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NAME | SYNOPSIS | DESCRIPTION | ARCHITECTURE | OPTIONS | EXIT STATUS | EXAMPLES | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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CHMEM(8) System Administration CHMEM(8)
chmem - configure memory
chmem [-h] [-V] [-v] [-c|-e|-d|-g] [SIZE|RANGE|-b BLOCKRANGE] [-z
ZONE] [-m MEMMAP-ON-MEMORY]
The chmem command sets a particular size or range of memory online
or offline.
• Specify SIZE as <size>[m|M|g|G]. With m or M, <size> specifies
the memory size in MiB (1024 x 1024 bytes). With g or G,
<size> specifies the memory size in GiB (1024 x 1024 x 1024
bytes). The default unit is MiB.
• Specify RANGE in the form 0x<start>-0x<end> as shown in the
output of the lsmem(1) command. <start> is the hexadecimal
address of the first byte and <end> is the hexadecimal address
of the last byte in the memory range.
• Specify BLOCKRANGE in the form <first>-<last> or <block> as
shown in the output of the lsmem(1) command. <first> is the
number of the first memory block and <last> is the number of
the last memory block in the memory range. Alternatively a
single block can be specified. BLOCKRANGE requires the
--blocks option.
• Specify ZONE as the name of a memory zone, as shown in the
output of the lsmem -o +ZONES command. The output shows one or
more valid memory zones for each memory range. If multiple
zones are shown, then the memory range currently belongs to
the first zone. By default, chmem will set memory online to
the zone Movable, if this is among the valid zones. This
default can be changed by specifying the --zone option with
another valid zone. For memory ballooning, it is recommended
to select the zone Movable for memory online and offline, if
possible. Memory in this zone is much more likely to be able
to be offlined again, but it cannot be used for arbitrary
kernel allocations, only for migratable pages (e.g., anonymous
and page cache pages). Use the --help option to see all
available zones.
• Specify MEMMAP-ON-MEMORY as 1 or 0. A value of 1 allocates
hotplug metadata (such as the struct pages array) from the
hotplug memory itself, enabling hot-add operations even under
memory pressure and without requiring additional system memory
to do so. A value of 0 stores hotplugged memory metadata in
regular system memory, which helps avoid issues related to
fragmentation of continuous physical memory across memory
blocks. The value can only be set when the memory block is in
a deconfigured state, and --memmap-on-memory is valid only
with --configure. If not specified, and if supported, chmem
uses the default value shown in lsmem output.
The --enable option configures the memory, if this is supported by
the architecture. If configuring memory is not supported by the
architecture, --enable still brings the memory online.
The --disable option brings the memory offline and performs an
optional deconfigure step if this is supported by the
architecture.
The --configure option requests memory from the hypervisor without
bringing it online, when supported by the architecture, allowing
explicit control and use of --memmap-on-memory.
The --deconfigure option returns memory resources to the
hypervisor if supported by the architecture.
SIZE and RANGE must be aligned to the Linux memory block size, as
shown in the output of the lsmem(1) command.
Setting memory online can fail for various reasons. On virtualized
systems it can fail if the hypervisor does not have enough memory
left, for example because memory was overcommitted. Setting memory
offline can fail if Linux cannot free the memory. If only part of
the requested memory can be set online or offline, a message tells
you how much memory was set online or offline instead of the
requested amount.
When setting memory online or when configuring memory, chmem
starts with the lowest memory block numbers. When setting memory
offline or deconfiguring memory, chmem starts with the highest
memory block numbers.
• s390 architecture:
MEMMAP-ON-MEMORY: For memory blocks configured online at boot, the
default value is 0 because they are added without memmap-on-memory
support. Memory added dynamically at runtime uses the default
value displayed in lsmem output.
-b, --blocks
Use a BLOCKRANGE parameter instead of RANGE or SIZE for the
--enable and --disable options.
-c, --configure
Set the specified RANGE, SIZE, or BLOCKRANGE of memory to be
configured.
-d, --disable
Set the specified RANGE, SIZE, or BLOCKRANGE of memory
offline.
-e, --enable
Set the specified RANGE, SIZE, or BLOCKRANGE of memory online.
-g, --deconfigure
Set the specified RANGE, SIZE, or BLOCKRANGE of memory to be
deconfigured.
-m, --memmap-on-memory
Select memmap-on-memory for the specified RANGE, SIZE, or
BLOCKRANGE of memory. This option is valid only with
--configure.
-z, --zone
Select the memory ZONE where to set the specified RANGE, SIZE,
or BLOCKRANGE of memory online or offline. By default, memory
will be set online to the zone Movable, if possible.
-v, --verbose
Verbose mode. Causes chmem to print debugging messages about
its progress.
-h, --help
Display help text and exit.
-V, --version
Display version and exit.
chmem has the following exit status values:
0
success
1
failure
64
partial success
chmem --enable 1024
This command requests 1024 MiB of memory to be set online.
chmem -e 2g
This command requests 2 GB of memory to be brought online and,
if supported by the architecture, configures the memory
beforehand.
chmem --disable 0x00000000e4000000-0x00000000f3ffffff
This command takes the memory range from 0x00000000e4000000 to
0x00000000f3ffffff offline and deconfigures it if supported by
the architecture.
chmem -b -d 10
This command takes memory block number 10 offline.
chmem -b -c 10 -m 1
This command configures memory block 10 with MEMMAP-ON-MEMORY
set. The block must be in a deconfigured state.
chmem -b -c 10
This command configures memory block 10 with the default
MEMMAP-ON-MEMORY setting. The default value is displayed in
lsmem --output-all. The block must be in a deconfigured state.
chmem -b -g 10
This command deconfigures memory block 10. The block must be
offline.
chmem -d 5g
This command takes 5 GB of memory offline and deconfigures it
if supported by the architecture. Blocks that are already
offline but still configured are skipped and must be
explicitly deconfigured with --deconfigure.
lsmem(1)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
The chmem command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
part of the util-linux (a random collection of Linux utilities)
project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2026-01-16. (At that time, the date of the most recent commit that
was found in the repository was 2026-01-14.) If you discover any
rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
util-linux 2.42-start-1036-e... 2025-12-04 CHMEM(8)
Pages that refer to this page: lsmem(1)
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HTML rendering created 2026-01-16 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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