52. VM for Solaris¶
Warning
As of 2013-05-26, the MPS is no longer supported on Solaris, so this document is only of historical interest.
52.1. Introduction¶
.intro: This is the design for the VM implementation on Solaris 2.x (see os.so for OS details). The implementation is in MMsrc!vmso.c (impl.c.vm). The design follows the design for and implements the contract of the generic VM interface (design.mps.vm). To summarize: The VM module provides a mechanism to reserve large (relative to the amount of RAM) amounts of address space, and functions to map (back with RAM) and unmap portions of this address space.
.source: Much of the implementation (and hence the design) was
inherited from the SunOS4 implementation. Not that there’s any design
for that. You’ll find the mmap(2) (for the system call mmap())
and the zero(7d) (for the device /dev/zero) man pages useful
as well. The generic interface and some generic design is in
design.mps.vm.
52.3. Overview¶
.over: The system calls mmap() and munmap() are used to
access the underlying functionality. They are used in slightly unusual
ways, typically to overcome baroque features or implementation details
of the operating system.
.over.reserve: In order to reserve address space, a mapping to a
file (/etc/passwd as it happens) is created with no protection
allowed.
.over.map: In order to map memory, a mapping to /dev/zero is
created.
.over.destroy: When the VM is destroyed, munmap() is used to
remove all the mappings previously created.
52.4. Implementation¶
.impl.create: VMCreate()
.impl.create.vmstruct: Enough pages to hold the VMStruct are
allocated by creating a mapping to /dev/zero (a read/write private
mapping), and using initializing the memory as a VMStruct.
.impl.create.reserve: The size parameter is rounded up to page size
and this amount of address space is reserved. The address space is
reserved by creating a shared mapping to /etc/passwd with no
access allowed (the prot argument is PROT_NONE, and the
flags argument is MAP_SHARED).
.impl.create.reserve.mmap.justify: mmap() gives us a flexible
way to allocate address space without interfering with any other
component in the process. Because we don’t specify MAP_FIXED we
are guaranteed to get a range of addresses that are not in use. Other
components must cooperate by not attempting to create mappings
specifying MAP_FIXED and an address in the range that the MPS has
reserved.
.impl.create.reserve.passwd.justify: Mapping /etc/passwd like
this worked on SunOS 4 (so this implementation inherited it). Mapping
/dev/zero with prot=PROT_NONE and flags=MAP_PRIVATE does
not work because Solaris gratuitously allocates swap (even though you
can’t use the memory).
.impl.create.reserve.improve: However, it would appears that or-ing
in MAP_NORESERVE mapping /dev/zero will reserve address space
without allocating swap, so this might be worth trying. That is, with
prot=PROT_NONE and flags=MAP_PRIVATE|MAP_NORESERVE. However
the following caveat comes from the original implementation:
“Experiments have shown that attempting to reserve address space by
mapping /dev/zero results in swap being reserved. This appears to
be a bug, so we work round it by using /etc/passwd, the only file
we can think of which is pretty much guaranteed to be around.” So that
might not work after all.
.impl.map.zero: A mapping to /dev/zero is created at the
relevant addresses (overriding the map to /etc/passwd that was
previously in place for those addresses). The prot argument is
specified as PROT_READ|PROT_WRITE|PROT_EXEC (so that any access is
allowed), the flags argument as MAP_PRIVATE|MAP_FIXED. The
flag MAP_PRIVATE means that the mapping is not shared with child
processes (child processes will have a mapping, but changes to the
memory will not be shared). The flag MAP_FIXED guarantees that we
get the mapping at the specified address). The zero(7d) man page
documents this as a way to create a “zero-initialized unnamed memory
object”.
.impl.map.error: If there’s not enough swap space for the mapping,
mmap() will return EAGAIN, not ENOMEM, although you might
not think so from the man page.
.impl.unmap.reserve: The relevant addresses are returned to the
reserved state by creating a mapping to /etc/passwd (overriding
the map /dev/zero that was previously in place for those
addresses). As for VMCreate() (see .impl.create.reserve above)
the prot argument is PROT_NONE, but the flags argument has
the addition MAP_FIXED flags (so is MAP_SHARED|MAP_FIXED).
.impl.unmap.reserve.offset: The offset argument is specified to be the offset of the addresses being unmapped from the base of the reserved VM area.
.impl.unmap.reserve.offset.justify: Not specifying the offset like
this makes Solaris create a separate mapping (in the kernel) each time
Unmap is used, eventually the call to mmap() will fail. Specifying
offset like this does not cause Solaris to create any extra mappings,
the existing mapping to /etc/passwd gets reused.
