mremap - remap a virtual memory address
Standard C library (
libc,
-lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/mman.h>
void *mremap(void old_address[.old_size], 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 to indicate the error.
- 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 glibc 2.4, glibc did not expose the definition of
MREMAP_FIXED,
and the prototype for
mremap() did not allow for the
new_address
argument.
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 use 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)