[PATCH mm-unstable] mm: rmap: abstract updating per-node and per-memcg stats

[Yosry Ahmed]

A lot of intricacies go into updating the stats when adding or removing
mappings: which stat index to use and which function. Abstract this away
into a new static helper in rmap.c, __folio_mod_stat().

This adds an unnecessary call to folio_test_anon() in
__folio_add_anon_rmap() and __folio_add_file_rmap(). However, the folio
struct should already be in the cache at this point, so it shouldn't
cause any noticeable overhead.

No functional change intended.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---

This applies on top of "mm: do not update memcg stats for
NR_{FILE/SHMEM}_PMDMAPPED":
https://lore.kernel.org/lkml/20240506192924.271999-1-yosryahmed@xxxxxxxxxx/

David, I was on the fence about adding a Suggested-by here. You did
suggest adding a helper, but the one with the extra folio_test_anon()
was my idea and I didn't want to blame it on you. So I'll leave this up
to you :)

---
 mm/rmap.c | 56 ++++++++++++++++++++++++++++---------------------------
 1 file changed, 29 insertions(+), 27 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index ed7f820369864..9ed995da47099 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1269,6 +1269,28 @@ static void __page_check_anon_rmap(struct folio *folio, struct page *page,
 		       page);
 }
 
+static void __folio_mod_stat(struct folio *folio, int nr, int nr_pmdmapped)
+{
+	int idx;
+
+	if (nr) {
+		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
+		__lruvec_stat_mod_folio(folio, idx, nr);
+	}
+	if (nr_pmdmapped) {
+		if (folio_test_anon(folio)) {
+			idx = NR_ANON_THPS;
+			__lruvec_stat_mod_folio(folio, idx, nr_pmdmapped);
+		} else {
+			/* NR_*_PMDMAPPED are not maintained per-memcg */
+			idx = folio_test_swapbacked(folio) ?
+				NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED;
+			__mod_node_page_state(folio_pgdat(folio), idx,
+					      nr_pmdmapped);
+		}
+	}
+}
+
 static __always_inline void __folio_add_anon_rmap(struct folio *folio,
 		struct page *page, int nr_pages, struct vm_area_struct *vma,
 		unsigned long address, rmap_t flags, enum rmap_level level)
@@ -1276,10 +1298,6 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
 	int i, nr, nr_pmdmapped = 0;
 
 	nr = __folio_add_rmap(folio, page, nr_pages, level, &nr_pmdmapped);
-	if (nr_pmdmapped)
-		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr_pmdmapped);
-	if (nr)
-		__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
 
 	if (unlikely(!folio_test_anon(folio))) {
 		VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
@@ -1297,6 +1315,8 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
 		__page_check_anon_rmap(folio, page, vma, address);
 	}
 
+	__folio_mod_stat(folio, nr, nr_pmdmapped);
+
 	if (flags & RMAP_EXCLUSIVE) {
 		switch (level) {
 		case RMAP_LEVEL_PTE:
@@ -1393,6 +1413,7 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 		unsigned long address)
 {
 	int nr = folio_nr_pages(folio);
+	int nr_pmdmapped = 0;
 
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 	VM_BUG_ON_VMA(address < vma->vm_start ||
@@ -1425,27 +1446,22 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 		atomic_set(&folio->_large_mapcount, 0);
 		atomic_set(&folio->_nr_pages_mapped, ENTIRELY_MAPPED);
 		SetPageAnonExclusive(&folio->page);
-		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
+		nr_pmdmapped = nr;
 	}
 
-	__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
+	__folio_mod_stat(folio, nr, nr_pmdmapped);
 }
 
 static __always_inline void __folio_add_file_rmap(struct folio *folio,
 		struct page *page, int nr_pages, struct vm_area_struct *vma,
 		enum rmap_level level)
 {
-	pg_data_t *pgdat = folio_pgdat(folio);
 	int nr, nr_pmdmapped = 0;
 
 	VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
 
 	nr = __folio_add_rmap(folio, page, nr_pages, level, &nr_pmdmapped);
-	if (nr_pmdmapped)
-		__mod_node_page_state(pgdat, folio_test_swapbacked(folio) ?
-			NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
-	if (nr)
-		__lruvec_stat_mod_folio(folio, NR_FILE_MAPPED, nr);
+	__folio_mod_stat(folio, nr, nr_pmdmapped);
 
 	/* See comments in folio_add_anon_rmap_*() */
 	if (!folio_test_large(folio))
@@ -1494,10 +1510,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		enum rmap_level level)
 {
 	atomic_t *mapped = &folio->_nr_pages_mapped;
-	pg_data_t *pgdat = folio_pgdat(folio);
 	int last, nr = 0, nr_pmdmapped = 0;
 	bool partially_mapped = false;
-	enum node_stat_item idx;
 
 	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
 
@@ -1541,20 +1555,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		break;
 	}
 
-	if (nr_pmdmapped) {
-		/* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
-		if (folio_test_anon(folio))
-			__lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
-		else
-			__mod_node_page_state(pgdat,
-					folio_test_swapbacked(folio) ?
-					NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED,
-					-nr_pmdmapped);
-	}
 	if (nr) {
-		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
-		__lruvec_stat_mod_folio(folio, idx, -nr);
-
 		/*
 		 * Queue anon large folio for deferred split if at least one
 		 * page of the folio is unmapped and at least one page
@@ -1566,6 +1567,7 @@ 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);
 
 	/*
 	 * It would be tidy to reset folio_test_anon mapping when fully
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog