[PATCH v4 0/2] mm: count the number of anonymous THPs per size

[PATCH v4 0/2] mm: count the number of anonymous THPs per size

[Barry Song]

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

Knowing the number of transparent anon THPs in the system is crucial
for performance analysis. It helps in understanding the ratio and
distribution of THPs versus small folios throughout the system.

Additionally, partial unmapping by userspace can lead to significant waste
of THPs over time and increase memory reclamation pressure. We need this
information for comprehensive system tuning.

-v4:
 * collect David's acked-by, thanks!
 * refine subjects, changelogs and docs according to David's comment,
   thanks very much!

-v3:
 https://lore.kernel.org/linux-mm/20240822224015.93186-1-21cnbao@gmail.com/
 * collect David's acked-by, thanks!
 * rename nr_split_deferred to meaningful name - nr_anon_partially_mapped,
   per David. Ryan, I assume you will like it after you come back :-)

-v2:
 https://lore.kernel.org/linux-mm/20240811224940.39876-1-21cnbao@gmail.com/
 * don't rely on rmap to implement - 1, uses folio_free, split etc.
   Thanks for David's comment;
 * rename sys counters and refine doc. Thanks for Ryan's comment;

-v1:
 https://lore.kernel.org/all/20240808010457.228753-1-21cnbao@gmail.com/

Barry Song (2):
  mm: count the number of anonymous THPs per size
  mm: count the number of partially mapped anonymous THPs per size

 Documentation/admin-guide/mm/transhuge.rst | 12 ++++++++++++
 include/linux/huge_mm.h                    | 16 ++++++++++++++--
 mm/huge_memory.c                           | 19 ++++++++++++++++---
 mm/migrate.c                               |  4 ++++
 mm/page_alloc.c                            |  5 ++++-
 mm/rmap.c                                  |  1 +
 6 files changed, 51 insertions(+), 6 deletions(-)

-- 
2.39.3 (Apple Git-146)

[PATCH v4 1/2] mm: count the number of anonymous THPs per size

[Barry Song]

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

Let's track for each anonymous THP size, how many of them are currently
allocated. We'll track the complete lifespan of an anon THP, starting
when it becomes an anon THP ("large anon folio") (->mapping gets set),
until it gets freed (->mapping gets cleared).

Introduce a new "nr_anon" counter per THP size and adjust the
corresponding counter in the following cases:
* We allocate a new THP and call folio_add_new_anon_rmap() to map
   it the first time and turn it into an anon THP.
* We split an anon THP into multiple smaller ones.
* We migrate an anon THP, when we prepare the destination.
* We free an anon THP back to the buddy.

Note that AnonPages in /proc/meminfo currently tracks the total number
of *mapped* anonymous *pages*, and therefore has slightly different
semantics. In the future, we might also want to track "nr_anon_mapped"
for each THP size, which might be helpful when comparing it to the
number of allocated anon THPs (long-term pinning, stuck in swapcache,
memory leaks, ...).

Further note that for now, we only track anon THPs after they got their
->mapping set, for example via folio_add_new_anon_rmap(). If we would
allocate some in the swapcache, they will only show up in the statistics
for now after they have been mapped to user space the first time, where
we call folio_add_new_anon_rmap().

Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst |  5 +++++
 include/linux/huge_mm.h                    | 15 +++++++++++++--
 mm/huge_memory.c                           | 13 ++++++++++---
 mm/migrate.c                               |  4 ++++
 mm/page_alloc.c                            |  5 ++++-
 mm/rmap.c                                  |  1 +
 6 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 79435c537e21..b78f2148b242 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -551,6 +551,11 @@ split_deferred
         it would free up some memory. Pages on split queue are going to
         be split under memory pressure, if splitting is possible.
 
+nr_anon
+       the number of transparent anon huge pages we have in the whole system.
+       These huge pages could be entirely mapped or have partially
+       unmapped/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 4c32058cacfe..2ee2971e4e10 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -126,6 +126,7 @@ enum mthp_stat_item {
 	MTHP_STAT_SPLIT,
 	MTHP_STAT_SPLIT_FAILED,
 	MTHP_STAT_SPLIT_DEFERRED,
+	MTHP_STAT_NR_ANON,
 	__MTHP_STAT_COUNT
 };
 
@@ -136,14 +137,24 @@ struct mthp_stat {
 
 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 513e7c87efee..26ad75fcda62 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -597,6 +597,7 @@ 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(nr_anon, MTHP_STAT_NR_ANON);
 
 static struct attribute *anon_stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -609,6 +610,7 @@ static struct attribute *anon_stats_attrs[] = {
 	&split_attr.attr,
 	&split_failed_attr.attr,
 	&split_deferred_attr.attr,
+	&nr_anon_attr.attr,
 	NULL,
 };
 
