[PATCH RFC 3/9] mm/huge_memory: add and use unmap_huge_pmd_entry()

[Yin Tirui <yintirui@gmail.com>]

Only a huge PMD entry mapping an anonymous folio is split. Everything else
is unmapped and comes back on the next fault. Put that in
unmap_huge_pmd_entry().

Old and new differ only for a device private entry in a non-anonymous VMA,
which nothing in the tree can produce.

Signed-off-by: Yin Tirui <yintirui@gmail.com>
---
 mm/huge_memory.c | 84 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 56 insertions(+), 28 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 90d84f761619..aefd62827139 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3148,6 +3148,52 @@ 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);
 }
 
+/**
+ * unmap_huge_pmd_entry() - Unmap a huge PMD entry rather than splitting it.
+ * @vma: The VMA @pmd belongs to.
+ * @haddr: The PMD-aligned address @pmd maps.
+ * @pmd: Pointer to the huge PMD entry.
+ * @folio: The folio @pmd describes, or NULL if it describes none.
+ * @is_present: Is @pmd a present entry rather than a softleaf entry?
+ *
+ * Only anonymous folios are rebuilt at PTE level when a huge PMD entry is
+ * split. Everything else is unmapped here and faulted back in on the next
+ * access.
+ */
+static void unmap_huge_pmd_entry(struct vm_area_struct *vma,
+		unsigned long haddr, pmd_t *pmd, struct folio *folio,
+		bool is_present)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	pmd_t old_pmd;
+
+	old_pmd = pmdp_huge_clear_flush(vma, haddr, pmd);
+	/*
+	 * We are going to unmap this huge page. So
+	 * just go ahead and zap it
+	 */
+	if (arch_needs_pgtable_deposit())
+		zap_deposited_table(mm, pmd);
+
+	if (!folio)
+		return;
+
+	if (is_present) {
+		struct page *page = pmd_page(old_pmd);
+
+		if (!folio_test_dirty(folio) && pmd_dirty(old_pmd))
+			folio_mark_dirty(folio);
+		if (!folio_test_referenced(folio) && pmd_young(old_pmd))
+			folio_set_referenced(folio);
+		folio_remove_rmap_pmd(folio, page, vma);
+	}
+
+	add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
+
+	if (is_present)
+		folio_put(folio);
+}
+
 static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long haddr, bool freeze)
 {
@@ -3187,34 +3233,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	}
 
 	if (!vma_is_anonymous(vma)) {
-		old_pmd = pmdp_huge_clear_flush(vma, haddr, pmd);
-		/*
-		 * We are going to unmap this huge page. So
-		 * just go ahead and zap it
-		 */
-		if (arch_needs_pgtable_deposit())
-			zap_deposited_table(mm, pmd);
-		if (vma_is_special_huge(vma))
-			return;
-		if (unlikely(pmd_is_migration_entry(old_pmd))) {
-			const softleaf_t old_entry = softleaf_from_pmd(old_pmd);
-
-			folio = softleaf_to_folio(old_entry);
-		} else if (is_huge_zero_pmd(old_pmd)) {
-			return;
-		} else {
-			page = pmd_page(old_pmd);
-			folio = page_folio(page);
-			if (!folio_test_dirty(folio) && pmd_dirty(old_pmd))
-				folio_mark_dirty(folio);
-			if (!folio_test_referenced(folio) && pmd_young(old_pmd))
-				folio_set_referenced(folio);
-			folio_remove_rmap_pmd(folio, page, vma);
-			add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
-			folio_put(folio);
-			return;
-		}
-		add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR);
+		const bool is_present = pmd_present(old_pmd);
+
+		if (vma_is_special_huge(vma) || is_huge_zero_pmd(old_pmd))
+			folio = NULL;
+		else if (is_present)
+			folio = page_folio(pmd_page(old_pmd));
+		else
+			folio = softleaf_to_folio(softleaf_from_pmd(old_pmd));
+
+		unmap_huge_pmd_entry(vma, haddr, pmd, folio, is_present);
 		return;
 	}
 
-- 
2.34.1