man2/remap_file_pages.2

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  25 .\" 2003-12-10 Initial creation, Michael Kerrisk <mtk.manpages@gmail.com>
  26 .\" 2004-10-28 aeb, corrected prototype, prot must be 0
  27 .\"
  28 .TH REMAP_FILE_PAGES 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  29 .SH NAME
  30 remap_file_pages \- create a nonlinear file mapping
  31 .SH SYNOPSIS
  32 .nf
  33 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  34 .B #include <sys/mman.h>
  35 .PP
  36 .BI "int remap_file_pages(void *" addr ", size_t " size ", int " prot ,
  37 .BI "                     size_t " pgoff ", int " flags );
  38 .fi
  39 .SH DESCRIPTION
  40 .BR Note :
  41 .\" commit 33041a0d76d3c3e0aff28ac95a2ffdedf1282dbc
  42 .\" http://lwn.net/Articles/597632/
  43 this system call was marked as deprecated starting with Linux 3.16.
  44 In Linux 4.0, the implementation was replaced
  45 .\" commit c8d78c1823f46519473949d33f0d1d33fe21ea16
  46 by a slower in-kernel emulation.
  47 Those few applications that use this system call should
  48 consider migrating to alternatives.
  49 This change was made because the kernel code for this system call was complex,
  50 and it is believed to be little used or perhaps even completely unused.
  51 While it had some use cases in database applications on 32-bit systems,
  52 those use cases don't exist on 64-bit systems.
  53 .PP
  54 The
  55 .BR remap_file_pages ()
  56 system call is used to create a nonlinear mapping, that is, a mapping
  57 in which the pages of the file are mapped into a nonsequential order
  58 in memory.
  59 The advantage of using
  60 .BR remap_file_pages ()
  61 over using repeated calls to
  62 .BR mmap (2)
  63 is that the former approach does not require the kernel to create
  64 additional VMA (Virtual Memory Area) data structures.
  65 .PP
  66 To create a nonlinear mapping we perform the following steps:
  67 .TP 3
  68 1.
  69 Use
  70 .BR mmap (2)
  71 to create a mapping (which is initially linear).
  72 This mapping must be created with the
  73 .B MAP_SHARED
  74 flag.
  75 .TP
  76 2.
  77 Use one or more calls to
  78 .BR remap_file_pages ()
  79 to rearrange the correspondence between the pages of the mapping
  80 and the pages of the file.
  81 It is possible to map the same page of a file
  82 into multiple locations within the mapped region.
  83 .PP
  84 The
  85 .I pgoff
  86 and
  87 .I size
  88 arguments specify the region of the file that is to be relocated
  89 within the mapping:
  90 .I pgoff
  91 is a file offset in units of the system page size;
  92 .I size
  93 is the length of the region in bytes.
  94 .PP
  95 The
  96 .I addr
  97 argument serves two purposes.
  98 First, it identifies the mapping whose pages we want to rearrange.
  99 Thus,
 100 .I addr
 101 must be an address that falls within
 102 a region previously mapped by a call to
 103 .BR mmap (2).
 104 Second,
 105 .I addr
 106 specifies the address at which the file pages
 107 identified by
 108 .I pgoff
 109 and
 110 .I size
 111 will be placed.
 112 .PP
 113 The values specified in
 114 .I addr
 115 and
 116 .I size
 117 should be multiples of the system page size.
 118 If they are not, then the kernel rounds
 119 .I both
 120 values
 121 .I down
 122 to the nearest multiple of the page size.
 123 .\" This rounding is weird, and not consistent with the treatment of
 124 .\" the analogous arguments for munmap()/mprotect() and for mlock().
 125 .\" MTK, 14 Sep 2005
 126 .PP
 127 The
 128 .I prot
 129 argument must be specified as 0.
 130 .PP
 131 The
 132 .I flags
 133 argument has the same meaning as for
 134 .BR mmap (2),
 135 but all flags other than
 136 .B MAP_NONBLOCK
 137 are ignored.
 138 .SH RETURN VALUE
 139 On success,
 140 .BR remap_file_pages ()
 141 returns 0.
 142 On error, \-1 is returned, and
 143 .I errno
 144 is set appropriately.
 145 .SH ERRORS
 146 .TP
 147 .B EINVAL
 148 .I addr
 149 does not refer to a valid mapping
 150 created with the
 151 .B MAP_SHARED
 152 flag.
 153 .TP
 154 .B EINVAL
 155 .IR addr ,
 156 .IR size ,
 157 .IR prot ,
 158 or
 159 .I pgoff
 160 is invalid.
 161 .\" And possibly others from vma->vm_ops->populate()
 162 .SH VERSIONS
 163 The
 164 .BR remap_file_pages ()
 165 system call appeared in Linux 2.5.46;
 166 glibc support was added in version 2.3.3.
 167 .SH CONFORMING TO
 168 The
 169 .BR remap_file_pages ()
 170 system call is Linux-specific.
 171 .SH NOTES
 172 Since Linux 2.6.23,
 173 .\" commit 3ee6dafc677a68e461a7ddafc94a580ebab80735
 174 .BR remap_file_pages ()
 175 creates non-linear mappings only
 176 on in-memory filesystems such as
 177 .BR tmpfs (5),
 178 hugetlbfs or ramfs.
 179 On filesystems with a backing store,
 180 .BR remap_file_pages ()
 181 is not much more efficient than using
 182 .BR mmap (2)
 183 to adjust which parts of the file are mapped to which addresses.
 184 .SH SEE ALSO
 185 .BR getpagesize (2),
 186 .BR mmap (2),
 187 .BR mmap2 (2),
 188 .BR mprotect (2),
 189 .BR mremap (2),
 190 .BR msync (2)