patches/old/mm-pgtable-remove-unnecessary-split-ptlock-for-kernel-pmd-page.patch

From: Qi Zheng <zhengqi.arch@bytedance.com>
Subject: mm: pgtable: remove unnecessary split ptlock for kernel PMD page
Date: Thu, 1 Feb 2024 16:05:41 +0800

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.

Link: https://lkml.kernel.org/r/63f0b3d2f9124ae5076963fb5505bd36daba0393.1706774109.git.zhengqi.arch@bytedance.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/asm-generic/pgalloc.h |   10 ++++++++--
 include/linux/mm.h            |   21 ++++++++++++++++-----
 2 files changed, 24 insertions(+), 7 deletions(-)

--- a/include/asm-generic/pgalloc.h~mm-pgtable-remove-unnecessary-split-ptlock-for-kernel-pmd-page
+++ a/include/asm-generic/pgalloc.h
@@ -139,7 +139,10 @@ static inline pmd_t *pmd_alloc_one(struc
 	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_st
 	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
--- a/include/linux/mm.h~mm-pgtable-remove-unnecessary-split-ptlock-for-kernel-pmd-page
+++ a/include/linux/mm.h
@@ -3048,26 +3048,37 @@ static inline spinlock_t *pmd_lock(struc
 	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
_