to pool_init. Untouched pools are ones that either in arch-specific
code, or aren't initialiased during initial system startup.
Convert struct session, ucred and lockf to pools.
the latter is not a appropriate place to do so and it broke vfork.
- deactivate pmap before calling cpu_exit() to keep a balance of
pmap_activate/deactivate.
which is zero by default.
perform rbtree sanity checks only when it isn't zero
because the check is very heavy weight especially when
there're many entries.
in the case that there's no cached entries,
if kmem_map is already up, allocate a entry from it
so that we won't try to vm_map_lock recursively.
XXX assuming usage pattern of kmem_map.
- for in-kernel maps, disable map entry merging so that
unmap operations won't block. (workaround for PR/24039)
- for in-kernel maps, allocate kva for vm_map_entry from
the map itsself and eliminate MAX_KMAPENT and
uvm_map_entry_kmem_pool.
(1) split the single list of pages allocated to a pool into three lists:
completely full, partially full, and completely empty.
there is no longer any need to traverse any list looking for a
certain type of page.
(2) replace the 8-element hash table for out-of-page page headers
with a splay tree.
these two changes (together with the recent enhancements to the wait code)
give us linear scaling for a fork+exit microbenchmark.
to improve scalability of operations on the map.
originally done by Niels Provos for OpenBSD.
tweaked for NetBSD by me with some advices from enami tsugutomo.
discussed on tech-kern@ and tech-perform@.
it may return space already in use as free space under some condition.
The symptom of the bug is that exec fails if stack is unlimited on
topdown VM kernel.
previous entry. (not if the current entry starts at the end of the new
space; that case doesn't take into account if the new space had a specified
alignment).
means that the dynamic linker gets mapped in at the top of available
user virtual memory (typically just below the stack), shared libraries
get mapped downwards from that point, and calls to mmap() that don't
specify a preferred address will get mapped in below those.
This means that the heap and the mmap()ed allocations will grow
towards each other, allowing one or the other to grow larger than
before. Previously, the heap was limited to MAXDSIZ by the placement
of the dynamic linker (and the process's rlimits) and the space
available to mmap was hobbled by this reservation.
This is currently only enabled via an *option* for the i386 platform
(though other platforms are expected to follow). Add "options
USE_TOPDOWN_VM" to your kernel config file, rerun config, and rebuild
your kernel to take advantage of this.
Note that the pmap_prefer() interface has not yet been modified to
play nicely with this, so those platforms require a bit more work
(most notably the sparc) before they can use this new memory
arrangement.
This change also introduces a VM_DEFAULT_ADDRESS() macro that picks
the appropriate default address based on the size of the allocation or
the size of the process's text segment accordingly. Several drivers
and the SYSV SHM address assignment were changed to use this instead
of each one picking their own "default".
malloc types into a structure, a pointer to which is passed around,
instead of an int constant. Allow the limit to be adjusted when the
malloc type is defined, or with a function call, as suggested by
Jonathan Stone.
uvm_map(). Change uvm_map() to honnor UVM_KMF_NOWAIT. For this, change
amap_extend() to take a flags parameter instead of just boolean for
direction, and introduce AMAP_EXTEND_FORWARDS and AMAP_EXTEND_NOWAIT flags
(AMAP_EXTEND_BACKWARDS is still defined as 0x0, to keep the code easier to
read).
Add a flag parameter to uvm_mapent_alloc().
This solves a problem a pool_get(PR_NOWAIT) could trigger a pool_get(PR_WAITOK)
in uvm_mapent_alloc().
Thanks to Chuck Silvers, enami tsugutomo, Andrew Brown and Jason R Thorpe
for feedback.
with:
Case #1 -- adjust offset: The slot offset in the aref can be
decremented to cover the required size addition.
Case #2 -- move pages and adjust offset: The slot offset is not large
enough, but the amap contains enough inactive space *after* the mapped
pages to make up the difference, so active slots are slid to the "end"
of the amap, and the slot offset is, again, adjusted to cover the
required size addition. This optimizes for hitting case #1 again on
the next small extension.
Case #3 -- reallocate, move pages, and adjust offset: There is not
enough inactive space in the amap, so the arrays are reallocated, and
the active pages are copied again to the "end" of the amap, and the
slot offset is adjusted to cover the required size. This also
optimizes for hitting case #1 on the next backwards extension.
