Understanding the Linux Kernel, 3rd Edition

Implementing the PFRA

The page frame reclaiming algorithm must take care of many kinds of pages owned by User Mode processes, disk caches and memory caches; moreover, it has to obey several heuristic rules. Thus, it is not surprising that the PFRA is composed of a large number of functions. Figure 17-3 shows the main PFRA functions; an arrow denotes a function invocation, thus for instance try_to_free_pages( ) invokes shrink_caches( ), shrink_slab( ), and out_of_memory( ).

As you can see, there are several “entry points” for the PFRA. Actually, page frame reclaiming is performed on essentially three occasions:

Low on memory reclaiming

The kernel detects a “low on memory” condition.

Hibernation reclaiming

The kernel must free memory because it is entering in the suspend-to-disk state (we don’t further discuss this case).

Periodic reclaiming

A kernel thread is activated periodically to perform memory reclaiming, if necessary.

Figure 17-3. The main functions of the PFRA

Low on memory reclaiming is activated in the following cases:

• The grow_buffers( ) function, invoked by _ _getblk( ), fails to allocate a new buffer page (see the section "Searching Blocks in the Page Cache" in Chapter 15).

• The alloc_page_buffers( ) function, invoked by create_empty_buffers( ), fails to allocate the temporary buffer heads for a page (see the section "Reading and Writing a File" in Chapter 16).

• The _ _alloc_pages( ...