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Understanding the Linux Kernel, Second Edition by Marco Cesati, Daniel P. Bovet

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Swapping Out Pages

The later section Section 16.7 explains what happens when pages are swapped out. As we indicated at the beginning of this chapter, swapping out pages is a last resort and appears as part of a general strategy to free memory that uses other tactics as well. In this section, we show how the kernel performs a swap out. This is achieved by a series of functions called in cascading fashion. Let’s start with the functions at the higher level.

The swap_out( ) function acts on a single classzone parameter that specifies the memory zone from which pages should be swapped out (see Section 7.1.2). Two other parameters, priority and gfp_mask, are not used.

The swap_out( ) function scans existing memory descriptors and tries to swap out the pages referenced in each process’s Page Tables. It terminates as soon as one of the following conditions occurs:

  • The function succeeds in releasing SWAP_CLUSTER_MAX page frames (by default, 32). A page frame is considered released when it is removed from the Page Tables of all processes that share it.

  • The function scans n memory descriptors, where n is the length of the memory descriptor list when the function starts.[111]

To ensure that all processes are evenly penalized by swap_out( ), the function starts scanning the list from the memory descriptor that was last analyzed in the previous invocation; the address of this memory descriptor is stored in the swap_mm global variable.

For each memory descriptor mm to be considered, the swap_out( ...

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