mremap

MREMAP(2)                   Linux Programmer's Manual                  MREMAP(2)



NAME
       mremap - remap a virtual memory address

SYNOPSIS
       #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 */);

DESCRIPTION
       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.

RETURN VALUE
       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.

ERRORS
       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.

CONFORMING TO
       This call is Linux-specific, and should not be used in programs intended
       to be portable.

NOTES
       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.

BUGS
       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.

SEE ALSO
       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)

COLOPHON
       This page is part of release 5.10 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)