Locking the Program Address Space

Locking the Program Address Space

The ULI handler must not reference a page of program text or data that is not present in memory. You prevent this by locking the pages of the program address space in memory. The simplest way to do this is to call the plock() system function:

   if (plock(PROCLOCK))
   { perror("plock"); exit();}

The plock() function has two possible difficulties. One is that the calling process must have superuser privilege (see the plock(2) reference page). This may not pose a problem if the program needs superuser privilege in any case, for example in order to open a device special file. The second is that it locks all text and data pages. In a very large program this may be so much memory that system performance is harmed.

The mpin() function can be used by unprivileged programs to lock a limited number of pages. The limit is set by the tunable system parameter maxlkmem. (Check its value--typically 2000--in /var/sysgen/mtune/kernel. See the systune(1) reference page for instructions on changing a tunable parameter.)

In order to use mpin(), you must specify the exact address ranges to be locked. Provided that the ULI handler refers only to global data and its own code, it is relatively simple to derive address ranges that encompass the needed pages. If the ULI handler calls any library functions, the library DSO needs to be locked as well. The smaller the scope of the ULI handler, the easier it is to use mpin().