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Memory Pool System Project

                             VM DESIGN FOR SOLARIS
                                design.mps.vmso
                                 incomplete doc
                                 drj 1998-05-08

    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.

    DEFINITIONS

    .def: See design.mps.vm.def.* for definitions common to all VMs.

    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 OS.  .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.

    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 (prot argument is PROT_NONE, 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 SunOS4 (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 ORing in
    MAP_NORESERVE mapping /dev/zero will reserve address space without allocating
    swap, so this might be worth trying.  I.e., with prot=PROT_NONE,
    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: VMMap

    .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
    (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).
    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: VMUnmap

    .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.

## A. References

## B. Document History

2002-06-07 | RB | Converted from MMInfo database design document.

## C. Copyright and License

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