``folio->flags`` and therefore has a negligible cost. A feedback loop
modeled after the PID controller monitors refaults over all the tiers
from anon and file types and decides which tiers from which types to
-evict or protect.
+evict or protect. The desired effect is to balance refault percentages
+between anon and file types proportional to the swappiness level.
There are two conceptually independent procedures: the aging and the
eviction. They form a closed-loop system, i.e., the page reclaim.
and memory sizes.
2. It is more reliable because it is directly wired to the OOM killer.
+``mm_struct`` list
+------------------
+An ``mm_struct`` list is maintained for each memcg, and an
+``mm_struct`` follows its owner task to the new memcg when this task
+is migrated.
+
+A page table walker iterates ``lruvec_memcg()->mm_list`` and calls
+``walk_page_range()`` with each ``mm_struct`` on this list to scan
+PTEs. When multiple page table walkers iterate the same list, each of
+them gets a unique ``mm_struct``, and therefore they can run in
+parallel.
+
+Page table walkers ignore any misplaced pages, e.g., if an
+``mm_struct`` was migrated, pages left in the previous memcg will be
+ignored when the current memcg is under reclaim. Similarly, page table
+walkers will ignore pages from nodes other than the one under reclaim.
+
+This infrastructure also tracks the usage of ``mm_struct`` between
+context switches so that page table walkers can skip processes that
+have been sleeping since the last iteration.
+
Rmap/PT walk feedback
---------------------
Searching the rmap for PTEs mapping each page on an LRU list (to test
adds the PMD entry pointing to the PTE table to the Bloom filter. This
forms a feedback loop between the eviction and the aging.
-Bloom Filters
+Bloom filters
-------------
Bloom filters are a space and memory efficient data structure for set
membership test, i.e., test if an element is not in the set or may be
which may yield hot pages anyway. Parameters of the filter itself can
control the false positive rate in the limit.
+PID controller
+--------------
+A feedback loop modeled after the Proportional-Integral-Derivative
+(PID) controller monitors refaults over anon and file types and
+decides which type to evict when both types are available from the
+same generation.
+
+The PID controller uses generations rather than the wall clock as the
+time domain because a CPU can scan pages at different rates under
+varying memory pressure. It calculates a moving average for each new
+generation to avoid being permanently locked in a suboptimal state.
+
Memcg LRU
---------
An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
* Generations
* Rmap walks
-* Page table walks
-* Bloom filters
-* PID controller
+* Page table walks via ``mm_struct`` list
+* Bloom filters for rmap/PT walk feedback
+* PID controller for refault feedback
The aging and the eviction form a producer-consumer model;
specifically, the latter drives the former by the sliding window over