In case the implementation ever changes from using a chain of next pointers,
then changing the macro definition will be necessary, but changing all the
files that iterate over vm_map entries will not.
Drop a counter in vm_object.c that would have an effect only if the
vm_map entry count was wrong.
Discussed with: alc
Reviewed by: markj
Tested by: pho (earlier version)
Differential Revision: https://reviews.freebsd.org/D21882
Currently writemapping accounting is only done for vnode_pager which does
some accounting on the underlying vnode.
Extend this to allow accounting to be possible for any of the pager types.
New pageops are added to update/release writecount that need to be
implemented for any pager wishing to do said accounting, and we implement
these methods now for both vnode_pager (unchanged) and swap_pager.
The primary motivation for this is to allow other systems with OBJT_SWAP
objects to check if their objects have any write mappings and reject
operations with EBUSY if so. posixshm will be the first to do so in order to
reject adding write seals to the shmfd if any writable mappings exist.
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D21456
neighbors, and is used in a way so that if entries a and b cannot be
merged, we consider them twice, first not-merging a with its successor
b, and then not-merging b with its predecessor a. This change replaces
vm_map_simplify_entry with vm_map_try_merge_entries, which compares
two adjacent entries only, and uses it to avoid duplicated
merge-checks.
Tested by: pho
Reviewed by: alc
Approved by: markj (implicit)
Differential Revision: https://reviews.freebsd.org/D20814
Since the only caller to vm_map_splay is vm_map_lookup_entry, move the
implementation of vm_map_splay into vm_map_lookup_helper, called by
vm_map_lookup_entry.
vm_map_lookup_entry returns the greatest entry less than or equal to a
given address, but in many cases the caller wants the least entry
greater than or equal to the address and uses the next pointer to get
to it. Provide an alternative interface to lookup,
vm_map_lookup_entry_ge, to provide the latter behavior, and let
callers use one or the other rather than having them use the next
pointer after a lookup miss to get what they really want.
In vm_map_growstack, the caller wants an entry that includes a given
address, and either the preceding or next entry depending on the value
of eflags in the first entry. Incorporate that behavior into
vm_map_lookup_helper, the function that implements all of these
lookups.
Eliminate some temporary variables used with vm_map_lookup_entry, but
inessential.
Reviewed by: markj (earlier version)
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20664
Spell all bits in the hex constants.
Since all lines are modified, consistently use <tab> after #define.
Reviewed by: alc (previous version), dougm
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D20560
Historically we have not distinguished between kernel wirings and user
wirings for accounting purposes. User wirings (via mlock(2)) were
subject to a global limit on the number of wired pages, so if large
swaths of physical memory were wired by the kernel, as happens with
the ZFS ARC among other things, the limit could be exceeded, causing
user wirings to fail.
The change adds a new counter, v_user_wire_count, which counts the
number of virtual pages wired by user processes via mlock(2) and
mlockall(2). Only user-wired pages are subject to the system-wide
limit which helps provide some safety against deadlocks. In
particular, while sources of kernel wirings typically support some
backpressure mechanism, there is no way to reclaim user-wired pages
shorting of killing the wiring process. The limit is exported as
vm.max_user_wired, renamed from vm.max_wired, and changed from u_int
to u_long.
The choice to count virtual user-wired pages rather than physical
pages was done for simplicity. There are mechanisms that can cause
user-wired mappings to be destroyed while maintaining a wiring of
the backing physical page; these make it difficult to accurately
track user wirings at the physical page layer.
The change also closes some holes which allowed user wirings to succeed
even when they would cause the system limit to be exceeded. For
instance, mmap() may now fail with ENOMEM in a process that has called
mlockall(MCL_FUTURE) if the new mapping would cause the user wiring
limit to be exceeded.
Note that bhyve -S is subject to the user wiring limit, which defaults
to 1/3 of physical RAM. Users that wish to exceed the limit must tune
vm.max_user_wired.