This provides the missing piece in the "forward extension of
vm_map_entries" logic, so the merge failure counters have been
removed.
Not many applications will make any use of this at this time (except
for jvms and perhaps gcc3), but a "top-down" memory allocator will use
it extensively.
backwards and forwards) if the previous entry was backed by an amap.
Fixes pr kern/18789, where netscape 7 + a java applet actually manage
to incur forward and bimerges in userspace.
Code reviewed by fvdl and thorpej.
allocations can be merged either forwards or backwards, meaning no new
entries will be added to the list, and some can even be merged in both
directions, resulting in a surplus entry.
This code typically reduces the number of map entries in the
kernel_map by an order of magnitude or more. It also makes possible
recovery from the pathological case of "5000 processes created and
then killed", which leaves behind a large number of map entries.
The only forward merge case not covered is the instance of an amap
that has to be extended backwards (WIP). Note that this only affects
processes, not the kernel (the kernel doesn't use amaps), and that
merge opportunities like this come up *very* rarely, if at all. Eg,
after being up for eight days, I see only three failures in this
regard, and even those are most likely due to programs I'm developing
to exercise this case.
Code reviewed by thorpej, matt, christos, mrg, chuq, chuck, perry,
tls, and probably others. I'd like to thank my mother, the Hollywood
Foreign Press...
tearing down a vm_map. use this to skip the pmap_update()
at the end of all the removes, which allows pmaps to optimize
pmap tear-down. also, use the new pmap_remove_all() hook to
let the pmap implemenation know what we're up to.
return failure if swap is full and there are no free physical pages.
have malloc() use this flag if M_CANFAIL is passed to it.
use M_CANFAIL to allow amap_extend() to fail when memory is scarce.
this should prevent most of the remaining hangs in low-memory situations.
deal with shortages of the VM maps where the backing pages are mapped
(usually kmem_map). Try to deal with this:
* Group all information about the backend allocator for a pool in a
separate structure. The pool references this structure, rather than
the individual fields.
* Change the pool_init() API accordingly, and adjust all callers.
* Link all pools using the same backend allocator on a list.
* The backend allocator is responsible for waiting for physical memory
to become available, but will still fail if it cannot callocate KVA
space for the pages. If this happens, carefully drain all pools using
the same backend allocator, so that some KVA space can be freed.
* Change pool_reclaim() to indicate if it actually succeeded in freeing
some pages, and use that information to make draining easier and more
efficient.
* Get rid of PR_URGENT. There was only one use of it, and it could be
dealt with by the caller.
From art@openbsd.org.
from VM_FAULT_WIRE in that when the pages being wired are faulted in,
the simulated fault is at the maximum protection allowed for the mapping
instead of the current protection. use this in uvm_map_pageable{,_all}()
to fix the problem where writing via ptrace() to shared libraries that
are also mapped with wired mappings in another process causes a
diagnostic panic when the wired mapping is removed.
this is a really obscure problem so it deserves some more explanation.
ptrace() writing to another process ends up down in uvm_map_extract(),
which for MAP_PRIVATE mappings (such as shared libraries) will cause
the amap to be copied or created. then the amap is made shared
(ie. the AMAP_SHARED flag is set) between the kernel and the ptrace()d
process so that the kernel can modify pages in the amap and have the
ptrace()d process see the changes. then when the page being modified
is actually faulted on, the object pages (from the shared library vnode)
is copied to a new anon page and inserted into the shared amap.
to make all the processes sharing the amap actually see the new anon
page instead of the vnode page that was there before, we need to
invalidate all the pmap-level mappings of the vnode page in the pmaps
of the processes sharing the amap, but we don't have a good way of
doing this. the amap doesn't keep track of the vm_maps which map it.
so all we can do at this point is to remove all the mappings of the
page with pmap_page_protect(), but this has the unfortunate side-effect
of removing wired mappings as well. removing wired mappings with
pmap_page_protect() is a legitimate operation, it can happen when a file
with a wired mapping is truncated. so the pmap has no way of knowing
whether a request to remove a wired mapping is normal or when it's due to
this weird situation. so the pmap has to remove the weird mapping.
the process being ptrace()d goes away and life continues. then,
much later when we go to unwire or remove the wired vm_map mapping,
we discover that the pmap mapping has been removed when it should
still be there, and we panic.
so where did we go wrong? the problem is that we don't have any way
to update just the pmap mappings that need to be updated in this
scenario. we could invent a mechanism to do this, but that is much
more complicated than this change and it doesn't seem like the right
way to go in the long run either.
