	obj = ma[0]->object;
  	VM_OBJECT_WLOCK(obj);
  	for (i = 0, pos = 0; pos < written; i++, pos += PAGE_SIZE) {
  		if (pos < trunc_page(written)) {
  			rtvals[i] = VM_PAGER_OK;
  			vm_page_undirty(ma[i]);
  		} else {
  			/* Partially written page. */
  			rtvals[i] = VM_PAGER_AGAIN;
  			vm_page_clear_dirty(ma[i], 0, written & PAGE_MASK);
  		}
  	}
  	VM_OBJECT_WUNLOCK(obj);
  }
  
  void
  vnode_pager_update_writecount(vm_object_t object, vm_offset_t start,
      vm_offset_t end)
  {
  	struct vnode *vp;
  	vm_ooffset_t old_wm;
  
  	VM_OBJECT_WLOCK(object);
  	if (object->type != OBJT_VNODE) {
  		VM_OBJECT_WUNLOCK(object);
  		return;
  	}
  	old_wm = object->un_pager.vnp.writemappings;
  	object->un_pager.vnp.writemappings += (vm_ooffset_t)end - start;
  	vp = object->handle;
  	if (old_wm == 0 && object->un_pager.vnp.writemappings != 0) {
  		ASSERT_VOP_ELOCKED(vp, "v_writecount inc");
  		VOP_ADD_WRITECOUNT(vp, 1);
  		CTR3(KTR_VFS, "%s: vp %p v_writecount increased to %d",
  		    __func__, vp, vp->v_writecount);
  	} else if (old_wm != 0 && object->un_pager.vnp.writemappings == 0) {
  		ASSERT_VOP_ELOCKED(vp, "v_writecount dec");
  		VOP_ADD_WRITECOUNT(vp, -1);
  		CTR3(KTR_VFS, "%s: vp %p v_writecount decreased to %d",
  		    __func__, vp, vp->v_writecount);
  	}
  	VM_OBJECT_WUNLOCK(object);
  }
  
  void
  vnode_pager_release_writecount(vm_object_t object, vm_offset_t start,
      vm_offset_t end)
  {
  	struct vnode *vp;
  	struct mount *mp;
  	vm_offset_t inc;
  
  	VM_OBJECT_WLOCK(object);
  
  	/*
  	 * First, recheck the object type to account for the race when
  	 * the vnode is reclaimed.
  	 */
  	if (object->type != OBJT_VNODE) {
  		VM_OBJECT_WUNLOCK(object);
  		return;
  	}
  
  	/*
  	 * Optimize for the case when writemappings is not going to
  	 * zero.
  	 */
  	inc = end - start;
  	if (object->un_pager.vnp.writemappings != inc) {
  		object->un_pager.vnp.writemappings -= inc;
  		VM_OBJECT_WUNLOCK(object);
  		return;
  	}
  
  	vp = object->handle;
  	vhold(vp);
  	VM_OBJECT_WUNLOCK(object);
  	mp = NULL;
  	vn_start_write(vp, &mp, V_WAIT);
  	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
  
  	/*
  	 * Decrement the object's writemappings, by swapping the start
  	 * and end arguments for vnode_pager_update_writecount().  If
  	 * there was not a race with vnode reclaimation, then the
  	 * vnode's v_writecount is decremented.
  	 */
  	vnode_pager_update_writecount(object, end, start);
  	VOP_UNLOCK(vp, 0);
  	vdrop(vp);
  	if (mp != NULL)
  		vn_finished_write(mp);
  }
  