remap_file_pages - create a nonlinear file mapping
Standard C library (
libc,
-lc)
#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);
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.
On success,
remap_file_pages() returns 0. On error, -1 is returned, and
errno is set to indicate the error.
- EINVAL
-
addr does not refer to a valid mapping created with
the MAP_SHARED flag.
- EINVAL
-
addr, size, prot, or pgoff is
invalid.
The
remap_file_pages() system call appeared in Linux 2.5.46; glibc
support was added in glibc 2.3.3.
The
remap_file_pages() system call is Linux-specific.
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.
getpagesize(2),
mmap(2),
mmap2(2),
mprotect(2),
mremap(2),
msync(2)