02-18-2013
Under VM, not dirty pages stay mapped in case anyone wants to read them through a mmap(). If you need a page, they can be taken, but they are not free. You can run a job that mmap()'s a file, read the whole file into ram, exit, run it again, and when it mmap()s the file again, the old pages are mapped into the new process memory range and it runs much faster. This is what happens with libc.so: everyone runs printf() from the same ram page.
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### snip
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LEARN ABOUT SUNOS
mprotect
mprotect(2) System Calls mprotect(2)
NAME
mprotect - set protection of memory mapping
SYNOPSIS
#include <sys/mman.h>
int mprotect(void *addr, size_t len, int prot);
DESCRIPTION
The mprotect() function changes the access protections on the mappings specified by the range [addr, addr + len), rounding len up to the
next multiple of the page size as returned by sysconf(3C), to be that specified by prot. Legitimate values for prot are the same as those
permitted for mmap(2) and are defined in <sys/mman.h> as:
PROT_READ /* page can be read */
PROT_WRITE /* page can be written */
PROT_EXEC /* page can be executed */
PROT_NONE /* page can not be accessed */
When mprotect() fails for reasons other than EINVAL, the protections on some of the pages in the range [addr, addr + len) may have been
changed. If the error occurs on some page at addr2, then the protections of all whole pages in the range [addr, addr2] will have been modi-
fied.
RETURN VALUES
Upon successful completion, mprotect() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.
ERRORS
The mprotect() function will fail if:
EACCES The prot argument specifies a protection that violates the access permission the process has to the underlying memory
object.
EINVAL The len argument has a value equal to 0, or addr is not a multiple of the page size as returned by sysconf(3C).
ENOMEM Addresses in the range [addr, addr + len) are invalid for the address space of a process, or specify one or more pages
which are not mapped.
The mprotect() function may fail if:
EAGAIN The address range [addr, addr + len) includes one or more pages that have been locked in memory and that were mapped
MAP_PRIVATE; prot includes PROT_WRITE; and the system has insufficient resources to reserve memory for the private pages
that may be created. These private pages may be created by store operations in the now-writable address range.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
SEE ALSO
mmap(2), plock(3C), mlock(3C), mlockall(3C), sysconf(3C), attributes(5), standards(5)
SunOS 5.10 12 Jan 1998 mprotect(2)