[PATCH 2/2] mm: pgtable: remove unnecessary split ptlock for kernel PMD page

[Qi Zheng <zhengqi.arch@bytedance.com>]

For kernel PMD entry, we use init_mm.page_table_lock to protect it, so
there is no need to allocate and initialize the split ptlock for kernel
PMD page.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 include/asm-generic/pgalloc.h | 10 ++++++++--
 include/linux/mm.h            | 21 ++++++++++++++++-----
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
index 908bd9140ac2..57bd41adf760 100644
--- a/include/asm-generic/pgalloc.h
+++ b/include/asm-generic/pgalloc.h
@@ -139,7 +139,10 @@ static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr)
 	ptdesc = pagetable_alloc(gfp, 0);
 	if (!ptdesc)
 		return NULL;
-	if (!pagetable_pmd_ctor(ptdesc)) {
+
+	if (mm == &init_mm) {
+		__pagetable_pmd_ctor(ptdesc);
+	} else if (!pagetable_pmd_ctor(ptdesc)) {
 		pagetable_free(ptdesc);
 		return NULL;
 	}
@@ -153,7 +156,10 @@ static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
 	struct ptdesc *ptdesc = virt_to_ptdesc(pmd);
 
 	BUG_ON((unsigned long)pmd & (PAGE_SIZE-1));
-	pagetable_pmd_dtor(ptdesc);
+	if (mm == &init_mm)
+		__pagetable_pmd_dtor(ptdesc);
+	else
+		pagetable_pmd_dtor(ptdesc);
 	pagetable_free(ptdesc);
 }
 #endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e37db032764e..68ca71407177 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3048,26 +3048,37 @@ static inline spinlock_t *pmd_lock(struct mm_struct *mm, pmd_t *pmd)
 	return ptl;
 }
 
-static inline bool pagetable_pmd_ctor(struct ptdesc *ptdesc)
+static inline void __pagetable_pmd_ctor(struct ptdesc *ptdesc)
 {
 	struct folio *folio = ptdesc_folio(ptdesc);
 
-	if (!pmd_ptlock_init(ptdesc))
-		return false;
 	__folio_set_pgtable(folio);
 	lruvec_stat_add_folio(folio, NR_PAGETABLE);
+}
+
+static inline bool pagetable_pmd_ctor(struct ptdesc *ptdesc)
+{
+	if (!pmd_ptlock_init(ptdesc))
+		return false;
+
+	__pagetable_pmd_ctor(ptdesc);
 	return true;
 }
 
-static inline void pagetable_pmd_dtor(struct ptdesc *ptdesc)
+static inline void __pagetable_pmd_dtor(struct ptdesc *ptdesc)
 {
 	struct folio *folio = ptdesc_folio(ptdesc);
 
-	pmd_ptlock_free(ptdesc);
 	__folio_clear_pgtable(folio);
 	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
 }
 
+static inline void pagetable_pmd_dtor(struct ptdesc *ptdesc)
+{
+	pmd_ptlock_free(ptdesc);
+	__pagetable_pmd_dtor(ptdesc);
+}
+
 /*
  * No scalability reason to split PUD locks yet, but follow the same pattern
  * as the PMD locks to make it easier if we decide to.  The VM should not be
-- 
2.30.2