A privileged process, either by running as root, or, better yet, by having the CAP_IPC_LOCK capability bit set in order to lock memory (we shall describe process credentials and capabilities in detail in their own chapters - Chapter 7, Process Credentials, and Chapter 8, Process Capabilities), can lock unlimited amounts of memory.
From Linux 2.6.9 onward, for a non-privileged process, it is limited by the RLIMIT_MEMLOCK soft resource limit (which, typically, is not set very high). Here is an example on an x86_64 Fedora box (as well as Ubuntu):
$ prlimit | grep MEMLOCKMEMLOCK max locked-in-memory address space 65536 65536 bytes$
It's just 64 KB (ditto on an embedded ARM Linux, by default).
At the time of writing this ...
