Limit memory usage for a single Linux process

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

I'm using the below script, which works great. It uses cgroups through cgmanager. 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.

#!/bin/sh

set -eu

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

limit="$1"
shift

cgname="limitmem_$$"
echo "limiting memory to $limit (cgroup $cgname) for command $@" >&2

cgm create memory "$cgname" >/dev/null
cgm setvalue memory "$cgname" memory.limit_in_bytes "$limit" >/dev/null
# try also limiting swap usage, but this fails if the system has no swap
cgm setvalue memory "$cgname" memory.memsw.limit_in_bytes "$limit" >/dev/null 2>&1 || true
bytes_limit=`cgm getvalue memory "$cgname" memory.limit_in_bytes | tail -1 | cut -f2 -d\"`

# spawn subshell to run in the cgroup
# set +e so a failing child does not prevent us from removing the cgroup
set +e
(
set -e
cgm movepid memory "$cgname" `sh -c 'echo $PPID'` > /dev/null
exec "$@"
)

# grab exit code 
exitcode=`echo $?`

set -e

peak_mem=`cgm getvalue memory "$cgname" memory.max_usage_in_bytes | tail -1 | cut -f2 -d\"`
failcount=`cgm getvalue memory "$cgname" memory.failcnt | tail -1 | cut -f2 -d\"`
percent=`expr "$peak_mem" / \( "$bytes_limit" / 100 \)`
echo "peak memory used: $peak_mem ($percent%); exceeded limit $failcount times" >&2

cgm remove memory "$cgname" >/dev/null

exit $exitcode