Ubuntu Manpage: remap_file_pages - create a nonlinear file mapping

NAME

       remap_file_pages - create a nonlinear file mapping

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

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

       [[deprecated]] int remap_file_pages(void addr[.size], 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 mapping, 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 correspondence 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 mapping 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 to
       indicate the error.

ERRORS

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

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

STANDARDS

       Linux.

HISTORY

       Linux 2.5.46, glibc 2.3.3.

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.