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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
SUBPAGE_PROT(2) Linux Programmer's Manual SUBPAGE_PROT(2)
subpage_prot - define a subpage protection for an address range
long subpage_prot(unsigned long addr, unsigned long len,
uint32_t *map);
Note: There is no glibc wrapper for this system call; see NOTES.
The PowerPC-specific subpage_prot() system call provides the facility
to control the access permissions on individual 4kB subpages on
systems configured with a page size of 64kB.
The protection map is applied to the memory pages in the region
starting at addr and continuing for len bytes. Both of these
arguments must be aligned to a 64-kB boundary.
The protection map is specified in the buffer pointed to by map. The
map has 2 bits per 4kB subpage; thus each 32-bit word specifies the
protections of 16 4kB subpages inside a 64kB page (so, the number of
32-bit words pointed to by map should equate to the number of 64-kB
pages specified by len). Each 2-bit field in the protection map is
either 0 to allow any access, 1 to prevent writes, or 2 or 3 to
prevent all accesses.
On success, subpage_prot() returns 0. Otherwise, one of the error
codes specified below is returned.
EFAULT The buffer referred to by map is not accessible.
EINVAL The addr or len arguments are incorrect. Both of these
arguments must be aligned to a multiple of the system page
size, and they must not refer to a region outside of the
address space of the process or to a region that consists of
huge pages.
ENOMEM Out of memory.
This system call is provided on the PowerPC architecture since Linux
2.6.25. The system call is provided only if the kernel is configured
with CONFIG_PPC_64K_PAGES. No library support is provided.
This system call is Linux-specific.
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
Normal page protections (at the 64-kB page level) also apply; the
subpage protection mechanism is an additional constraint, so putting
0 in a 2-bit field won't allow writes to a page that is otherwise
write-protected.
Rationale
This system call is provided to assist writing emulators that operate
using 64-kB pages on PowerPC systems. When emulating systems such as
x86, which uses a smaller page size, the emulator can no longer use
the memory-management unit (MMU) and normal system calls for
controlling page protections. (The emulator could emulate the MMU by
checking and possibly remapping the address for each memory access in
software, but that is slow.) The idea is that the emulator supplies
an array of protection masks to apply to a specified range of virtual
addresses. These masks are applied at the level where hardware page-
table entries (PTEs) are inserted into the hardware page table based
on the Linux PTEs, so the Linux PTEs are not affected. Implicit in
this is that the regions of the address space that are protected are
switched to use 4-kB hardware pages rather than 64-kB hardware pages
(on machines with hardware 64-kB page support).
mprotect(2), syscall(2)
Documentation/vm/hugetlbpage.txt in the Linux kernel source tree
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2012-07-13 SUBPAGE_PROT(2)
Pages that refer to this page: syscalls(2)