NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

SUBPAGE_PROT(2)           Linux Programmer's Manual          SUBPAGE_PROT(2)

NAME         top

       subpage_prot - define a subpage protection for an address range

SYNOPSIS         top

       long subpage_prot(unsigned long addr, unsigned long len,
                         uint32_t *map);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       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.

RETURN VALUE         top

       On success, subpage_prot() returns 0.  Otherwise, one of the error
       codes specified below is returned.

ERRORS         top

       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.

VERSIONS         top

       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.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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).

SEE ALSO         top

       mprotect(2), syscall(2)

       Documentation/vm/hugetlbpage.txt in the Linux kernel source tree

COLOPHON         top

       This page is part of release 4.07 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)