subpage_prot - Man Page

Standard C library (libc, -lc)

#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

int syscall(SYS_subpage_prot, unsigned long addr, unsigned long len,
            uint32_t *map);

Note: glibc provides no wrapper for subpage_prot(), necessitating the use of syscall(2).

The PowerPC-specific subpage_prot() 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 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 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 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.

Linux.

Linux 2.6.25 (PowerPC).

The system call is provided only if the kernel is configured with CONFIG_PPC_64K_PAGES.

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.

mprotect(2), syscall(2)

Documentation/admin-guide/mm/hugetlbpage.rst in the Linux kernel source tree

syscalls(2).