subpage_prot(2) — Linux manual page

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

       int 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 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.

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

COLOPHON         top

       This page is part of release 5.10 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                          2020-12-21                SUBPAGE_PROT(2)

Pages that refer to this page: syscalls(2)