the real underlying problem here is that wired pmap mappings just
aren't a good concept. one of the original properties of the pmap
design was supposed to be that all the information in the pmap could
be thrown away at any time and the VM system could regenerate it all
through fault processing, but wired pmap mappings don't allow that.
a better design for UVM would not require wired pmap mappings,
and Chuck C. and I are talking about this, but it won't be done
anytime soon, so this change will do for now.
this change has the effect of causing MAP_PRIVATE mappings to be
copied to anonymous memory when they are mlock()d, so that uvm_fault()
doesn't need to copy these pages later when called from ptrace(), thus
avoiding the call to pmap_page_protect() and the panic that results
from this when the mlock()d region is unlocked or freed. note that
this change doesn't help the case where the wired mapping is MAP_SHARED.
discussed at great length with Chuck Cranor.
fixes PRs 10363, 12554, 12604, 13041, 13487, 14580 and 14853.
we need to make sure that vnode pages are written to disk at least once,
otherwise processes could gain access to whatever data was previously stored
in disk blocks which are freshly allocated to a file.
uobject and uanon pointers rather than at the PQ_ANON flag to determine
which lock to hold, since PQ_ANON can be clear even when the anon's lock
is the one which we should hold (if the page was loaned from an object
and then freed by the object).
(either the current protection or the max protection) that reference
vnodes associated with a file system mounted with the NOEXEC option.
uvm_mmap(): Don't allow PROT_EXEC mappings to be established of vnodes
which are associated with a file system mounted with the NOEXEC option.
executable mappings. Stop overloading VTEXT for this purpose (VTEXT
also has another meaning).
- Rename vn_marktext() to vn_markexec(), and use it when executable
mappings of a vnode are established.
- In places where we want to set VTEXT, set it in v_flag directly, rather
than making a function call to do this (it no longer makes sense to
use a function call, since we no longer overload VTEXT with VEXECMAP's
meaning).
VEXECMAP suggested by Chuq Silvers.
- remove special treatment of pager_map mappings in pmaps. this is
required now, since I've removed the globals that expose the address range.
pager_map now uses pmap_kenter_pa() instead of pmap_enter(), so there's
no longer any need to special-case it.
- eliminate struct uvm_vnode by moving its fields into struct vnode.
- rewrite the pageout path. the pager is now responsible for handling the
high-level requests instead of only getting control after a bunch of work
has already been done on its behalf. this will allow us to UBCify LFS,
which needs tighter control over its pages than other filesystems do.
writing a page to disk no longer requires making it read-only, which
allows us to write wired pages without causing all kinds of havoc.
- use a new PG_PAGEOUT flag to indicate that a page should be freed
on behalf of the pagedaemon when it's unlocked. this flag is very similar
to PG_RELEASED, but unlike PG_RELEASED, PG_PAGEOUT can be cleared if the
pageout fails due to eg. an indirect-block buffer being locked.
this allows us to remove the "version" field from struct vm_page,
and together with shrinking "loan_count" from 32 bits to 16,
struct vm_page is now 4 bytes smaller.
- no longer use PG_RELEASED for swap-backed pages. if the page is busy
because it's being paged out, we can't release the swap slot to be
reallocated until that write is complete, but unlike with vnodes we
don't keep a count of in-progress writes so there's no good way to
know when the write is done. instead, when we need to free a busy
swap-backed page, just sleep until we can get it busy ourselves.
- implement a fast-path for extending writes which allows us to avoid
zeroing new pages. this substantially reduces cpu usage.
- encapsulate the data used by the genfs code in a struct genfs_node,
which must be the first element of the filesystem-specific vnode data
for filesystems which use genfs_{get,put}pages().
- eliminate many of the UVM pagerops, since they aren't needed anymore
now that the pager "put" operation is a higher-level operation.
- enhance the genfs code to allow NFS to use the genfs_{get,put}pages
instead of a modified copy.
- clean up struct vnode by removing all the fields that used to be used by
the vfs_cluster.c code (which we don't use anymore with UBC).
- remove kmem_object and mb_object since they were useless.
instead of allocating pages to these objects, we now just allocate
pages with no object. such pages are mapped in the kernel until they
are freed, so we can use the mapping to find the page to free it.
this allows us to remove splvm() protection in several places.
