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

C. Copyright and License

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