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



NAME
       subpage_prot - define a subpage protection for an address range

SYNOPSIS
       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
       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 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.

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

ERRORS
       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
       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
       This system call is Linux-specific.

NOTES
       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.

   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  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).

SEE ALSO
       mprotect(2), syscall(2)

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

COLOPHON
       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2012-07-13                            SUBPAGE_PROT(2)