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,
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
The protection map is applied to the memory pages in the region starting at addr and con-
tinuing 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
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 mech-
anism is an additional constraint, so putting 0 in a 2-bit field won't allow writes to a
page that is otherwise write-protected.
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 possi-
bly 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).
Documentation/vm/hugetlbpage.txt in the Linux kernel source tree
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2012-07-13 SUBPAGE_PROT(2)