linux-kernel - [PATCH v2 2/2] mm: collect the number of anon large folios on split

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Message-Id: <20240811224940.39876-3-21cnbao@gmail.com>
Date: Mon, 12 Aug 2024 10:49:40 +1200
From: Barry Song <21cnbao@...il.com>
To: akpm@...ux-foundation.org,
	linux-mm@...ck.org
Cc: baolin.wang@...ux.alibaba.com,
	chrisl@...nel.org,
	david@...hat.com,
	hanchuanhua@...o.com,
	ioworker0@...il.com,
	kaleshsingh@...gle.com,
	kasong@...cent.com,
	linux-kernel@...r.kernel.org,
	ryan.roberts@....com,
	v-songbaohua@...o.com,
	ziy@...dia.com,
	yuanshuai@...o.com
Subject: [PATCH v2 2/2] mm: collect the number of anon large folios on split_deferred list

From: Barry Song <v-songbaohua@...o.com>

When an mTHP is added to the deferred_list, its partial pages
are unused, leading to wasted memory and potentially increasing
memory reclamation pressure.

Detailing the specifics of how unmapping occurs is quite difficult
and not that useful, so we adopt a simple approach: each time an
mTHP enters the deferred_list, we increment the count by 1; whenever
it leaves for any reason, we decrement the count by 1.

Signed-off-by: Barry Song <v-songbaohua@...o.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 5 +++++
 include/linux/huge_mm.h                    | 1 +
 mm/huge_memory.c                           | 6 ++++++
 3 files changed, 12 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 9fdfb46e4560..7072469de8a8 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -532,6 +532,11 @@ nr_anon
        These huge pages could be entirely mapped or have partially
        unmapped/unused subpages.
 
+nr_split_deferred
+       the number of anon huge pages which have been partially unmapped
+       and put onto split queue. Those unmapped subpages are also unused
+       and temporarily wasting memory.
+
 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 2ee2971e4e10..1e2d5dbe82c5 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -127,6 +127,7 @@ enum mthp_stat_item {
 	MTHP_STAT_SPLIT_FAILED,
 	MTHP_STAT_SPLIT_DEFERRED,
 	MTHP_STAT_NR_ANON,
+	MTHP_STAT_NR_SPLIT_DEFERRED,
 	__MTHP_STAT_COUNT
 };
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afb911789df8..1a12c011e2df 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -597,6 +597,7 @@ 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(nr_anon, MTHP_STAT_NR_ANON);
+DEFINE_MTHP_STAT_ATTR(nr_split_deferred, MTHP_STAT_NR_SPLIT_DEFERRED);
 
 static struct attribute *anon_stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -610,6 +611,7 @@ static struct attribute *anon_stats_attrs[] = {
 	&split_failed_attr.attr,
 	&split_deferred_attr.attr,
 	&nr_anon_attr.attr,
+	&nr_split_deferred_attr.attr,
 	NULL,
 };
 
@@ -3359,6 +3361,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		if (folio_order(folio) > 1 &&
 		    !list_empty(&folio->_deferred_list)) {
 			ds_queue->split_queue_len--;
+			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_SPLIT_DEFERRED, -1);
 			/*
 			 * Reinitialize page_deferred_list after removing the
 			 * page from the split_queue, otherwise a subsequent
@@ -3425,6 +3428,7 @@ void __folio_undo_large_rmappable(struct folio *folio)
 	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
 	if (!list_empty(&folio->_deferred_list)) {
 		ds_queue->split_queue_len--;
+		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_SPLIT_DEFERRED, -1);
 		list_del_init(&folio->_deferred_list);
 	}
 	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
@@ -3466,6 +3470,7 @@ void deferred_split_folio(struct folio *folio)
 		if (folio_test_pmd_mappable(folio))
 			count_vm_event(THP_DEFERRED_SPLIT_PAGE);
 		count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
+		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_SPLIT_DEFERRED, 1);
 		list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
 		ds_queue->split_queue_len++;
 #ifdef CONFIG_MEMCG
@@ -3513,6 +3518,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 			list_move(&folio->_deferred_list, &list);
 		} else {
 			/* We lost race with folio_put() */
+			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_SPLIT_DEFERRED, -1);
 			list_del_init(&folio->_deferred_list);
 			ds_queue->split_queue_len--;
 		}
-- 
2.34.1