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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
MREMAP(2) Linux Programmer's Manual MREMAP(2)
mremap - remap a virtual memory address
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(void *old_address, size_t old_size,
size_t new_size, int flags, ... /* void *new_address */);
mremap() expands (or shrinks) an existing memory mapping, potentially
moving it at the same time (controlled by the flags argument and the
available virtual address space).
old_address is the old address of the virtual memory block that you
want to expand (or shrink). Note that old_address has to be page
aligned. old_size is the old size of the virtual memory block.
new_size is the requested size of the virtual memory block after the
resize. An optional fifth argument, new_address, may be provided;
see the description of MREMAP_FIXED below.
If the value of old_size is zero, and old_address refers to a
shareable mapping (see mmap(2) MAP_SHARED), then mremap() will create
a new mapping of the same pages. new_size will be the size of the
new mapping and the location of the new mapping may be specified with
new_address; see the description of MREMAP_FIXED below. If a new
mapping is requested via this method, then the MREMAP_MAYMOVE flag
must also be specified.
The flags bit-mask argument may be 0, or include the following flags:
MREMAP_MAYMOVE
By default, if there is not sufficient space to expand a
mapping at its current location, then mremap() fails. If this
flag is specified, then the kernel is permitted to relocate
the mapping to a new virtual address, if necessary. If the
mapping is relocated, then absolute pointers into the old
mapping location become invalid (offsets relative to the
starting address of the mapping should be employed).
MREMAP_FIXED (since Linux 2.3.31)
This flag serves a similar purpose to the MAP_FIXED flag of
mmap(2). If this flag is specified, then mremap() accepts a
fifth argument, void *new_address, which specifies a page-
aligned address to which the mapping must be moved. Any
previous mapping at the address range specified by new_address
and new_size is unmapped.
If MREMAP_FIXED is specified, then MREMAP_MAYMOVE must also be
specified.
MREMAP_DONTUNMAP (since Linux 5.7)
This flag, which must be used in conjunction with
MREMAP_MAYMOVE, remaps a mapping to a new address but does not
unmap the mapping at old_address.
The MREMAP_DONTUNMAP flag can be used only with private
anonymous mappings (see the description of MAP_PRIVATE and
MAP_ANONYMOUS in mmap(2)).
After completion, any access to the range specified by
old_address and old_size will result in a page fault. The
page fault will be handled by a userfaultfd(2) handler if the
address is in a range previously registered with
userfaultfd(2). Otherwise, the kernel allocates a zero-filled
page to handle the fault.
The MREMAP_DONTUNMAP flag may be used to atomically move a
mapping while leaving the source mapped. See NOTES for some
possible applications of MREMAP_DONTUNMAP.
If the memory segment specified by old_address and old_size is locked
(using mlock(2) or similar), then this lock is maintained when the
segment is resized and/or relocated. As a consequence, the amount of
memory locked by the process may change.
On success mremap() returns a pointer to the new virtual memory area.
On error, the value MAP_FAILED (that is, (void *) -1) is returned,
and errno is set appropriately.
EAGAIN The caller tried to expand a memory segment that is locked,
but this was not possible without exceeding the RLIMIT_MEMLOCK
resource limit.
EFAULT Some address in the range old_address to old_address+old_size
is an invalid virtual memory address for this process. You
can also get EFAULT even if there exist mappings that cover
the whole address space requested, but those mappings are of
different types.
EINVAL An invalid argument was given. Possible causes are:
* old_address was not page aligned;
* a value other than MREMAP_MAYMOVE or MREMAP_FIXED or
MREMAP_DONTUNMAP was specified in flags;
* new_size was zero;
* new_size or new_address was invalid;
* the new address range specified by new_address and new_size
overlapped the old address range specified by old_address
and old_size;
* MREMAP_FIXED or MREMAP_DONTUNMAP was specified without also
specifying MREMAP_MAYMOVE;
* MREMAP_DONTUNMAP was specified, but one or more pages in
the range specified by old_address and old_size were not
private anonymous;
* MREMAP_DONTUNMAP was specified and old_size was not equal
to new_size;
* old_size was zero and old_address does not refer to a
shareable mapping (but see BUGS);
* old_size was zero and the MREMAP_MAYMOVE flag was not
specified.
ENOMEM Not enough memory was available to complete the operation.
Possible causes are:
* The memory area cannot be expanded at the current virtual
address, and the MREMAP_MAYMOVE flag is not set in flags.
Or, there is not enough (virtual) memory available.
* MREMAP_DONTUNMAP was used causing a new mapping to be
created that would exceed the (virtual) memory available.
Or, it would exceed the maximum number of allowed mappings.
This call is Linux-specific, and should not be used in programs
intended to be portable.
mremap() changes the mapping between virtual addresses and memory
pages. This can be used to implement a very efficient realloc(3).
In Linux, memory is divided into pages. A process has (one or)
several linear virtual memory segments. Each virtual memory segment
has one or more mappings to real memory pages (in the page table).
Each virtual memory segment has its own protection (access rights),
which may cause a segmentation violation (SIGSEGV) if the memory is
accessed incorrectly (e.g., writing to a read-only segment).
Accessing virtual memory outside of the segments will also cause a
segmentation violation.
If mremap() is used to move or expand an area locked with mlock(2) or
equivalent, the mremap() call will make a best effort to populate the
new area but will not fail with ENOMEM if the area cannot be
populated.
Prior to version 2.4, glibc did not expose the definition of
MREMAP_FIXED, and the prototype for mremap() did not allow for the
new_address argument.
MREMAP_DONTUNMAP use cases
Possible applications for MREMAP_DONTUNMAP include:
* Non-cooperative userfaultfd(2): an application can yank out a
virtual address range using MREMAP_DONTUNMAP and then employ a
userfaultfd(2) handler to handle the page faults that subsequently
occur as other threads in the process touch pages in the yanked
range.
* Garbage collection: MREMAP_DONTUNMAP can be used in conjunction
with userfaultfd(2) to implement garbage collection algorithms
(e.g., in a Java virtual machine). Such an implementation can be
cheaper (and simpler) than conventional garbage collection
techniques that involve marking pages with protection PROT_NONE in
conjunction with the of a SIGSEGV handler to catch accesses to
those pages.
Before Linux 4.14, if old_size was zero and the mapping referred to
by old_address was a private mapping (mmap(2) MAP_PRIVATE), mremap()
created a new private mapping unrelated to the original mapping.
This behavior was unintended and probably unexpected in user-space
applications (since the intention of mremap() is to create a new
mapping based on the original mapping). Since Linux 4.14, mremap()
fails with the error EINVAL in this scenario.
brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2),
malloc(3), realloc(3)
Your favorite text book on operating systems for more information on
paged memory (e.g., Modern Operating Systems by Andrew S. Tanenbaum,
Inside Linux by Randolph Bentson, The Design of the UNIX Operating
System by Maurice J. Bach)
This page is part of release 5.08 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 2020-06-09 MREMAP(2)
Pages that refer to this page: memusage(1) , getrlimit(2) , ioctl_userfaultfd(2) , mmap2(2) , mmap(2) , munmap(2) , prctl(2) , prlimit(2) , prlimit64(2) , remap_file_pages(2) , setrlimit(2) , syscalls(2) , ugetrlimit(2) , userfaultfd(2) , mmap64(3) , vlimit(3)
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