My /tmp folder isn't a partition but has fixed size. Why?

Question

I was compiling 2 kernels at once but the compiling process stopped. Reason: /tmp directory full. I checked with df and... /tmp has a fixed size of 2GB! (My PC has 4GB of RAM) Why? I don't even have a /tmp partition... how can I change that?

    File system    1K-blocks    Usati Disponib. Uso% Montato su
/dev/sda5       35431432 13651652  19973276  41% /
dev              1989592        0   1989592   0% /dev
run              1992432      976   1991456   1% /run
tmpfs            1992432      280   1992152   1% /dev/shm
tmpfs            1992432        0   1992432   0% /sys/fs/cgroup
tmpfs            1992432  1436412    556020  73% /tmp

Answer 1

Putting a limit on /tmp size if it is mounted as tmpfs is to prevent a DOS attack. If one service was compromised or one user malicious they could simply write to /tmp until all of the swap and memory would be occupied.

Since the OOM condition does not apply to files the processes would be killed until the malicious user shell would be killed (and he would not be able to log in due to lack of memory) or the compromised service would be killed. At this stage the system would be too slow for administrator to actually do anything other then restart, which would wipe the evidence thus making it effective DOS technique.

Not having /tmp as a separate mount then a slightly different problems would happen. Making the / partition full may render system unbootable and in very bad situation (say a glibc update) it may require much more repairing.

While those problems can be significant for multi-system there is also a reason to prevent it for desktop users. For example I once used a program which tried to write several GB of files in times when 512 MiB was large amount of RAM, which would render system unusable had the tmpfs been unlimited. In such way it only cause problems for the program itself and others trying to write to /tmp.

50% of RAM for tmpfs allows to use 50% of RAM for other things like running programs thus preventing the large swapping. Since it is generally assumed that minimal amount of information is kept there (41M on my system), but it needs to be quickly accessible is the reason of using tmpfs in the first place (no need to preserve it across reboots is enabling it). Thus 50% is 'reasonable' to both having large slack in case a peak /tmp usage happens while at the same time preventing the system slowdown.

As mentioned by abdelsaid, you can change the settings in /etc/fstab:

none /tmp tmpfs size=2G 0 0 # Change the size= parameter to whatever you want

PS. Where are you trying to compile the kernels (in /tmp)? Usually you compile them with user rights AND save the results in case you need 3rd party modules etc. gcc should keep minimal amount of files on /tmp thus avoiding the whole problem.

Once you use 4 GB of /tmp on 4GB system it is possible (especially when you are using heavyweight desktop environment) that your system will slow down significantly.

Answer 2

By default installation, RHEL and most Linux distributions mount tmpfs (a RAM-based temporarily filesystem) to /dev/shm directory and this temporarily filesystem size is always set to be half of the installed memory.

source: http://www.walkernews.net/2010/05/04/how-to-resize-devshm-filesystem-in-linux/

You can change that by specifying size=... in /etc/fstab:

e.g. 2GB:

tmpfs      /dev/shm      tmpfs   defaults,size=2g   0   0

e.g. 4GB:

tmpfs      /dev/shm      tmpfs   defaults,size=4g   0   0

Answer 3

If you prefer to not have /tmp mounted in tmpfs, look in /etc/fstab for a line similar to this:

tmpfs /tmp tmpfs defaults,noexec,nosuid 0 0

Remove it if present.

Depending on the distribution, it may also be configured in /etc/default/tmpfs or /etc/default/rcS. Edit that file and if the following line is present, comment it (prepend a #):

RAMTMP=yes