[PATCH RFC 2/9] mm/huge_memory: add and use huge_zero_pmd_can_split()

[Yin Tirui <yintirui@gmail.com>]

Only a huge zero PMD in an anonymous VMA is split, into a page table of
shared zero page mappings. Any other huge zero PMD is simply unmapped.

vm_normal_folio_pmd() returns NULL for a huge zero PMD, and the split path
unmaps any entry which has no folio. So make the decision before the folio
is looked up, in huge_zero_pmd_can_split().

No functional change intended.

Signed-off-by: Yin Tirui <yintirui@gmail.com>
---
 mm/huge_memory.c | 36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8feabdcf6307..90d84f761619 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3139,6 +3139,15 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
 	pmd_populate(mm, pmd, pgtable);
 }
 
+/*
+ * Only a huge zero PMD in an anonymous VMA is split, into a page table of
+ * shared zero page mappings. Any other huge zero PMD is simply unmapped.
+ */
+static bool huge_zero_pmd_can_split(struct vm_area_struct *vma, pmd_t pmdval)
+{
+	return is_huge_zero_pmd(pmdval) && vma_is_anonymous(vma);
+}
+
 static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long haddr, bool freeze)
 {
@@ -3163,6 +3172,20 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 
 	count_vm_event(THP_SPLIT_PMD);
 
+	/*
+	 * FIXME: Do we want to invalidate secondary mmu by calling
+	 * mmu_notifier_arch_invalidate_secondary_tlbs() see comments below
+	 * inside __split_huge_pmd() ?
+	 *
+	 * We are going from a zero huge page write protected to zero small
+	 * page also write protected so it does not seems useful to invalidate
+	 * secondary mmu at this time.
+	 */
+	if (huge_zero_pmd_can_split(vma, old_pmd)) {
+		__split_huge_zero_page_pmd(vma, haddr, pmd);
+		return;
+	}
+
 	if (!vma_is_anonymous(vma)) {
 		old_pmd = pmdp_huge_clear_flush(vma, haddr, pmd);
 		/*
@@ -3195,19 +3218,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		return;
 	}
 
-	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
-		 * inside __split_huge_pmd() ?
-		 *
-		 * We are going from a zero huge page write protected to zero
-		 * small page also write protected so it does not seems useful
-		 * to invalidate secondary mmu at this time.
-		 */
-		return __split_huge_zero_page_pmd(vma, haddr, pmd);
-	}
-
 	if (pmd_is_migration_entry(old_pmd)) {
 		softleaf_t entry;
 
-- 
2.34.1