@@ -3314,8 +3316,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
 	/* reset xarray order to new order after split */
 	XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
-	struct anon_vma *anon_vma = NULL;
+	bool is_anon = folio_test_anon(folio);
 	struct address_space *mapping = NULL;
+	struct anon_vma *anon_vma = NULL;
 	int order = folio_order(folio);
 	int extra_pins, ret;
 	pgoff_t end;
@@ -3327,7 +3330,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (new_order >= folio_order(folio))
 		return -EINVAL;
 
-	if (folio_test_anon(folio)) {
+	if (is_anon) {
 		/* order-1 is not supported for anonymous THP. */
 		if (new_order == 1) {
 			VM_WARN_ONCE(1, "Cannot split to order-1 folio");
@@ -3367,7 +3370,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (folio_test_writeback(folio))
 		return -EBUSY;
 
-	if (folio_test_anon(folio)) {
+	if (is_anon) {
 		/*
 		 * The caller does not necessarily hold an mmap_lock that would
 		 * prevent the anon_vma disappearing so we first we take a
@@ -3480,6 +3483,10 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 			}
 		}
 
+		if (is_anon) {
+			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
+			mod_mthp_stat(new_order, MTHP_STAT_NR_ANON, 1 << (order - new_order));
+		}
 		__split_huge_page(page, list, end, new_order);
 		ret = 0;
 	} else {
diff --git a/mm/migrate.c b/mm/migrate.c
index 4f55f4930fe8..3cc8555de6d6 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -450,6 +450,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 		/* No turning back from here */
 		newfolio->index = folio->index;
 		newfolio->mapping = folio->mapping;
+		if (folio_test_anon(folio) && folio_test_large(folio))
+			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
 		if (folio_test_swapbacked(folio))
 			__folio_set_swapbacked(newfolio);
 
@@ -474,6 +476,8 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 	 */
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
+	if (folio_test_anon(folio) && folio_test_large(folio))
+		mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON, 1);
 	folio_ref_add(newfolio, nr); /* add cache reference */
 	if (folio_test_swapbacked(folio)) {
 		__folio_set_swapbacked(newfolio);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8a67d760b71a..7dcb0713eb57 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1084,8 +1084,11 @@ __always_inline bool free_pages_prepare(struct page *page,
 			(page + i)->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
 		}
 	}
-	if (PageMappingFlags(page))
+	if (PageMappingFlags(page)) {
+		if (PageAnon(page))
+			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
 		page->mapping = NULL;
+	}
 	if (is_check_pages_enabled()) {
 		if (free_page_is_bad(page))
 			bad++;
diff --git a/mm/rmap.c b/mm/rmap.c
index 1103a536e474..78529cf0fd66 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,
-- 
2.39.3 (Apple Git-146)

[PATCH v4 2/2] mm: count the number of partially mapped anonymous THPs per size

[Barry Song]

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

When a THP is added to the deferred_list due to partially mapped,
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 a
THP 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@oppo.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 7 +++++++
 include/linux/huge_mm.h                    | 1 +
 mm/huge_memory.c                           | 6 ++++++
 3 files changed, 14 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index b78f2148b242..6630f2ed14ee 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -556,6 +556,13 @@ nr_anon
        These huge pages could be entirely mapped or have partially
        unmapped/unused subpages.
 
+nr_anon_partially_mapped
+       the number of anonymous THP which are likely partially mapped, possibly
+       wasting memory, and have been queued for deferred memory reclamation.
+       Note that in corner some cases (e.g., failed migration), we might detect
+       an anonymous THP as "partially mapped" and count it here, even though it
+       is not actually partially mapped anymore.
+
 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..4902e2f7e896 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_ANON_PARTIALLY_MAPPED,
 	__MTHP_STAT_COUNT
 };
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 26ad75fcda62..a81eab98d6b8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -598,6 +598,7 @@ 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_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED);
 
 static struct attribute *anon_stats_attrs[] = {
 	&anon_fault_alloc_attr.attr,
@@ -611,6 +612,7 @@ static struct attribute *anon_stats_attrs[] = {
 	&split_failed_attr.attr,
 	&split_deferred_attr.attr,
 	&nr_anon_attr.attr,
+	&nr_anon_partially_mapped_attr.attr,
 	NULL,
 };
 
@@ -3457,6 +3459,7 @@ 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_ANON_PARTIALLY_MAPPED, -1);
 			/*
 			 * Reinitialize page_deferred_list after removing the
 			 * page from the split_queue, otherwise a subsequent
@@ -3523,6 +3526,7 @@ 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_ANON_PARTIALLY_MAPPED, -1);
 		list_del_init(&folio->_deferred_list);
 	}
 	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
@@ -3564,6 +3568,7 @@ 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_ANON_PARTIALLY_MAPPED, 1);
 		list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
 		ds_queue->split_queue_len++;
 #ifdef CONFIG_MEMCG
@@ -3611,6 +3616,7 @@ 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_ANON_PARTIALLY_MAPPED, -1);
 			list_del_init(&folio->_deferred_list);
 			ds_queue->split_queue_len--;
 		}
-- 
2.39.3 (Apple Git-146)

