man2/remap_file_pages.2

   1 .\" Copyright (C) 2003, Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" 2003-12-10 Initial creation, Michael Kerrisk <mtk.manpages@gmail.com>
   6 .\" 2004-10-28 aeb, corrected prototype, prot must be 0
   7 .\"
   8 .TH remap_file_pages 2 (date) "Linux man-pages (unreleased)"
   9 .SH NAME
  10 remap_file_pages \- create a nonlinear file mapping
  11 .SH LIBRARY
  12 Standard C library
  13 .RI ( libc ", " \-lc )
  14 .SH SYNOPSIS
  15 .nf
  16 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  17 .B #include <sys/mman.h>
  18 .P
  19 .BI "[[deprecated]] int remap_file_pages(void " addr [. size "], size_t " size ,
  20 .BI "                                    int " prot ", size_t " pgoff ", \
  21 int " flags );
  22 .fi
  23 .SH DESCRIPTION
  24 .BR Note :
  25 .\" commit 33041a0d76d3c3e0aff28ac95a2ffdedf1282dbc
  26 .\" http://lwn.net/Articles/597632/
  27 this system call was marked as deprecated starting with Linux 3.16.
  28 In Linux 4.0, the implementation was replaced
  29 .\" commit c8d78c1823f46519473949d33f0d1d33fe21ea16
  30 by a slower in-kernel emulation.
  31 Those few applications that use this system call should
  32 consider migrating to alternatives.
  33 This change was made because the kernel code for this system call was complex,
  34 and it is believed to be little used or perhaps even completely unused.
  35 While it had some use cases in database applications on 32-bit systems,
  36 those use cases don't exist on 64-bit systems.
  37 .P
  38 The
  39 .BR remap_file_pages ()
  40 system call is used to create a nonlinear mapping, that is, a mapping
  41 in which the pages of the file are mapped into a nonsequential order
  42 in memory.
  43 The advantage of using
  44 .BR remap_file_pages ()
  45 over using repeated calls to
  46 .BR mmap (2)
  47 is that the former approach does not require the kernel to create
  48 additional VMA (Virtual Memory Area) data structures.
  49 .P
  50 To create a nonlinear mapping we perform the following steps:
  51 .TP 3
  52 1.
  53 Use
  54 .BR mmap (2)
  55 to create a mapping (which is initially linear).
  56 This mapping must be created with the
  57 .B MAP_SHARED
  58 flag.
  59 .TP
  60 2.
  61 Use one or more calls to
  62 .BR remap_file_pages ()
  63 to rearrange the correspondence between the pages of the mapping
  64 and the pages of the file.
  65 It is possible to map the same page of a file
  66 into multiple locations within the mapped region.
  67 .P
  68 The
  69 .I pgoff
  70 and
  71 .I size
  72 arguments specify the region of the file that is to be relocated
  73 within the mapping:
  74 .I pgoff
  75 is a file offset in units of the system page size;
  76 .I size
  77 is the length of the region in bytes.
  78 .P
  79 The
  80 .I addr
  81 argument serves two purposes.
  82 First, it identifies the mapping whose pages we want to rearrange.
  83 Thus,
  84 .I addr
  85 must be an address that falls within
  86 a region previously mapped by a call to
  87 .BR mmap (2).
  88 Second,
  89 .I addr
  90 specifies the address at which the file pages
  91 identified by
  92 .I pgoff
  93 and
  94 .I size
  95 will be placed.
  96 .P
  97 The values specified in
  98 .I addr
  99 and
 100 .I size
 101 should be multiples of the system page size.
 102 If they are not, then the kernel rounds
 103 .I both
 104 values
 105 .I down
 106 to the nearest multiple of the page size.
 107 .\" This rounding is weird, and not consistent with the treatment of
 108 .\" the analogous arguments for munmap()/mprotect() and for mlock().
 109 .\" MTK, 14 Sep 2005
 110 .P
 111 The
 112 .I prot
 113 argument must be specified as 0.
 114 .P
 115 The
 116 .I flags
 117 argument has the same meaning as for
 118 .BR mmap (2),
 119 but all flags other than
 120 .B MAP_NONBLOCK
 121 are ignored.
 122 .SH RETURN VALUE
 123 On success,
 124 .BR remap_file_pages ()
 125 returns 0.
 126 On error, \-1 is returned, and
 127 .I errno
 128 is set to indicate the error.
 129 .SH ERRORS
 130 .TP
 131 .B EINVAL
 132 .I addr
 133 does not refer to a valid mapping
 134 created with the
 135 .B MAP_SHARED
 136 flag.
 137 .TP
 138 .B EINVAL
 139 .IR addr ,
 140 .IR size ,
 141 .IR prot ,
 142 or
 143 .I pgoff
 144 is invalid.
 145 .\" And possibly others from vma->vm_ops->populate()
 146 .SH STANDARDS
 147 Linux.
 148 .SH HISTORY
 149 Linux 2.5.46,
 150 glibc 2.3.3.
 151 .SH NOTES
 152 Since Linux 2.6.23,
 153 .\" commit 3ee6dafc677a68e461a7ddafc94a580ebab80735
 154 .BR remap_file_pages ()
 155 creates non-linear mappings only
 156 on in-memory filesystems such as
 157 .BR tmpfs (5),
 158 hugetlbfs or ramfs.
 159 On filesystems with a backing store,
 160 .BR remap_file_pages ()
 161 is not much more efficient than using
 162 .BR mmap (2)
 163 to adjust which parts of the file are mapped to which addresses.
 164 .SH SEE ALSO
 165 .BR getpagesize (2),
 166 .BR mmap (2),
 167 .BR mmap2 (2),
 168 .BR mprotect (2),
 169 .BR mremap (2),
 170 .BR msync (2)