Reviewed by: kib, ngie (mlock() test changes)
Tested by: pho (earlier version)
MFC after: 45 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19908
kern_execve() locks text vnode exclusive to be able to set and clear
VV_TEXT flag. VV_TEXT is mutually exclusive with the v_writecount > 0
condition.
The change removes VV_TEXT, replacing it with the condition
v_writecount <= -1, and puts v_writecount under the vnode interlock.
Each text reference decrements v_writecount. To clear the text
reference when the segment is unmapped, it is recorded in the
vm_map_entry backed by the text file as MAP_ENTRY_VN_TEXT flag, and
v_writecount is incremented on the map entry removal
The operations like VOP_ADD_WRITECOUNT() and VOP_SET_TEXT() check that
v_writecount does not contradict the desired change. vn_writecheck()
is now racy and its use was eliminated everywhere except access.
Atomic check for writeability and increment of v_writecount is
performed by the VOP. vn_truncate() now increments v_writecount
around VOP_SETATTR() call, lack of which is arguably a bug on its own.
nullfs bypasses v_writecount to the lower vnode always, so nullfs
vnode has its own v_writecount correct, and lower vnode gets all
references, since object->handle is always lower vnode.
On the text vnode' vm object dealloc, the v_writecount value is reset
to zero, and deadfs vop_unset_text short-circuit the operation.
Reclamation of lowervp always reclaims all nullfs vnodes referencing
lowervp first, so no stray references are left.
Reviewed by: markj, trasz
Tested by: mjg, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D19923
Drop the adj_free field from vm_map_entry_t. Refine the max_free field
so that p->max_free is the size of the largest gap with one endpoint
in the subtree rooted at p. Change vm_map_findspace so that, first,
the address-based splay is restricted to tree nodes with large-enough
max_free value, to avoid searching for the right starting point in a
subtree where all the gaps are too small. Second, when the address
search leads to a tree search for the first large-enough gap, that gap
is the subject of a splay-search that brings the gap to the top of the
tree, so that an immediate insertion will take constant time.
Break up the splay code into separate components, one for searching
and breaking up the tree and another for reassembling it. Use these
components, and not splay itself, for linking and unlinking. Drop the
after-where parameter to link, as it is computed as a side-effect of
the splay search.
Submitted by: Doug Moore <dougm@rice.edu>
Reviewed by: markj
Tested by: pho
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D17794
With this change, randomization can be enabled for all non-fixed
mappings. It means that the base address for the mapping is selected
with a guaranteed amount of entropy (bits). If the mapping was
requested to be superpage aligned, the randomization honours the
superpage attributes.
Although the value of ASLR is diminshing over time as exploit authors
work out simple ASLR bypass techniques, it elimintates the trivial
exploitation of certain vulnerabilities, at least in theory. This
implementation is relatively small and happens at the correct
architectural level. Also, it is not expected to introduce
regressions in existing cases when turned off (default for now), or
cause any significant maintaince burden.
The randomization is done on a best-effort basis - that is, the
allocator falls back to a first fit strategy if fragmentation prevents
entropy injection. It is trivial to implement a strong mode where
failure to guarantee the requested amount of entropy results in
mapping request failure, but I do not consider that to be usable.
I have not fine-tuned the amount of entropy injected right now. It is
only a quantitive change that will not change the implementation. The
current amount is controlled by aslr_pages_rnd.
To not spoil coalescing optimizations, to reduce the page table
fragmentation inherent to ASLR, and to keep the transient superpage
promotion for the malloced memory, locality clustering is implemented
for anonymous private mappings, which are automatically grouped until
fragmentation kicks in. The initial location for the anon group range
is, of course, randomized. This is controlled by vm.cluster_anon,
enabled by default.
The default mode keeps the sbrk area unpopulated by other mappings,
but this can be turned off, which gives much more breathing bits on
architectures with small address space, such as i386. This is tied
with the question of following an application's hint about the mmap(2)
base address. Testing shows that ignoring the hint does not affect the
function of common applications, but I would expect more demanding
code could break. By default sbrk is preserved and mmap hints are
satisfied, which can be changed by using the
kern.elf{32,64}.aslr.honor_sbrk sysctl.
