[PATCH RFC 1/2] mm: collect the number of anon large folios

[Barry Song <21cnbao@xxxxxxxxx>]

From: Barry Song <v-songbaohua@xxxxxxxx>

When a new anonymous mTHP is added to the rmap, we increase the count.
We reduce the count whenever an mTHP is completely unmapped.

Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 Documentation/admin-guide/mm/transhuge.rst |  5 +++++
 include/linux/huge_mm.h                    | 15 +++++++++++++--
 mm/huge_memory.c                           |  2 ++
 mm/rmap.c                                  |  3 +++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 058485daf186..715f181543f6 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -527,6 +527,11 @@ split_deferred
         it would free up some memory. Pages on split queue are going to
         be split under memory pressure, if splitting is possible.
 
+anon_num
+       the number of anon huge pages we have in the whole system.
+       These huge pages could be still entirely mapped and have partially
+       unmapped and unused subpages.
+
 As the system ages, allocating huge pages may be expensive as the
 system uses memory compaction to copy data around memory to free a
 huge page for use. There are some counters in ``/proc/vmstat`` to help
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e25d9ebfdf89..294c348fe3cc 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -281,6 +281,7 @@ enum mthp_stat_item {
 	MTHP_STAT_SPLIT,
 	MTHP_STAT_SPLIT_FAILED,
 	MTHP_STAT_SPLIT_DEFERRED,
+	MTHP_STAT_NR_ANON,
 	__MTHP_STAT_COUNT
 };
 
@@ -291,14 +292,24 @@ struct mthp_stat {
 #ifdef CONFIG_SYSFS
 DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
 
-static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+static inline void mod_mthp_stat(int order, enum mthp_stat_item item, int delta)
 {
 	if (order <= 0 || order > PMD_ORDER)
 		return;
 
-	this_cpu_inc(mthp_stats.stats[order][item]);
+	this_cpu_add(mthp_stats.stats[order][item], delta);
+}
+
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+	mod_mthp_stat(order, item, 1);
 }
+
 #else
+static inline void mod_mthp_stat(int order, enum mthp_stat_item item, int delta)
+{
+}
+
 static inline void count_mthp_stat(int order, enum mthp_stat_item item)
 {
 }
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 697fcf89f975..b6bc2a3791e3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -578,6 +578,7 @@ DEFINE_MTHP_STAT_ATTR(shmem_fallback_charge, MTHP_STAT_SHMEM_FALLBACK_CHARGE);
 DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
 DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
 DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
+DEFINE_MTHP_STAT_ATTR(anon_num, MTHP_STAT_NR_ANON);
 
 static struct attribute *stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -591,6 +592,7 @@ static struct attribute *stats_attrs[] = {
 	&split_attr.attr,
 	&split_failed_attr.attr,
 	&split_deferred_attr.attr,
+	&anon_num_attr.attr,
 	NULL,
 };
 
diff --git a/mm/rmap.c b/mm/rmap.c
index 901950200957..2b722f26224c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1467,6 +1467,7 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 	}
 
 	__folio_mod_stat(folio, nr, nr_pmdmapped);
+	mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
 }
 
 static __always_inline void __folio_add_file_rmap(struct folio *folio,
@@ -1582,6 +1583,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 	    list_empty(&folio->_deferred_list))
 		deferred_split_folio(folio);
 	__folio_mod_stat(folio, -nr, -nr_pmdmapped);
+	if (folio_test_anon(folio) && !atomic_read(mapped))
+		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, -1);
 
 	/*
 	 * It would be tidy to reset folio_test_anon mapping when fully
-- 
2.34.1