Section: Linux Programmer's Manual (2)
subpage_prot - define a subpage protection for an address range
long subpage_prot(unsigned long addr, unsigned long len,
There is no glibc wrapper for this system call; see NOTES.
system call provides the facility to control the access
permissions on individual 4 kB subpages on systems configured with
a page size of 64 kB.
The protection map is applied to the memory pages in the region starting at
and continuing for
Both of these arguments must be aligned to a 64-kB boundary.
The protection map is specified in the buffer pointed to by
The map has 2 bits per 4 kB subpage;
thus each 32-bit word specifies the protections of 16 4 kB subpages
inside a 64 kB page
(so, the number of 32-bit words pointed to by
should equate to the number of 64-kB pages specified by
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.
Otherwise, one of the error codes specified below is returned.
The buffer referred to by
is not accessible.
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.
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
No library support is provided.
This system call is Linux-specific.
Glibc does not provide a wrapper for this system call; call it using
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
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).
in the Linux kernel source tree
This page is part of release 5.04 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at