ASLR is enabled on per-ABI basis, and currently it is only allowed on
FreeBSD native i386 and amd64 (including compat 32bit) ABIs. Support
for additional architectures will be added after further testing.
Both per-process and per-image controls are implemented:
- procctl(2) adds PROC_ASLR_CTL/PROC_ASLR_STATUS;
- NT_FREEBSD_FCTL_ASLR_DISABLE feature control note bit makes it possible
to force ASLR off for the given binary. (A tool to edit the feature
control note is in development.)
Global controls are:
- kern.elf{32,64}.aslr.enable - for non-fixed mappings done by mmap(2);
- kern.elf{32,64}.aslr.pie_enable - for PIE image activation mappings;
- kern.elf{32,64}.aslr.honor_sbrk - allow to use sbrk area for mmap(2);
- vm.cluster_anon - enables anon mapping clustering.
PR: 208580 (exp runs)
Exp-runs done by: antoine
Reviewed by: markj (previous version)
Discussed with: emaste
Tested by: pho
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D5603
Based on the description in Linux man page.
Reviewed by: markj, ngie (previous version)
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18837
Initializing the eflags field of the map->header entry to a value with a
unique new bit set makes a few comparisons to &map->header unnecessary.
Submitted by: Doug Moore <dougm@rice.edu>
Reviewed by: alc, kib
Tested by: pho
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14005
Exposing max_offset and min_offset defines in public headers is
causing clashes with variable names, for example when building QEMU.
Based on the submission by: royger
Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 1 week
Approved by: re (marius)
Differential revision: https://reviews.freebsd.org/D16881
- inline atomics in modules on i386 and amd64 (they were always
inline on other arches)
- allow modules to opt in to inlining locks by specifying
MODULE_TIED=1 in the makefile
Reviewed by: kib
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D16079
There are out of tree consumers of vm_map_min() and vm_map_max(), and
I believe there are consumers of vm_map_pmap(), although the later is
arguably less in the need of KBI-stable interface. For the consumers
benefit, make modules using this KPI not depended on the struct vm_map
layout.
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14902
In several places, entry start and end field are checked, after
excluding the possibility that the entry is map->header. By assigning
max and min values to the start and end fields of map->header in
vm_map_init, the explicit map->header checks become unnecessary.
Submitted by: Doug Moore <dougm@rice.edu>
Reviewed by: alc, kib, markj (previous version)
Tested by: pho (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13735
Mainly focus on files that use BSD 3-Clause license.
The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.
Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.
a hint.
Right now, for non-fixed mmap(2) calls, addr is de-facto interpreted
as the absolute minimal address of the range where the mapping is
created. The VA allocator only allocates in the range [addr,
VM_MAXUSER_ADDRESS]. This is too restrictive, the mmap(2) call might
unduly fail if there is no free addresses above addr but a lot of
usable space below it.
Lift this implementation limitation by allocating VA in two passes.
First, try to allocate above addr, as before. If that fails, do the
second pass with less restrictive constraints for the start of
allocation by specifying minimal allocation address at the max bss
end, if this limit is less than addr.
One important case where this change makes a difference is the
allocation of the stacks for new threads in libthr. Under some
configuration conditions, libthr tries to hint kernel to reuse the
main thread stack grow area for the new stacks. This cannot work by
design now after grow area is converted to stack, and there is no
unallocated VA above the main stack. Interpreting requested stack
base address as the hint provides compatibility with old libthr and
with (mis-)configured current libthr.
Reviewed by: alc
Tested by: dim (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Guard, requested by the MAP_GUARD mmap(2) flag, prevents the reuse of
the allocated address space, but does not allow instantiation of the
pages in the range. It is useful for more explicit support for usual
two-stage reserve then commit allocators, since it prevents accidental
instantiation of the mapping, e.g. by mprotect(2).
