Function vop_stdadvise

freebsd/kern/vfs_default.c:1080–1150

}

int
vop_stdadvise(struct vop_advise_args *ap)
{
	struct vnode *vp;
	struct bufobj *bo;
	daddr_t startn, endn;
	off_t bstart, bend, start, end;
	int bsize, error;

	vp = ap->a_vp;
	switch (ap->a_advice) {
	case POSIX_FADV_WILLNEED:
		/*
		 * Do nothing for now.  Filesystems should provide a
		 * custom method which starts an asynchronous read of
		 * the requested region.
		 */
		error = 0;
		break;
	case POSIX_FADV_DONTNEED:
		error = 0;
		vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
		if (VN_IS_DOOMED(vp)) {
			VOP_UNLOCK(vp);
			break;
		}

		/*
		 * Round to block boundaries (and later possibly further to
		 * page boundaries).  Applications cannot reasonably be aware  
		 * of the boundaries, and the rounding must be to expand at
		 * both extremities to cover enough.  It still doesn't cover
		 * read-ahead.  For partial blocks, this gives unnecessary
		 * discarding of buffers but is efficient enough since the
		 * pages usually remain in VMIO for some time.
		 */
		bsize = vp->v_bufobj.bo_bsize;
		bstart = rounddown(ap->a_start, bsize);
		bend = roundup(ap->a_end, bsize);

		/*
		 * Deactivate pages in the specified range from the backing VM
		 * object.  Pages that are resident in the buffer cache will
		 * remain wired until their corresponding buffers are released
		 * below.
		 */
		if (vp->v_object != NULL) {
			start = trunc_page(bstart);
			end = round_page(bend);
			VM_OBJECT_RLOCK(vp->v_object);
			vm_object_page_noreuse(vp->v_object, OFF_TO_IDX(start),
			    OFF_TO_IDX(end));
			VM_OBJECT_RUNLOCK(vp->v_object);
		}

		bo = &vp->v_bufobj;
		BO_RLOCK(bo);
		startn = bstart / bsize;