Ubuntu Manpage: mremap - remap a virtual memory address

NAME

       mremap - remap a virtual memory address

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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 */);

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 to indicate the error.

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.

STANDARDS

       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 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_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
          use 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.