Use guards to reimplement stack grow code. Explicitely track stack
grow area with the guard, including the stack guard page. On stack
grow, trivial shift of the guard map entry and stack map entry limits
makes the stack expansion. Move the code to detect stack grow and
call vm_map_growstack(), from vm_fault() into vm_map_lookup().
As result, it is impossible to get random mapping to occur in the
stack grow area, or to overlap the stack guard page.
Enable stack guard page by default.
Reviewed by: alc, markj
Man page update reviewed by: alc, bjk, emaste, markj, pho
Tested by: pho, Qualys
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D11306 (man pages)
Renumber cluase 4 to 3, per what everybody else did when BSD granted
them permission to remove clause 3. My insistance on keeping the same
numbering for legal reasons is too pedantic, so give up on that point.
Submitted by: Jan Schaumann <jschauma@stevens.edu>
Pull Request: https://github.com/freebsd/freebsd/pull/96
vm_offset_t. (This field is used to detect sequential access to the virtual
address range represented by the map entry.) There are three reasons to
make this change. First, a vm_offset_t is smaller on 32-bit architectures.
Consequently, a struct vm_map_entry is now smaller on 32-bit architectures.
Second, a vm_offset_t can be written atomically, whereas it may not be
possible to write a vm_pindex_t atomically on a 32-bit architecture. Third,
using a vm_pindex_t makes the next_read field dependent on which object in
the shadow chain is being read from.
Replace an "XXX" comment.
Reviewed by: kib
Approved by: re (gjb)
Sponsored by: EMC / Isilon Storage Division
the VM_FAULT_CHANGE_WIRING flag to VM_FAULT_WIRE. Assert that the
flag is only passed when faulting on the wired map entry. Remove the
vm_page_unwire() call, which should be never reachable.
Since VM_FAULT_WIRE flag implies wired map entry, the TRYPAGER() macro
is reduced to the testing of the fs.object having a default pager.
Inline the check.
Suggested and reviewed by: alc
Tested by: pho (previous version)
MFC after: 1 week
and prevents the request from deleting existing mappings in the
region, failing instead.
Reviewed by: alc
Discussed with: jhb
Tested by: markj, pho (previous version, as part of the bigger patch)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
an address in the first 2GB of the process's address space. This flag should
have the same semantics as the same flag on Linux.
To facilitate this, add a new parameter to vm_map_find() that specifies an
optional maximum virtual address. While here, fix several callers of
vm_map_find() to use a VMFS_* constant for the findspace argument instead of
TRUE and FALSE.
Reviewed by: alc
Approved by: re (kib)
address alignment of mappings.
- MAP_ALIGNED(n) requests a mapping aligned on a boundary of (1 << n).
Requests for n >= number of bits in a pointer or less than the size of
a page fail with EINVAL. This matches the API provided by NetBSD.
- MAP_ALIGNED_SUPER is a special case of MAP_ALIGNED. It can be used
to optimize the chances of using large pages. By default it will align
the mapping on a large page boundary (the system is free to choose any
large page size to align to that seems best for the mapping request).
However, if the object being mapped is already using large pages, then
it will align the virtual mapping to match the existing large pages in
the object instead.
- Internally, VMFS_ALIGNED_SPACE is now renamed to VMFS_SUPER_SPACE, and
VMFS_ALIGNED_SPACE(n) is repurposed for specifying a specific alignment.
MAP_ALIGNED(n) maps to using VMFS_ALIGNED_SPACE(n), while
MAP_ALIGNED_SUPER maps to VMFS_SUPER_SPACE.
- mmap() of a device object now uses VMFS_OPTIMAL_SPACE rather than
explicitly using VMFS_SUPER_SPACE. All device objects are forced to
use a specific color on creation, so VMFS_OPTIMAL_SPACE is effectively
equivalent.
Reviewed by: alc
MFC after: 1 month
transparent layering and better fragmentation.
- Normalize functions that allocate memory to use kmem_*
- Those that allocate address space are named kva_*
- Those that operate on maps are named kmap_*
- Implement recursive allocation handling for kmem_arena in vmem.
