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.