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 the description of
MAP_SHARED in mmap(2)), 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:
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).
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.
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.
The caller tried to expand a memory segment that is locked, but this was not possible without exceeding the RLIMIT_MEMLOCK resource limit.
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.
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.
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.
Linux.
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.
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.
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 (see the
description of MAP_PRIVATE in
mmap(2)), 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)