Function vm_fault_readahead

freebsd/vm/vm_fault.c:745–791

* Calculate the desired readahead. Handle drop-behind. * * Returns the number of readahead blocks to pass to the pager. */

 * Returns the number of readahead blocks to pass to the pager.
 */
static int
vm_fault_readahead(struct faultstate *fs)
{
	int era, nera;
	u_char behavior;

	KASSERT(fs->lookup_still_valid, ("map unlocked"));
	era = fs->entry->read_ahead;
	behavior = vm_map_entry_behavior(fs->entry);
	if (behavior == MAP_ENTRY_BEHAV_RANDOM) {
		nera = 0;
	} else if (behavior == MAP_ENTRY_BEHAV_SEQUENTIAL) {
		nera = VM_FAULT_READ_AHEAD_MAX;
		if (fs->vaddr == fs->entry->next_read)
			vm_fault_dontneed(fs, fs->vaddr, nera);
	} else if (fs->vaddr == fs->entry->next_read) {
		/*
		 * This is a sequential fault.  Arithmetically
		 * increase the requested number of pages in
		 * the read-ahead window.  The requested
		 * number of pages is "# of sequential faults
		 * x (read ahead min + 1) + read ahead min"
		 */
		nera = VM_FAULT_READ_AHEAD_MIN;
		if (era > 0) {
			nera += era + 1;
			if (nera > VM_FAULT_READ_AHEAD_MAX)
				nera = VM_FAULT_READ_AHEAD_MAX;
		}
		if (era == VM_FAULT_READ_AHEAD_MAX)
			vm_fault_dontneed(fs, fs->vaddr, nera);
	} else {
		/*
		 * This is a non-sequential fault.
		 */
		nera = 0;
	}
	if (era != nera) {
		/*
		 * A read lock on the map suffices to update
		 * the read ahead count safely.
		 */
		fs->entry->read_ahead = nera;
	}

	return (nera);
}

static int
vm_fault_lookup(struct faultstate *fs)