Linux Kernel Architecture

Chapter 18: Page Reclaim and Swapping

set, but when is it going to be removed again? Either the kernel does not remove it automatically, but

then the page would remain in the active state forever even if it would only be used very little, or not at

all anymore. To remove the flag automatically after some specific time-out would require a huge number

of kernel timers because appropriate hardware support is not available on all CPUs supported by Linux.

Considering the large number of pages that are present in a typical system, this approach is also doomed

to fail.

CPU 1

CPU 0

lru_inactive lru_active

lru_add_pvecs lru_add_active_pvecs

1

1

4 3 3 2

SetPageActive

activate_page

shrink_active_list

_ _pagevec_lru_add

2 _ _pagevec_lru_add_active

3 SetPageLRU

4 SetPageActive

Figure 18-12: Page movements between the per-CPU page lists and

the global LRU lists. To simplify matters, only a single zone is used as

the basis of the global lists. Only the most important functions that

move pages between the active and inactive lists are shown.

Having two flags allows for implementing a more sophisticated approach to determining page activity.

The core idea is to use one flag to denote the current activity rating, and another one that signals if the

page has been recently referenced. Both bits need to be set in close cooperation. Figure 18-13 illustrates

the corresponding algorithm. Essentially the following steps are necessary:

1. If the page is deemed active, thePG_activeflagisset;otherwise,not.Theflagdirectlycorre-
   sponds to the LRU list the page is on, namely, the (zone-specific)inactiveoractivelist.


2. Each time the page is accessed, the flagPG_referencedis set. The function responsible for
   this ismark_page_accessed, and the kernel must make sure to call it appropriately.


3. ThePG_referencedflag and information provided by reversemappingareusedtodeter-
   mine page activity. The crucial point is that thePG_referencedflag isremovedeach time an
   activity check is performed.page_referencedis the function that implements this behavior.


4. Entermark_page_accessedagain. When it finds that thePG_accessedbitisalreadysetwhen
   it checks the page, this means thatnocheck was performed bypage_referenced. The calls