The sum of all these changes improves write throughput on my
decstation 5000/200 to within 1% of the rate of NetBSD 1.5
and reduces the elapsed time for "make release" of a NetBSD 1.5
source tree on my 128MB pc to 10% less than a 1.5 kernel took.
This will allow improvements to the pmaps so that they can more easily defer expensive operations, eg tlb/cache flush, til the last possible moment.
Currently this is a no-op on most platforms, so they should see no difference.
Reviewed by Jason.
kernel_map. use this instead of the static map entries when allocating
map entries for kernel_map. this greatly reduces the number of static
map entries used and should eliminate the problems with running out.
algorithm (Solaris calls this "Bin Hopping").
This implementation currently relies on MD code to define a
constant defining the number of buckets. This will change
reasonably soon (MD code will be able to dynamically size
the bucket array).
- pmap_enter()
- pmap_remove()
- pmap_protect()
- pmap_kenter_pa()
- pmap_kremove()
as described in pmap(9).
These calls are relatively conservative. It may be possible to
optimize these a little more.
space is already torn down in uvmspace_free() when the vmspace
refrence count reaches 0. Move the shmexit() call into uvmspace_free().
Note that there is a beneficial side-effect of deferring the unmap
to uvmspace_free() -- on systems where TLB invalidations are
particularly expensive, the unmapping of the address space won't
have to cause TLB invalidations; uvmspace_free() is going to be
run in a context other than the exiting process's, so the "pmap is
active" test will evaluate to FALSE in the pmap module.
entry in the map. the old code would walk around the end of the linked list,
through the header entry, and keep going from the first map entry until it
found a gap in the map, at which point it would return an error. if the map
had no gaps then it would loop forever. reported by k-abe@cs.utah.edu.
while I'm here, clean up this function a bit.
also, use MIN() instead of min(), since the latter takes arguments of
type "int" but we're passing it values of type "vaddr_t", which can be
a larger size.
Mach VM's now. Specific changes:
- Pages now need not have all of their mappings removed before being
put on the inactive list. They only need to have the "referenced"
attribute cleared. This makes putting pages onto the inactive list
much more efficient. In order to eliminate redundant clearings of
"refrenced", callers of uvm_pagedeactivate() must now do this
themselves.
- When checking the "modified" attribute for a page (for clearing
PG_CLEAN), make sure to only do it if PG_CLEAN is currently set on
the page (saves a potentially expensive pmap operation).
- When scanning the inactive list, if a page is referenced, reactivate
it (this part was actually added in uvm_pdaemon.c,v 1.27). This
now works properly now that pages on the inactive list are allowed to
have mappings.
- When scanning the inactive list and considering a page for freeing,
remove all mappings, and then check the "modified" attribute if the
page is marked PG_CLEAN.
- When scanning the active list, if the page was referenced since its
last sweep by the scanner, don't deactivate it. (This part was
actually added in uvm_pdaemon.c,v 1.28.)
These changes greatly improve interactive performance during
moderate to high memory and I/O load.
use queue.h macros and KASSERT().
address amap offsets in pages instead of bytes.
make amap_ref() and amap_unref() take an amap, offset and length
instead of a vm_map_entry_t.
improve whitespace and comments.
it and free it as appropriate. Activate p2's new address space once
it references p1's.
- uvm_fork(): Make sure the child's vmspace is NULL before calling
uvmspace_share() (the child doens't have one already in this case).
These changes do not change the behavior for the current use of
uvmspace_share() (vfork(2)), but make it possible for an already
running process (such as a kernel thread) to properly attach to
another process's address space.
to the contents of the hint in the map, and the hint saved in the
map only if the two values match. When an unconditional save is
required, the "check" value passed should be map->hint (and the
compiler will optimize the test away). When deleting a map entry,
the new SAVE_HINT() will only change the hint if the entry being
deleted was the hint value (thus preserving any meaningful hint
that may have been there previously, rather than stomping on it).
- Add a missing hint update when deleting the map entry in
uvm_map_entry_unlink(). This is the fix for kern/11125, from
ITOH Yasufumi <itohy@netbsd.org>.
routine. Works similarly fto pmap_prefer(), but allows callers
to specify a minimum power-of-two alignment of the region.
How we ever got along without this for so long is beyond me.
