[PATCH RFC 1/9] mm/huge_memory: read the huge PMD entry once when splitting it

[Yin Tirui]

__split_huge_pmd_locked() re-reads *pmd several times to classify the same
entry. The PMD page table lock is held throughout, so read it once into
old_pmd and classify from that.

No functional change intended.

Signed-off-by: Yin Tirui <yintirui@xxxxxxxxx>
---
 mm/huge_memory.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afbb5974bd22..8feabdcf6307 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3143,10 +3143,11 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long haddr, bool freeze)
 {
 	struct mm_struct *mm = vma->vm_mm;
+	pmd_t old_pmd = *pmd;
 	struct folio *folio;
 	struct page *page;
 	pgtable_t pgtable;
-	pmd_t old_pmd, _pmd;
+	pmd_t _pmd;
 	bool soft_dirty, uffd_wp = false, young = false, write = false;
 	bool anon_exclusive = false, dirty = false;
 	unsigned long addr;
@@ -3157,7 +3158,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
 	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
 
-	VM_WARN_ON_ONCE(!pmd_is_valid_softleaf(*pmd) && !pmd_trans_huge(*pmd));
+	VM_WARN_ON_ONCE(!pmd_is_valid_softleaf(old_pmd) &&
+			!pmd_trans_huge(old_pmd));
 
 	count_vm_event(THP_SPLIT_PMD);
 
@@ -3193,7 +3195,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		return;
 	}
 
-	if (is_huge_zero_pmd(*pmd)) {
+	if (is_huge_zero_pmd(old_pmd)) {
 		/*
 		 * FIXME: Do we want to invalidate secondary mmu by calling
 		 * mmu_notifier_arch_invalidate_secondary_tlbs() see comments below
@@ -3206,10 +3208,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		return __split_huge_zero_page_pmd(vma, haddr, pmd);
 	}
 
-	if (pmd_is_migration_entry(*pmd)) {
+	if (pmd_is_migration_entry(old_pmd)) {
 		softleaf_t entry;
 
-		old_pmd = *pmd;
 		entry = softleaf_from_pmd(old_pmd);
 		page = softleaf_to_page(entry);
 		folio = page_folio(page);
@@ -3222,10 +3223,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			anon_exclusive = softleaf_is_migration_read_exclusive(entry);
 		young = softleaf_is_migration_young(entry);
 		dirty = softleaf_is_migration_dirty(entry);
-	} else if (pmd_is_device_private_entry(*pmd)) {
+	} else if (pmd_is_device_private_entry(old_pmd)) {
 		softleaf_t entry;
 
-		old_pmd = *pmd;
 		entry = softleaf_from_pmd(old_pmd);
 		page = softleaf_to_page(entry);
 		folio = page_folio(page);
@@ -3417,7 +3417,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	}
 	pte_unmap(pte);
 
-	if (!pmd_is_migration_entry(*pmd))
+	if (!pmd_is_migration_entry(old_pmd))
 		folio_remove_rmap_pmd(folio, page, vma);
 	if (freeze)
 		put_page(page);
-- 
2.34.1