mlockall disables paging for all pages mapped into the address space of
the calling process. This includes the pages of the code, data and stack
segment, as well as shared libraries, user space kernel data, shared memory
and memory mapped files. All mapped pages are guaranteed to be resident in RAM
when the mlockall system call returns successfully and they are
guaranteed to stay in RAM until the pages are unlocked again by munlock
or munlockall or until the process terminates or starts another program
with exec. Child processes do not inherit page locks across a
Memory locking has two main applications: real-time algorithms and high-security
data processing. Real-time applications require deterministic timing, and,
like scheduling, paging is one major cause of unexpected program execution
delays. Real-time applications will usually also switch to a real-time
scheduler with sched_setscheduler. Cryptographic security software
often handles critical bytes like passwords or secret keys as data structures.
As a result of paging, these secrets could be transfered onto a persistent
swap store medium, where they might be accessible to the enemy long after the
security software has erased the secrets in RAM and terminated. For security
applications, only small parts of memory have to be locked, for which
mlock is available.
The flags parameter can be constructed from the bitwise OR of the
Lock all pages which are currently mapped into the address
space of the process.
Lock all pages which will become mapped into the address
space of the process in the future. These could be for instance new pages
required by a growing heap and stack as well as new memory mapped files or
shared memory regions.
If MCL_FUTURE has been specified and the number of locked pages exceeds
the upper limit of allowed locked pages, then the system call which caused the
new mapping will fail with ENOMEM. If these new pages have been mapped
by the the growing stack, then the kernel will deny stack expansion and send a
Real-time processes should reserve enough locked stack pages before entering the
time-critical section, so that no page fault can be caused by function calls.
This can be achieved by calling a function which has a sufficiently large
automatic variable and which writes to the memory occupied by this large array
in order to touch these stack pages. This way, enough pages will be mapped for
the stack and can be locked into RAM. The dummy writes ensure that not even
copy-on-write page faults can occur in the critical section.
Memory locks do not stack, i.e., pages which have been locked several times by
calls to mlockall or mlock will be unlocked by a single call to
munlockall. Pages which are mapped to several locations or by several
processes stay locked into RAM as long as they are locked at least at one
location or by at least one process.