mm: multi-gen LRU: remove MM_LEAF_OLD and MM_NONLEAF_TOTAL stats

Patch series "mm: multi-gen LRU: Have secondary MMUs participate in
MM_WALK".

Today, the MM_WALK capability causes MGLRU to clear the young bit from
PMDs and PTEs during the page table walk before eviction, but MGLRU does
not call the clear_young() MMU notifier in this case.  By not calling this
notifier, the MM walk takes less time/CPU, but it causes pages that are
accessed mostly through KVM / secondary MMUs to appear younger than they
should be.

We do call the clear_young() notifier today, but only when attempting to
evict the page, so we end up clearing young/accessed information less
frequently for secondary MMUs than for mm PTEs, and therefore they appear
younger and are less likely to be evicted.  Therefore, memory that is
*not* being accessed mostly by KVM will be evicted *more* frequently,
worsening performance.

ChromeOS observed a tab-open latency regression when enabling MGLRU with a
setup that involved running a VM:

		Tab-open latency histogram (ms)
Version		p50	mean	p95	p99	max
base		1315	1198	2347	3454	10319
mglru		2559	1311	7399	12060	43758
fix		1119	926	2470	4211	6947

This series replaces the final non-selftest patchs from this series[1],
which introduced a similar change (and a new MMU notifier) with KVM
optimizations.  I'll send a separate series (to Sean and Paolo) for the
KVM optimizations.

This series also makes proactive reclaim with MGLRU possible for KVM
memory.  I have verified that this functions correctly with the selftest
from [1], but given that that test is a KVM selftest, I'll send it with
the rest of the KVM optimizations later.  Andrew, let me know if you'd
like to take the test now anyway.

[1]: https://lore.kernel.org/linux-mm/20240926013506.860253-18-jthoughton@google.com/


This patch (of 2):

The removed stats, MM_LEAF_OLD and MM_NONLEAF_TOTAL, are not very helpful
and become more complicated to properly compute when adding
test/clear_young() notifiers in MGLRU's mm walk.

Link: https://lkml.kernel.org/r/20241019012940.3656292-1-jthoughton@google.com
Link: https://lkml.kernel.org/r/20241019012940.3656292-2-jthoughton@google.com
Fixes: bd74fda ("mm: multi-gen LRU: support page table walks")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: James Houghton <jthoughton@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: David Stevens <stevensd@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Yu Zhao

include/linux/mmzone.h

@@ -458,9 +458,7 @@ struct lru_gen_folio {
 
 enum {
 	MM_LEAF_TOTAL,		/* total leaf entries */
-	MM_LEAF_OLD,		/* old leaf entries */
 	MM_LEAF_YOUNG,		/* young leaf entries */
-	MM_NONLEAF_TOTAL,	/* total non-leaf entries */
 	MM_NONLEAF_FOUND,	/* non-leaf entries found in Bloom filters */
 	MM_NONLEAF_ADDED,	/* non-leaf entries added to Bloom filters */
 	NR_MM_STATS

mm/vmscan.c

@@ -3399,7 +3399,6 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 			continue;
 
 		if (!pte_young(ptent)) {
-			walk->mm_stats[MM_LEAF_OLD]++;
 			continue;
 		}
 
@@ -3552,7 +3551,6 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 			walk->mm_stats[MM_LEAF_TOTAL]++;
 
 			if (!pmd_young(val)) {
-				walk->mm_stats[MM_LEAF_OLD]++;
 				continue;
 			}
 
@@ -3564,8 +3562,6 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 			continue;
 		}
 
-		walk->mm_stats[MM_NONLEAF_TOTAL]++;
-
 		if (!walk->force_scan && should_clear_pmd_young()) {
 			if (!pmd_young(val))
 				continue;
@@ -5254,11 +5250,11 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec,
 	for (tier = 0; tier < MAX_NR_TIERS; tier++) {
 		seq_printf(m, "            %10d", tier);
 		for (type = 0; type < ANON_AND_FILE; type++) {
-			const char *s = "   ";
+			const char *s = "xxx";
 			unsigned long n[3] = {};
 
 			if (seq == max_seq) {
-				s = "RT ";
+				s = "RTx";
 				n[0] = READ_ONCE(lrugen->avg_refaulted[type][tier]);
 				n[1] = READ_ONCE(lrugen->avg_total[type][tier]);
 			} else if (seq == min_seq[type] || NR_HIST_GENS > 1) {
@@ -5280,14 +5276,14 @@ static void lru_gen_seq_show_full(struct seq_file *m, struct lruvec *lruvec,
 
 	seq_puts(m, "                      ");
 	for (i = 0; i < NR_MM_STATS; i++) {
-		const char *s = "      ";
+		const char *s = "xxxx";
 		unsigned long n = 0;
 
 		if (seq == max_seq && NR_HIST_GENS == 1) {
-			s = "LOYNFA";
+			s = "TYFA";
 			n = READ_ONCE(mm_state->stats[hist][i]);
 		} else if (seq != max_seq && NR_HIST_GENS > 1) {
-			s = "loynfa";
+			s = "tyfa";
 			n = READ_ONCE(mm_state->stats[hist][i]);
 		}