Re: [PATCH v4 1/2] mm: count the number of anonymous THPs per size

[David Hildenbrand]

On 24.08.24 03:04, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Let's track for each anonymous THP size, how many of them are currently
> allocated. We'll track the complete lifespan of an anon THP, starting
> when it becomes an anon THP ("large anon folio") (->mapping gets set),
> until it gets freed (->mapping gets cleared).
> 
> Introduce a new "nr_anon" counter per THP size and adjust the
> corresponding counter in the following cases:
> * We allocate a new THP and call folio_add_new_anon_rmap() to map
>     it the first time and turn it into an anon THP.
> * We split an anon THP into multiple smaller ones.
> * We migrate an anon THP, when we prepare the destination.
> * We free an anon THP back to the buddy.
> 
> Note that AnonPages in /proc/meminfo currently tracks the total number
> of *mapped* anonymous *pages*, and therefore has slightly different
> semantics. In the future, we might also want to track "nr_anon_mapped"
> for each THP size, which might be helpful when comparing it to the
> number of allocated anon THPs (long-term pinning, stuck in swapcache,
> memory leaks, ...).
> 
> Further note that for now, we only track anon THPs after they got their
> ->mapping set, for example via folio_add_new_anon_rmap(). If we would
> allocate some in the swapcache, they will only show up in the statistics
> for now after they have been mapped to user space the first time, where
> we call folio_add_new_anon_rmap().
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
>   Documentation/admin-guide/mm/transhuge.rst |  5 +++++
>   include/linux/huge_mm.h                    | 15 +++++++++++++--
>   mm/huge_memory.c                           | 13 ++++++++++---
>   mm/migrate.c                               |  4 ++++
>   mm/page_alloc.c                            |  5 ++++-
>   mm/rmap.c                                  |  1 +
>   6 files changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 79435c537e21..b78f2148b242 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -551,6 +551,11 @@ split_deferred
>           it would free up some memory. Pages on split queue are going to
>           be split under memory pressure, if splitting is possible.
> 

In light of documentation of patch #2, small nits:


> +nr_anon
> +       the number of transparent anon huge pages we have in the whole system.

s/transparent anon huge pages/anonymous THP/

> +       These huge pages could be entirely mapped or have partially

s/huge pages/THPs/

s/could be/might be currently/

Maybe Andrew can just fix it up if there are no other comments.

Thanks!

-- 
Cheers,

David / dhildenb

Re: [PATCH v4 1/2] mm: count the number of anonymous THPs per size

[chunpeng lee]

> Let's track for each anonymous THP size, how many of them are currently
> allocated. We'll track the complete lifespan of an anon THP, starting
> when it becomes an anon THP ("large anon folio") (->mapping gets set),
> until it gets freed (->mapping gets cleared).

IIUC, If an anon THP is swapped out as a whole, it is still being counted, correct?

> Note that AnonPages in /proc/meminfo currently tracks the total number
> of *mapped* anonymous *pages*, and therefore has slightly different
> semantics. In the future, we might also want to track "nr_anon_mapped"
> for each THP size, which might be helpful when comparing it to the
> number of allocated anon THPs (long-term pinning, stuck in swapcache,
> memory leaks, ...).

If we do not consider tracking each THP size, Can we expand the NR_ANON_THPS
statistic to include pte-mapped thp as well?

---
 mm/memcontrol-v1.c | 2 +-
 mm/rmap.c | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index 44803cbea38a..3e44175db81f 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -786,7 +786,7 @@ static int mem_cgroup_move_account(struct folio *folio,
 if (folio_mapped(folio)) {
 __mod_lruvec_state(from_vec, NR_ANON_MAPPED, -nr_pages);
 __mod_lruvec_state(to_vec, NR_ANON_MAPPED, nr_pages);
- if (folio_test_pmd_mappable(folio)) {
+ if (folio_test_large(folio)) {
 __mod_lruvec_state(from_vec, NR_ANON_THPS,
 -nr_pages);
 __mod_lruvec_state(to_vec, NR_ANON_THPS,
diff --git a/mm/rmap.c b/mm/rmap.c
index a8797d1b3d49..97eb25d023ba 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1291,6 +1291,11 @@ static void __folio_mod_stat(struct folio *folio, int nr, int nr_pmdmapped)
 if (nr) {
 idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
 __lruvec_stat_mod_folio(folio, idx, nr);
+
+ if (folio_test_anon(folio) &&
+ folio_test_large(folio) &&
+ nr == 1 << folio_order(folio))
+ __lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
 }
 if (nr_pmdmapped) {
 if (folio_test_anon(folio)) {
--