.. mode: -*- rst -*- VM for Solaris ============== :Tag: design.mps.vmso :Author: David Jones :Date: 1998-05-08 :Status: incomplete document :Revision: $Id: //info.ravenbrook.com/project/mps/version/1.116/design/vmso.txt#1 $ :Copyright: See `Copyright and License`_. :Index terms: pair: VM for Solaris; design .. warning:: As of 2013-05-26, the MPS is no longer supported on Solaris, so this document is only of historical interest. 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. .. _design.mps.vm: vm _`.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. .. _design.mps.vm.def: vm#def 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. 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`: ``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``. 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`: ``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. Document History ---------------- - 1998-05-08 David Jones. Incomplete document. - 2002-06-07 RB_ Converted from MMInfo database design document. - 2013-05-26 GDR_ Converted to reStructuredText. .. _RB: http://www.ravenbrook.com/consultants/rb/ .. _GDR: http://www.ravenbrook.com/consultants/gdr/ Copyright and License --------------------- Copyright © 2013-2014 Ravenbrook Limited . All rights reserved. This is an open source license. Contact Ravenbrook for commercial licensing options. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: #. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. #. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. #. 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