I'm running pdftoppm to convert a user-provided PDF into a 300DPI image. This works great, except if the user provides an PDF with a very large page size. pdftoppm will allocate enough memory to hold a 300DPI image of that size in memory, which for a 100 inch square page is 100*300 * 100*300 * 4 bytes per pixel = 3.5GB. A malicious user could just give me a silly-large PDF and cause all kinds of problems.

So what I'd like to do is put some kind of hard limit on memory usage for a child process I'm about to run--just have the process die if it tries to allocate more than, say, 500MB of memory. Is that possible?

I don't think ulimit can be used for this, but is there a one-process equivalent?

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  • Maybe docker? – user7000 May 12 '16 at 23:40
up vote 53 down vote accepted

There's some problems with ulimit. Here's a useful read on the topic: Limiting time and memory consumption of a program in Linux, which lead to the timeout tool, which lets you cage a process (and its forks) by time or memory consumption.

The timeout tool requires Perl 5+ and the /proc filesystem mounted. After that you copy the tool to e.g. /usr/local/bin like so:

curl https://raw.githubusercontent.com/pshved/timeout/master/timeout | \
  sudo tee /usr/local/bin/timeout && sudo chmod 755 /usr/local/bin/timeout

After that, you can 'cage' your process by memory consumption as in your question like so:

timeout -m 500 pdftoppm Sample.pdf

Alternatively you could use -t <seconds> and -x <hertz> to respectively limit the process by time or CPU constraints.

The way this tool works is by checking multiple times per second if the spawned process has not oversubscribed its set boundaries. This means there actually is a small window where a process could potentially be oversubscribing before timeout notices and kills the process.

A more correct approach would hence likely involve cgroups, but that is much more involved to set up, even if you'd use Docker or runC, which among things, offer a more user-friendly abstraction around cgroups.

  • Seems to be working for me now (again?) but here's the google cache version: webcache.googleusercontent.com/… – kvz Apr 27 '17 at 12:32
  • Can we use timeout together with taskset (we need to limit both memory and cores) ? – ransh Oct 24 '17 at 12:47
  • 3
    It should be noted that this answer is not referring to the linux standard coreutils utility of the same name! Thus, the answer is potentially dangerous if anywhere on your system, some package has a script expecting timeout to be the linux standard coreutils package! I am unaware of this tool being packaged for distributions such as debian. – user1404316 Apr 8 at 7:03
  • Does -t <seconds> constraint kill the process after that many seconds? – Roger That Nov 26 at 2:05

Another way to limit this is to use Linux's control groups. This is especially useful if you want to limit a process's (or group of processes') allocation of physical memory distinctly from virtual memory. For example:

cgcreate -g memory:/myGroup
echo $(( 500 * 1024 * 1024 )) > /sys/fs/cgroup/memory/myGroup/memory.limit_in_bytes
echo $(( 5000 * 1024 * 1024 )) > /sys/fs/cgroup/memory/myGroup/memory.memsw.limit_in_bytes

will create a control group named myGroup, cap the set of processes run under myGroup up to 500 MB of physical memory and up to 5000 MB of swap. To run a process under the control group:

cgexec -g memory:myGroup pdftoppm

Note that on a modern Ubuntu distribution this example requires installing the cgroup-bin package and editing /etc/default/grub to change GRUB_CMDLINE_LINUX_DEFAULT to:

GRUB_CMDLINE_LINUX_DEFAULT="cgroup_enable=memory swapaccount=1"

and then running sudo update-grub and rebooting to boot with the new kernel boot parameters.

  • 3
    The firejail program will also let you start a process with memory limits (using cgroups and namespaces to limit more than just memory). On my systems I did not have to change the kernel command line for this to work! – Ned64 Feb 15 at 12:20
  • Do you need the GRUB_CMDLINE_LINUX_DEFAULT modification to make the setting persistent? I found another way to make it persistent here. – stason Aug 5 at 18:36

If your process doesn't spawn more children that consume the most memory, you may use setrlimit function. More common user interface for that is using ulimit command of the shell:

$ ulimit -Sv 500000     # Set ~500 mb limit
$ pdftoppm ...

This will only limit "virtual" memory of your process, taking into account—and limiting—the memory the process being invoked shares with other processes, and the memory mapped but not reserved (for instance, Java's large heap). Still, virtual memory is the closest approximation for processes that grow really large, making the said errors insignificant.

If your program spawns children, and it's them which allocate memory, it becomes more complex, and you should write auxiliary scripts to run processes under your control. I wrote in my blog, why and how.

  • 2
    why is setrlimit more complex for more children? man setrlimit tells me that "A child process created via fork(2) inherits its parents resource limits. Resource limits are preserved across execve(2)" – akira Feb 13 '11 at 8:13
  • 6
    Because the kernel does not sum the vm size for all child processes; if it did it would get the answer wrong anyway. The limit is per-process, and is virtual address space, not memory usage. Memory usage is harder to measure. – MarkR Feb 13 '11 at 8:17
  • 1
    if i understand the question correctly then OP whats the limit per subprocess (child) .. not in total. – akira Feb 13 '11 at 8:21
  • @MarkR, anyway, virtual address space is a good approximation for the memory used, especially if you run a program that's not controlled by a virtual machine (say, Java). At least I don't know any better metric. – Pavel Shved Feb 13 '11 at 8:23
  • 2
    Just wanted to say thanks - this ulimit approach helped me with firefox's bug 622816 – Loading a large image can "freeze" firefox, or crash the system; which on a USB boot (from RAM) tends to freeze the OS, requiring hard restart; now at least firefox crashes itself, leaving the OS alive... Cheers! – sdaau Apr 4 '13 at 15:51

In addition to the tools from daemontools, suggested by Mark Johnson, you can also consider chpst which is found in runit. Runit itself is bundled in busybox, so you might already have it installed.