<vm/pglist.h> -> <uvm/uvm_pglist.h>
<vm/vm_inherit.h> -> <uvm/uvm_inherit.h>
<vm/vm_kern.h> -> into <uvm/uvm_extern.h>
<vm/vm_object.h> -> nothing
<vm/vm_pager.h> -> into <uvm/uvm_pager.h>
also includes a bunch of <vm/vm_page.h> include removals (due to redudancy
with <vm/vm.h>), and a scattering of other similar headers.
- Make page free lists have two actual queues: known-zero pages and
pages with unknown contents.
- Implement uvm_pageidlezero(). This function attempts to zero up to
the target number of pages until the target has been reached (currently
target is `all free pages') or until whichqs becomes non-zero (indicating
that a process is ready to run).
- Define a new hook for the pmap module for pre-zero'ing pages. This is
used to zero the pages using uncached access. This allows us to zero
as many pages as we want without polluting the cache.
In order to use this feature, each platform must add the appropropriate
glue in their idle loop.
one pmap and activating another. this isn't actually necessary (since
pmap_activate() and pmap_deactivate() affect only user-level mappings,
which cannot be accessed from interrupts anyway), and pmap_activate()
is very slow on old sun4c sparcs so we can't block interrupts for this long.
this fixes PR 8322.
Add a new type voff_t (defined as a synonym for off_t) to describe offsets
into uvm objects, and update the appropriate interfaces to use it, the
most visible effect being the ability to mmap() file offsets beyond
the range of a vaddr_t.
Originally by Chuck Silvers; blame me for problems caused by merging this
into non-UBC.
- Fix some locking bugs; a couple of places would return an error condition
without unlocking the map.
- Deal with maps marked WIREFUTURE; if making an entry VM_PROT_NONE ->
anything else, and it is not already marked as wired, wire it.
of some functions. Use these flags in uvm_map_pageable() to determine
if the map is locked on entry (replaces an already present boolean_t
argument `islocked'), and if the function should return with the map
still locked.
pages.
XXX This should be handled better in the future, probably by marking the
XXX page as released, and making uvm_pageunwire() free the page when
XXX the wire count on a released page reaches zero.
* Implement MADV_DONTNEED: deactivate pages in the specified range,
semantics similar to Solaris's MADV_DONTNEED.
* Add MADV_FREE: free pages and swap resources associated with the
specified range, causing the range to be reloaded from backing
store (vnodes) or zero-fill (anonymous), semantics like FreeBSD's
MADV_FREE and like Digital UNIX's MADV_DONTNEED (isn't it SO GREAT
that madvise(2) isn't standardized!?)
As part of this, move the non-map-modifying advice handling out of
uvm_map_advise(), and into sys_madvise().
As another part, implement general amap cleaning in uvm_map_clean(), and
change uvm_map_clean() to only push dirty pages to disk if PGO_CLEANIT
is set in its flags (and update sys___msync13() accordingly). XXX Add
a patchable global "amap_clean_works", defaulting to 1, which can disable
the amap cleaning code, just in case problems are unearthed; this gives
a developer/user a quick way to recover and send a bug report (e.g. boot
into DDB and change the value).
XXX Still need to implement a real uao_flush().
XXX Need to update the manual page.
With these changes, rebuilding libc will automatically cause the new
malloc(3) to use MADV_FREE to actually release pages and swap resources
when it decides that can be done.
the map be at least read-locked to call this function. This requirement
will be taken advantage of in a future commit.
* Write a uvm_fault_unwire() wrapper which read-locks the map and calls
uvm_fault_unwire_locked().
* Update the comments describing the locking contraints of uvm_fault_wire()
and uvm_fault_unwire().
semantics. That is, regardless of the number of mlock/mlockall calls,
an munlock/munlockall actually unlocks the region (i.e. sets wiring count
to 0).
Add a comment describing why uvm_map_pageable() should not be used for
transient page wirings (e.g. for physio) -- note, it's currently only
(ab)used in this way by a few pieces of code which are known to be
broken, i.e. the Amiga and Atari pmaps, and i386 and pc532 if PMAP_NEW is
not used. The i386 GDT code uses uvm_map_pageable(), but in a safe
way, and could be trivially converted to use uvm_fault_wire() instead.
* Provide POSIX 1003.1b mlockall(2) and munlockall(2) system calls.
MCL_CURRENT is presently implemented. MCL_FUTURE is not fully
implemented. Also, the same one-unlock-for-every-lock caveat
currently applies here as it does to mlock(2). This will be
addressed in a future commit.
