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RAMFS in Linux
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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 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 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 support 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. SEE ALSO
getpagesize(2), mmap(2), mmap2(2), mprotect(2), mremap(2), msync(2) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 REMAP_FILE_PAGES(2)
