mm: multi-gen LRU: section for working set protection

Patch series "mm: multi-gen LRU: improve".

This patch series improves a few MGLRU functions, collects related
functions, and adds additional documentation.


This patch (of 7):

Add a section for working set protection in the code and the design doc. 
The admin doc already contains its usage.

Link: https://lkml.kernel.org/r/20230118001827.1040870-1-talumbau@google.com
Link: https://lkml.kernel.org/r/20230118001827.1040870-2-talumbau@google.comSigned-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/multigen_lru.rst

@@ -141,6 +141,21 @@ loop has detected outlying refaults from the tier this page is in. To
 this end, the feedback loop uses the first tier as the baseline, for
 the reason stated earlier.
 
+Working set protection
+----------------------
+Each generation is timestamped at birth. If ``lru_gen_min_ttl`` is
+set, an ``lruvec`` is protected from the eviction when its oldest
+generation was born within ``lru_gen_min_ttl`` milliseconds. In other
+words, it prevents the working set of ``lru_gen_min_ttl`` milliseconds
+from getting evicted. The OOM killer is triggered if this working set
+cannot be kept in memory.
+
+This time-based approach has the following advantages:
+
+1. It is easier to configure because it is agnostic to applications
+   and memory sizes.
+2. It is more reliable because it is directly wired to the OOM killer.
+
 Summary
 -------
 The multi-gen LRU can be disassembled into the following parts:

mm/vmscan.c

@@ -4475,6 +4475,10 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long max_seq,
 	return true;
 }
 
+/******************************************************************************
+ *                          working set protection
+ ******************************************************************************/
+
 static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
 {
 	int gen, type, zone;