Reviewed by: alc
Tested by: pho
Sponsored by: EMC / Isilon Storage Division
We cannot busy a page before doing pagefaults.
Infact, it can deadlock against vnode lock, as it tries to vget().
Other functions, right now, have an opposite lock ordering, like
vm_object_sync(), which acquires the vnode lock first and then
sleeps on the busy mechanism.
Before this patch is reinserted we need to break this ordering.
Sponsored by: EMC / Isilon storage division
Reported by: kib
- It does not let pages respect the LRU policy
- It bloats the active/inactive queues of few pages
Try to avoid it as much as possible with the long-term target to
completely remove it.
Use the soft-busy mechanism to protect page content accesses during
short-term operations (like uiomove_fromphys()).
After this change only vm_fault_quick_hold_pages() is still using the
hold mechanism for page content access.
There is an additional complexity there as the quick path cannot
immediately access the page object to busy the page and the slow path
cannot however busy more than one page a time (to avoid deadlocks).
Fixing such primitive can bring to complete removal of the page hold
mechanism.
Sponsored by: EMC / Isilon storage division
Discussed with: alc
Reviewed by: jeff
Tested by: pho
- Add a new address space allocation method (VMFS_OPTIMAL_SPACE) for
vm_map_find() that will try to alter the alignment of a mapping to match
any existing superpage mappings of the object being mapped. If no
suitable address range is found with the necessary alignment,
vm_map_find() will fall back to using the simple first-fit strategy
(VMFS_ANY_SPACE).
- Change mmap() without MAP_FIXED, shmat(), and the GEM mapping ioctl to
use VMFS_OPTIMAL_SPACE instead of VMFS_ANY_SPACE.
Reviewed by: alc (earlier version)
MFC after: 2 weeks
parallel creation of the map entries, e.g. by mmap() or stack growing.
It also breaks when other entry is wired in parallel.
The vm_map_wire() iterates over the map entries in the region, and
assumes that map entries it finds are marked as in transition before,
also that any entry marked as in transition, are marked by the current
invocation of vm_map_wire(). This is not true for new entries in the
holes.
Add the thread owner of the MAP_ENTRY_IN_TRANSITION flag to struct
vm_map_entry. In vm_map_wire() and vm_map_unwire(), only process the
entries which transition owner is the current thread.
Reported and tested by: pho
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
VM_KMEM_MAX_SIZE.
The code was not taking into account the size of the kernel_map, which
the kmem_map is allocated from, so it could produce a sub-map size too
large to fit. The simplest solution is to ignore VM_KMEM_MAX entirely
and base the memguard map's size off the kernel_map's size, since this
is always relevant and always smaller.
Found by: Justin Hibbits
There are two aspects to the sequential access optimization: (1) read ahead
of pages that are expected to be accessed in the near future and (2) unmap
and cache behind of pages that are not expected to be accessed again. This
revision changes both aspects.
The read ahead optimization is now more effective. It starts with the same
initial read window as before, but arithmetically grows the window on
sequential page faults. This can yield increased read bandwidth. For
example, on one of my machines, a program using mmap() to read a file that
is several times larger than the machine's physical memory takes about 17%
less time to complete.
The unmap and cache behind optimization is now more selectively applied.
The read ahead window must grow to its maximum size before unmap and cache
behind is performed. This significantly reduces the number of times that
pages are unmapped and cached only to be reactivated a short time later.
The unmap and cache behind optimization now clears each page's referenced
flag. Previously, in the case of dirty pages, if the containing file was
still mapped at the time that the page daemon examined the dirty pages,
they would be reactivated.
From a stylistic standpoint, this revision also cleanly separates the
implementation of the read ahead and unmap/cache behind optimizations.
Glanced at: kib
MFC after: 2 weeks
v_writecount. Keep the amount of the virtual address space used by
the mappings in the new vm_object un_pager.vnp.writemappings
counter. The vnode v_writecount is incremented when writemappings gets
non-zero value, and decremented when writemappings is returned to
zero.
