mm: multi-gen LRU: section for rmap/PT walk feedback

Add a section for lru_gen_look_around() in the code and the design doc.

Link: https://lkml.kernel.org/r/20230118001827.1040870-3-talumbau@google.com
Signed-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

@@ -156,6 +156,20 @@ This time-based approach has the following advantages:
    and memory sizes.
 2. It is more reliable because it is directly wired to the OOM killer.
 
+Rmap/PT walk feedback
+---------------------
+Searching the rmap for PTEs mapping each page on an LRU list (to test
+and clear the accessed bit) can be expensive because pages from
+different VMAs (PA space) are not cache friendly to the rmap (VA
+space). For workloads mostly using mapped pages, searching the rmap
+can incur the highest CPU cost in the reclaim path.
+
+``lru_gen_look_around()`` exploits spatial locality to reduce the
+trips into the rmap. It scans the adjacent PTEs of a young PTE and
+promotes hot pages. If the scan was done cacheline efficiently, it
+adds the PMD entry pointing to the PTE table to the Bloom filter. This
+forms a feedback loop between the eviction and the aging.
+
 Summary
 -------
 The multi-gen LRU can be disassembled into the following parts:

mm/vmscan.c

@@ -4569,6 +4569,10 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 	}
 }
 
+/******************************************************************************
+ *                          rmap/PT walk feedback
+ ******************************************************************************/
+
 /*
  * This function exploits spatial locality when shrink_folio_list() walks the
  * rmap. It scans the adjacent PTEs of a young PTE and promotes hot pages. If