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

NAME

     remap_file_pages - create a nonlinear file mapping

SYNOPSIS

     #define _GNU_SOURCE         /* See feature_test_macros(7) */
     #include <sys/mman.h>

     int remap_file_pages(void *addr, size_t size, int prot,
                          size_t pgoff, int flags);

DESCRIPTION

     Note:  this  system  call  was marked as deprecated starting with Linux
     3.16.  In Linux 4.0, the implementation was replaced by  a  slower  in-
     kernel  emulation.   Those  few  applications that use this system call
     should consider  migrating  to  alternatives.   This  change  was  made
     because  the  kernel  code  for this system call was complex, and it is
     believed to be little used or perhaps even completely unused.  While it
     had  some  use  cases in database applications on 32-bit systems, those
     use cases don't exist on 64-bit systems.

     The remap_file_pages() system call is used to create a  nonlinear  map-
     ping, that is, a mapping in which the pages of the file are mapped into
     a  nonsequential   order   in   memory.    The   advantage   of   using
     remap_file_pages()  over  using  repeated  calls to mmap(2) is that the
     former approach does not require the kernel to  create  additional  VMA
     (Virtual Memory Area) data structures.

     To create a nonlinear mapping we perform the following steps:

     1. Use  mmap(2)  to create a mapping (which is initially linear).  This
        mapping must be created with the MAP_SHARED flag.

     2. Use one or more calls to remap_file_pages() to rearrange the  corre-
        spondence  between  the  pages  of  the mapping and the pages of the
        file.  It is possible to map the same page of a file  into  multiple
        locations within the mapped region.

     The  pgoff and size arguments specify the region of the file that is to
     be relocated within the mapping: pgoff is a file offset in units of the
     system page size; size is the length of the region in bytes.

     The  addr  argument serves two purposes.  First, it identifies the map-
     ping whose pages we want to rearrange.  Thus, addr must be  an  address
     that  falls  within  a  region  previously mapped by a call to mmap(2).
     Second, addr specifies the address at which the file  pages  identified
     by pgoff and size will be placed.

     The values specified in addr and size should be multiples of the system
     page size.  If they are not, then the kernel rounds both values down to
     the nearest multiple of the page size.

     The prot argument must be specified as 0.

     The  flags  argument has the same meaning as for mmap(2), but all flags
     other than MAP_NONBLOCK are ignored.

RETURN VALUE

     On success, remap_file_pages() returns 0.  On error,  -1  is  returned,
     and errno is set appropriately.

ERRORS

     EINVAL addr  does  not  refer  to  a  valid  mapping  created  with the
            MAP_SHARED flag.

     EINVAL addr, size, prot, or pgoff is invalid.

VERSIONS

     The remap_file_pages() system call appeared in Linux 2.5.46; glibc sup-
     port was added in version 2.3.3.

CONFORMING TO

     The remap_file_pages() system call is Linux-specific.

NOTES

     Since Linux 2.6.23, remap_file_pages() creates non-linear mappings only
     on in-memory filesystems such as  tmpfs(5),  hugetlbfs  or  ramfs.   On
     filesystems  with  a backing store, remap_file_pages() is not much more
     efficient than using mmap(2) to adjust which  parts  of  the  file  are
     mapped to which addresses.