Writeable shared vnode-backed mappings are accounted for in vm_mmap(),
and vm_map_insert() is instructed to set MAP_ENTRY_VN_WRITECNT flag on
the created map entry. During deferred map entry deallocation,
vm_map_process_deferred() checks for MAP_ENTRY_VN_WRITECOUNT and
decrements writemappings for the vm object.
Now, the writeable mount cannot be demoted to read-only while
writeable shared mappings of the vnodes from the mount point
exist. Also, execve(2) fails for such files with ETXTBUSY, as it
should be.
Noted by: tegge
Reviewed by: tegge (long time ago, early version), alc
Tested by: pho
MFC after: 3 weeks
for a shared mapping and marking the entry for inheritance.
Other thread might execute vmspace_fork() in between (e.g. by fork(2)),
resulting in the mapping becoming private.
Noted and reviewed by: alc
MFC after: 1 week
condition in proc_rwmem() and to (2) simplify the implementation of the
cxgb driver's vm_fault_hold_user_pages(). Specifically, in proc_rwmem()
the requested read or write could fail because the targeted page could be
reclaimed between the calls to vm_fault() and vm_page_hold().
In collaboration with: kib@
MFC after: 6 weeks
vmspace_fork and vm_map_wire that would lead to "vm_fault_copy_wired: page
missing" panics. While faulting in pages for a map entry that is being
wired down, mark the containing map as busy. In vmspace_fork wait until the
map is unbusy, before we try to copy the entries.
Reviewed by: kib
MFC after: 5 days
Sponsored by: Isilon Systems, Inc.
in "struct vm_object". This is required to make it possible to account
for per-jail swap usage.
Reviewed by: kib@
Tested by: pho@
Sponsored by: FreeBSD Foundation
its value as a loop invariant. Currently this is a no-op because
'atomic_cmpset_int()' clobbers all memory on current architectures.
- Use atomic_fetchadd_int() instead of an atomic_cmpset_int() loop to drop
a reference in vmspace_free().
Reviewed by: alc
MFC after: 1 month
vm_map_unlock_nodefer() part of the synchronization interface for maps.
Add comments to vm_map_unlock_and_wait() and vm_map_wakeup() describing
how they should be used. In particular, describe the deferred deallocations
issue with vm_map_unlock_and_wait().
Redo the implementation of vm_map_unlock_and_wait() so that it passes
along the caller's file and line information, just like the other map
locking primitives.
Reviewed by: kib
X-MFC after: r212824
on map unlock to the lock downgrade and later read unlock operation.
System map entries cannot be backed by OBJT_VNODE objects, no need to
defer deallocation for them. Map entries from user maps do not require
the owner map for deallocation, and can be accumulated in the
thread-local list for freeing when a user map is unlocked.
Move the collection of entries for deferred reclamation into
vm_map_delete(). Create helper vm_map_process_deferred(), that is
called from locations where processing is feasible. Do not process
deferred entries in vm_map_unlock_and_wait() since map_sleep_mtx is
held.
Reviewed by: alc, rstone (previous versions)
Tested by: pho
MFC after: 2 weeks
address space for an address as aligned by the new pmap_align_tlb()
function, which is for constraints imposed by the TLB. [1]
o) Add a kmem_alloc_nofault_space() function, which acts like
kmem_alloc_nofault() but allows the caller to specify which find-space
option to use. [1]
o) Use kmem_alloc_nofault_space() with VMFS_TLB_ALIGNED_SPACE to allocate the
kernel stack address on MIPS. [1]
o) Make pmap_align_tlb() on MIPS align addresses so that they do not start on
an odd boundary within the TLB, so that they are suitable for insertion as
wired entries and do not have to share a TLB entry with another mapping,
assuming they are appropriately-sized.
o) Eliminate md_realstack now that the kstack will be appropriately-aligned on
MIPS.
o) Increase the number of guard pages to 2 so that we retain the proper
alignment of the kstack address.
Reviewed by: [1] alc
X-MFC-after: Making sure alc has not come up with a better interface.
