mm: change per-VMA lock statistics to be disabled by default

Change CONFIG_PER_VMA_LOCK_STATS to be disabled by default, as most users
don't need it.  Add configuration help to clarify its usage.

Link: https://lkml.kernel.org/r/20230428173533.18158-1-surenb@google.com
Fixes: 52f23865 ("mm: introduce per-VMA lock statistics")
Signed-off-by: NSuren Baghdasaryan <surenb@google.com>
Suggested-by: NLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NLorenzo Stoakes <lstoakes@gmail.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

mm/Kconfig.debug

@@ -274,6 +274,12 @@ config DEBUG_KMEMLEAK_AUTO_SCAN
 config PER_VMA_LOCK_STATS
 	bool "Statistics for per-vma locks"
 	depends on PER_VMA_LOCK
-	default y
 	help
-	  Statistics for per-vma locks.
+	  Say Y here to enable success, retry and failure counters of page
+	  faults handled under protection of per-vma locks. When enabled, the
+	  counters are exposed in /proc/vmstat. This information is useful for
+	  kernel developers to evaluate effectiveness of per-vma locks and to
+	  identify pathological cases. Counting these events introduces a small
+	  overhead in the page fault path.
+
+	  If in doubt, say N.