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.
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,
gfp_mask, are not
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.
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,