* Provide the mincore(2) system call, with the same semantics as
Solaris.
* Clean up the error recovery in uvm_map_pageable().
* Fix a bug where a process would hang if attempting to mlock a
zero-fill region where none of the pages in that region are resident.
[ This fix has been submitted for inclusion in 1.4.1 ]
setting recursive has no effect! The kernel lock manager doesn't allow
an exclusive recursion into a shared lock. This situation must simply
be avoided. The only place where this might be a problem is the (ab)use
of uvm_map_pageable() in the Utah-derived pmaps for m68k (they should
either toss the iffy scheme they use completely, or use something like
uvm_fault_wire()).
In addition, once we have looped over uvm_fault_wire(), only upgrade to
an exclusive (write) lock if we need to modify the map again (i.e.
wiring a page failed).
don't unlock a kernel map (!!!) and then relock it later; a recursive lock,
as it used in the user map case, is fine. Also, don't change map entries
while only holding a read lock on the map. Instead, if we fail to wire
a page, clear recursive locking, and upgrade back to a write lock before
dropping the wiring count on the remaining map entries.
locks (and thus, never shared locks). Move the "set/clear recursive"
functions to uvm_map.c, which is the only placed they're used (and
they should go away anyhow). Delete some unused cruft.
right access_type to pass to uvm_fault_wire(). This way, if the entry has
VM_PROT_WRITE, and the entry is marked COW, the copy will happen immediately
in uvm_fault(), as if the access were performed.
has PAGEABLE and INTRSAFE flags. PAGEABLE now really means "pageable",
not "allocate vm_map_entry's from non-static pool", so update all map
creations to reflect that. INTRSAFE maps are maps that are used in
interrupt context (e.g. kmem_map, mb_map), and thus use the static
map entry pool (XXX as does kernel_map, for now). This will eventually
change now these maps are locked, as well.
which can be used in an interrupt context. Use pmap_kenter*() and
pmap_kremove() only for mappings owned by these objects.
Fixes some locking protocol issues related to MP support, and eliminates
all of the pmap_enter vs. pmap_kremove inconsistencies.
end of the mappable kernel virtual address space. Previously, it would
get called more often than necessary, because the caller only new what
was requested.
Also, export uvm_maxkaddr so that uvm_pageboot_alloc() can grow the
kernel pmap if necessary, as well. Note that pmap_growkernel() must
now be able to handle being called before pmap_init().
uvmspace_fork().
pmap_fork() is used to "fork a pmap", that is copy data from one pmap
to the other that is NOT related to actual mappings in the pmap, but is
otherwise logically coupled to the address space.
- break anon related functions out of uvm_amap.c and put them in their own
file (uvm_anon.c). includes break up uvm_anon_init into an amap and an
an anon init function
- ensure that only functions within the amap module access amap structure
fields (add macros to amap api as needed)
- replace map checks with submap checks
- get rid of unused 'mainonly' arg in uvm_unmap/uvm_unmap_remove, simplify
code. update all calls to reflect this.
- don't worry about unmapping or changing the protection of shared share
map mappings (is_main_map no longer used).
- remove unused uvm_map_sharemapcopy() function from fork code.
default free list, and 0 - N additional free list, in order of descending
priority.
A new page allocation function, uvm_pagealloc_strat(), has been added,
providing three page allocation strategies:
- normal: high -> low priority free list walk, taking the
page off the first free list that has one.
- only: attempt to allocate a page only from the specified free
list, failing if that free list has none available.
- fallback: if `only' fails, fall back on `normal'.
uvm_pagealloc(...) is provided for normal use (and is a synonym for
uvm_pagealloc_strat(..., UVM_PGA_STRAT_NORMAL, 0); the free list argument
is ignored for the `normal' case).
uvm_page_physload() now specified which free list the pages will be
loaded onto. This means that some platforms which have multiple physical
memory segments may define additional vm_physsegs if they wish to break
individual physical segments into differing priorities.
Machine-dependent code must define _at least_ the following constants
in <machine/vmparam.h>:
VM_NFREELIST: the number of free lists the system will have
VM_FREELIST_DEFAULT: the default freelist (should always be 0,
but is defined in machdep code so that it's with all of the
other free list-related constants).
Additional free list names may be defined by machine-dependent code, but
they will only be used by machine-dependent code (e.g. for loading the
vm_physsegs).
petrov