man2/subpage_prot.2

   1 .\" Copyright (c) 2010 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" based on a proposal from Stephan Mueller <smueller@atsec.com>
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  23 .\"
  24 .\" Various pieces of text taken from the kernel source and the commentary
  25 .\" in kernel commit fa28237cfcc5827553044cbd6ee52e33692b0faa
  26 .\" both written by Paul Mackerras <paulus@samba.org>
  27 .\"
  28 .TH SUBPAGE_PROT 2 2012-07-13 "Linux" "Linux Programmer's Manual"
  29 .SH NAME
  30 subpage_prot \- define a subpage protection for an address range
  31 .SH SYNOPSIS
  32 .nf
  33 .BI "long subpage_prot(unsigned long " addr ", unsigned long " len ,
  34 .BI "                  uint32_t *" map ");
  35 .fi
  36
  37 .IR Note :
  38 There is no glibc wrapper for this system call; see NOTES.
  39 .SH DESCRIPTION
  40 The PowerPC-specific
  41 .BR subpage_prot ()
  42 system call provides the facility to control the access
  43 permissions on individual 4kB subpages on systems configured with
  44 a page size of 64kB.
  45
  46 The protection map is applied to the memory pages in the region starting at
  47 .I addr
  48 and continuing for
  49 .I len
  50 bytes.
  51 Both of these arguments must be aligned to a 64-kB boundary.
  52
  53 The protection map is specified in the buffer pointed to by
  54 .IR map .
  55 The map has 2 bits per 4kB subpage;
  56 thus each 32-bit word specifies the protections of 16 4kB subpages
  57 inside a 64kB page
  58 (so, the number of 32-bit words pointed to by
  59 .I map
  60 should equate to the number of 64-kB pages specified by
  61 .IR len ).
  62 Each 2-bit field in the protection map is either 0 to allow any access,
  63 1 to prevent writes, or 2 or 3 to prevent all accesses.
  64 .SH RETURN VALUE
  65 On success,
  66 .BR subpage_prot ()
  67 returns 0.
  68 Otherwise, one of the error codes specified below is returned.
  69 .SH ERRORS
  70 .TP
  71 .B EFAULT
  72 The buffer referred to by
  73 .I map
  74 is not accessible.
  75 .TP
  76 .B EINVAL
  77 The
  78 .I addr
  79 or
  80 .I len
  81 arguments are incorrect.
  82 Both of these arguments must be aligned to a multiple of the system page size,
  83 and they must not refer to a region outside of the
  84 address space of the process or to a region that consists of huge pages.
  85 .TP
  86 .B ENOMEM
  87 Out of memory.
  88 .SH VERSIONS
  89 This system call is provided on the PowerPC architecture
  90 since Linux 2.6.25.
  91 The system call is provided only if the kernel is configured with
  92 .BR CONFIG_PPC_64K_PAGES .
  93 No library support is provided.
  94 .SH CONFORMING TO
  95 This system call is Linux-specific.
  96 .SH NOTES
  97 Glibc does not provide a wrapper for this system call; call it using
  98 .BR syscall (2).
  99
 100 Normal page protections (at the 64-kB page level) also apply;
 101 the subpage protection mechanism is an additional constraint,
 102 so putting 0 in a 2-bit field won't allow writes to a page that is otherwise
 103 write-protected.
 104 .SS Rationale
 105 This system call is provided to assist writing emulators that
 106 operate using 64-kB pages on PowerPC systems.
 107 When emulating systems such as x86, which uses a smaller page size,
 108 the emulator can no longer use the memory-management unit (MMU)
 109 and normal system calls for controlling page protections.
 110 (The emulator could emulate the MMU by checking and possibly remapping
 111 the address for each memory access in software, but that is slow.)
 112 The idea is that the emulator supplies an array of protection masks
 113 to apply to a specified range of virtual addresses.
 114 These masks are applied at the level where hardware page-table entries (PTEs)
 115 are inserted into the hardware page table based on the Linux PTEs,
 116 so the Linux PTEs are not affected.
 117 Implicit in this is that the regions of the address space that are
 118 protected are switched to use 4-kB hardware pages rather than 64-kB
 119 hardware pages (on machines with hardware 64-kB page support).
 120 .\" In the initial implementation, it was the case that:
 121 .\"     In fact the whole process is switched to use 4k hardware pages when the
 122 .\"     subpage_prot system call is used, but this could be improved in future
 123 .\"     to switch only the affected segments.
 124 .\" But Paul Mackerass says (Oct 2010): I'm pretty sure we now only switch
 125 .\" the affected segment, not the whole process.
 126 .SH SEE ALSO
 127 .BR mprotect (2),
 128 .BR syscall (2);
 129 .br
 130 the kernel source file
 131 .IR Documentation/vm/hugetlbpage.txt .