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| <h4 class="subsection">3.4.2 Locked Memory Details</h4> |
| |
| <p>A memory lock is associated with a virtual page, not a real frame. The |
| paging rule is: If a frame backs at least one locked page, don't page it |
| out. |
| |
| <p>Memory locks do not stack. I.e., you can't lock a particular page twice |
| so that it has to be unlocked twice before it is truly unlocked. It is |
| either locked or it isn't. |
| |
| <p>A memory lock persists until the process that owns the memory explicitly |
| unlocks it. (But process termination and exec cause the virtual memory |
| to cease to exist, which you might say means it isn't locked any more). |
| |
| <p>Memory locks are not inherited by child processes. (But note that on a |
| modern Unix system, immediately after a fork, the parent's and the |
| child's virtual address space are backed by the same real page frames, |
| so the child enjoys the parent's locks). See <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>. |
| |
| <p>Because of its ability to impact other processes, only the superuser can |
| lock a page. Any process can unlock its own page. |
| |
| <p>The system sets limits on the amount of memory a process can have locked |
| and the amount of real memory it can have dedicated to it. See <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>. |
| |
| <p>In Linux, locked pages aren't as locked as you might think. |
| Two virtual pages that are not shared memory can nonetheless be backed |
| by the same real frame. The kernel does this in the name of efficiency |
| when it knows both virtual pages contain identical data, and does it |
| even if one or both of the virtual pages are locked. |
| |
| <p>But when a process modifies one of those pages, the kernel must get it a |
| separate frame and fill it with the page's data. This is known as a |
| <dfn>copy-on-write page fault</dfn>. It takes a small amount of time and in |
| a pathological case, getting that frame may require I/O. |
| <a name="index-copy_002don_002dwrite-page-fault-357"></a><a name="index-page-fault_002c-copy_002don_002dwrite-358"></a> |
| To make sure this doesn't happen to your program, don't just lock the |
| pages. Write to them as well, unless you know you won't write to them |
| ever. And to make sure you have pre-allocated frames for your stack, |
| enter a scope that declares a C automatic variable larger than the |
| maximum stack size you will need, set it to something, then return from |
| its scope. |
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