[PATCH v2 8/9] mm/rmap: Add batched version of folio_try_share_anon_rmap_pte

[Dev Jain]

To enabe batched unmapping of anonymous folios, we need to handle the
sharing of exclusive pages. Hence, a batched version of
folio_try_share_anon_rmap_pte is required.

Currently, the sole purpose of nr_pages in __folio_try_share_anon_rmap is
to do some rmap sanity checks. Add helpers to set and clear the
PageAnonExclusive bit on a batch of nr_pages. Note that
__folio_try_share_anon_rmap can receive nr_pages == HPAGE_PMD_NR from the
PMD path, but currently we only clear the bit on the head page. Retain
this behaviour by setting nr_pages = 1 in case the caller is
folio_try_share_anon_rmap_pmd.

While at it, convert nr_pages to unsigned long to future-proof from
overflow in case P4D-huge mappings etc get supported down the road.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
 include/linux/mm.h   | 11 +++++++++++
 include/linux/rmap.h | 27 ++++++++++++++++++++-------
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 633bbf9a184a6..2d20954da652a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -243,6 +243,17 @@ static inline unsigned long folio_page_idx(const struct folio *folio,
 	return page - &folio->page;
 }
 
+static __always_inline void folio_clear_pages_anon_exclusive(struct page *page,
+		unsigned long nr_pages)
+{
+	for (;;) {
+		ClearPageAnonExclusive(page);
+		if (--nr_pages == 0)
+			break;
+		++page;
+	}
+}
+
 static inline struct folio *lru_to_folio(struct list_head *head)
 {
 	return list_entry((head)->prev, struct folio, lru);
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 8dc0871e5f001..f3b3ee3955afc 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -706,15 +706,19 @@ static inline int folio_try_dup_anon_rmap_pmd(struct folio *folio,
 }
 
 static __always_inline int __folio_try_share_anon_rmap(struct folio *folio,
-		struct page *page, int nr_pages, enum pgtable_level level)
+		struct page *page, unsigned long nr_pages, enum pgtable_level level)
 {
 	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
 	VM_WARN_ON_FOLIO(!PageAnonExclusive(page), folio);
 	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
 
+	/* We only clear anon-exclusive from head page of PMD folio */
+	if (level == PGTABLE_LEVEL_PMD)
+		nr_pages = 1;
+
 	/* device private folios cannot get pinned via GUP. */
 	if (unlikely(folio_is_device_private(folio))) {
-		ClearPageAnonExclusive(page);
+		folio_clear_pages_anon_exclusive(page, nr_pages);
 		return 0;
 	}
 
@@ -766,7 +770,7 @@ static __always_inline int __folio_try_share_anon_rmap(struct folio *folio,
 
 	if (unlikely(folio_maybe_dma_pinned(folio)))
 		return -EBUSY;
-	ClearPageAnonExclusive(page);
+	folio_clear_pages_anon_exclusive(page, nr_pages);
 
 	/*
 	 * This is conceptually a smp_wmb() paired with the smp_rmb() in
@@ -778,11 +782,12 @@ static __always_inline int __folio_try_share_anon_rmap(struct folio *folio,
 }
 
 /**
- * folio_try_share_anon_rmap_pte - try marking an exclusive anonymous page
- *				   mapped by a PTE possibly shared to prepare
+ * folio_try_share_anon_rmap_ptes - try marking exclusive anonymous pages
+ *				   mapped by PTEs possibly shared to prepare
  *				   for KSM or temporary unmapping
  * @folio:	The folio to share a mapping of
- * @page:	The mapped exclusive page
+ * @page:	The first mapped exclusive page of the batch in the folio
+ * @nr_pages:	The number of pages to share in the folio (batch size)
  *
  * The caller needs to hold the page table lock and has to have the page table
  * entries cleared/invalidated.
@@ -797,11 +802,19 @@ static __always_inline int __folio_try_share_anon_rmap(struct folio *folio,
  *
  * Returns 0 if marking the mapped page possibly shared succeeded. Returns
  * -EBUSY otherwise.
+ *
+ * The caller needs to hold the page table lock.
  */
+static inline int folio_try_share_anon_rmap_ptes(struct folio *folio,
+		struct page *page, unsigned long nr_pages)
+{
+	return __folio_try_share_anon_rmap(folio, page, nr_pages, PGTABLE_LEVEL_PTE);
+}
+
 static inline int folio_try_share_anon_rmap_pte(struct folio *folio,
 		struct page *page)
 {
-	return __folio_try_share_anon_rmap(folio, page, 1, PGTABLE_LEVEL_PTE);
+	return folio_try_share_anon_rmap_ptes(folio, page, 1);
 }
 
 /**
-- 
2.34.1