The man page of chpst shows the option:

-m bytes limit memory. Limit the data segment, stack segment, locked physical pages, and total of all segment per process to bytes bytes each.

I'm using the below script, which works great. It uses cgroups through cgmanager. Update: it now uses the commands from cgroup-tools. Name this script limitmem and put it in your $PATH and you can use it like limitmem 100M bash. This will limit both memory and swap usage. To limit just memory remove the line with memory.memsw.limit_in_bytes.

Disclaimer: I wouldn't be surprised if cgroup-tools also breaks in the future. The correct solution would be to use the systemd api's for cgroup management but there are no command line tools for that a.t.m.

#!/bin/sh

# This script uses commands from the cgroup-tools package. The cgroup-tools commands access the cgroup filesystem directly which is against the (new-ish) kernel's requirement that cgroups are managed by a single entity (which usually will be systemd). Additionally there is a v2 cgroup api in development which will probably replace the existing api at some point. So expect this script to break in the future. The correct way forward would be to use systemd's apis to create the cgroups, but afaik systemd currently (feb 2018) only exposes dbus apis for which there are no command line tools yet, and I didn't feel like writing those.

# strict mode: error if commands fail or if unset variables are used
set -eu

if [ "$#" -lt 2 ]
then
    echo Usage: `basename $0` "<limit> <command>..."
    echo or: `basename $0` "<memlimit> -s <swaplimit> <command>..."
    exit 1
fi

cgname="limitmem_$$"

# parse command line args and find limits

limit="$1"
swaplimit="$limit"
shift

if [ "$1" = "-s" ]
then
    shift
    swaplimit="$1"
    shift
fi

if [ "$1" = -- ]
then
    shift
fi

if [ "$limit" = "$swaplimit" ]
then
    memsw=0
    echo "limiting memory to $limit (cgroup $cgname) for command $@" >&2
else
    memsw=1
    echo "limiting memory to $limit and total virtual memory to $swaplimit (cgroup $cgname) for command $@" >&2
fi

# create cgroup
sudo cgcreate -g "memory:$cgname"
sudo cgset -r memory.limit_in_bytes="$limit" "$cgname"
bytes_limit=`cgget -g "memory:$cgname" | grep memory.limit_in_bytes | cut -d\  -f2`

# try also limiting swap usage, but this fails if the system has no swap
if sudo cgset -r memory.memsw.limit_in_bytes="$swaplimit" "$cgname"
then
    bytes_swap_limit=`cgget -g "memory:$cgname" | grep memory.memsw.limit_in_bytes | cut -d\  -f2`
else
    echo "failed to limit swap"
    memsw=0
fi

# create a waiting sudo'd process that will delete the cgroup once we're done. This prevents the user needing to enter their password to sudo again after the main command exists, which may take longer than sudo's timeout.
tmpdir=${XDG_RUNTIME_DIR:-$TMPDIR}
tmpdir=${tmpdir:-/tmp}
fifo="$tmpdir/limitmem_$$_cgroup_closer"
mkfifo --mode=u=rw,go= "$fifo"
sudo -b sh -c "head -c1 '$fifo' >/dev/null ; cgdelete -g 'memory:$cgname'"

# spawn subshell to run in the cgroup. If the command fails we still want to remove the cgroup so unset '-e'.
set +e
(
set -e
# move subshell into cgroup
sudo cgclassify -g "memory:$cgname" --sticky `sh -c 'echo $PPID'`  # $$ returns the main shell's pid, not this subshell's.
exec "$@"
)

# grab exit code 
exitcode=$?

set -e

# show memory usage summary

peak_mem=`cgget -g "memory:$cgname" | grep memory.max_usage_in_bytes | cut -d\  -f2`
failcount=`cgget -g "memory:$cgname" | grep memory.failcnt | cut -d\  -f2`
percent=`expr "$peak_mem" / \( "$bytes_limit" / 100 \)`

echo "peak memory used: $peak_mem ($percent%); exceeded limit $failcount times" >&2

if [ "$memsw" = 1 ]
then
    peak_swap=`cgget -g "memory:$cgname" | grep memory.memsw.max_usage_in_bytes | cut -d\  -f2`
    swap_failcount=`cgget -g "memory:$cgname" |grep memory.memsw.failcnt | cut -d\  -f2`
    swap_percent=`expr "$peak_swap" / \( "$bytes_swap_limit" / 100 \)`

    echo "peak virtual memory used: $peak_swap ($swap_percent%); exceeded limit $swap_failcount times" >&2
fi

# remove cgroup by sending a byte through the pipe
echo 1 > "$fifo"
rm "$fifo"

exit $exitcode
  • 1
    call to cgmanager_create_sync failed: invalid request for every process I try to run with limitmem 100M processname. I'm on Xubuntu 16.04 LTS and that package is installed. – Aaron Franke Mar 12 '17 at 9:58
  • Ups, I get this error message: $ limitmem 400M rstudio limiting memory to 400M (cgroup limitmem_24575) for command rstudio Error org.freedesktop.DBus.Error.InvalidArgs: invalid request any idea? – R Kiselev Feb 15 at 7:19
  • @RKiselev cgmanager is deprecated now, and not even available in Ubuntu 17.10. The systemd api that it uses was changed at some point, so that's probably the reason. I have updated the script to use cgroup-tools commands. – JanKanis Feb 15 at 11:39
  • if the calculation for percent results in zero, the expr status code is 1, and this script exits prematurely. recommend changing the line to: percent=$(( "$peak_mem" / $(( "$bytes_limit" / 100 )) )) (ref: unix.stackexchange.com/questions/63166/…) – Willi Ballenthin May 23 at 22:46

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