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I've always thought this possible, so today I tried it:
I have 2 gigs of physical RAM, and no swap partitions, so my memory limit is a hard 2 gigs.
I made 2 1-gig tmpfs mounts; nothing complained.
I filled them up one at a time, and kept checking both free memory (with free) and free space on the tmpfs mounts -- as I started to get near the limit of my system's RAM, free still showed plenty of RAM free in the +/- row.
Neither free nor df gave me any indication that the system was dangerously low on memory. (I assume from a bit of Googling that "free" considers the size of tmpfs's to be part of the "cached" column, as nothing else was big enough to contain them.)
Anyhow, I added a few more files to the tmpfs and the system locked up for a while... After about 3 minutes of hard drive thrashing (which was a bit distressing, but since my files look OK I guess it was desperately running sync again and again to try to free RAM for the tmpfs), a couple programs died and everything went back to normal.
So, in an effort to have that not happen again (since I use tmpfs heavily; it's a nice way to screw around with big files without waiting on hard drives), I wrote this little shell script:
#!/bin/bash
#prints worst-case free memory, from /proc/meminfo
total=`grep MemTotal /proc/meminfo | gawk '{print $2}'`
used=`grep Committed_AS /proc/meminfo | gawk '{print $2}'`
echo "Free RAM in the worst case: " $(($total - $used)) "kB"and put this in my .bashrc:
alias free='free; wcfree'
alias df='df; wcfree'~Felix.
PS: If it's not possible to make stuff crash with only a single tmpfs, please let me know -- that would be good knowledge, and make me feel silly for writing this :P
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