[PATCH 8/9] mm/rmap: introduce folio_try_share_anon_rmap_ptes

[Dev Jain]

In the quest of enabling 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.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
 include/linux/page-flags.h | 11 +++++++++++
 include/linux/rmap.h       | 28 ++++++++++++++++++++++++++--
 mm/rmap.c                  |  2 +-
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 0e03d816e8b9d..1d74ed9a28c41 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1178,6 +1178,17 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
 	__clear_bit(PG_anon_exclusive, &PF_ANY(page, 1)->flags.f);
 }
 
+static __always_inline void ClearPagesAnonExclusive(struct page *page,
+		unsigned int nr)
+{
+	for (;;) {
+		ClearPageAnonExclusive(page);
+		if (--nr == 0)
+			break;
+		++page;
+	}
+}
+
 #ifdef CONFIG_MMU
 #define __PG_MLOCKED		(1UL << PG_mlocked)
 #else
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 1b7720c66ac87..7a67776dca3fe 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -712,9 +712,13 @@ static __always_inline int __folio_try_share_anon_rmap(struct 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);
+		ClearPagesAnonExclusive(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);
+	ClearPagesAnonExclusive(page, nr_pages);
 
 	/*
 	 * This is conceptually a smp_wmb() paired with the smp_rmb() in
@@ -804,6 +808,26 @@ static inline int folio_try_share_anon_rmap_pte(struct folio *folio,
 	return __folio_try_share_anon_rmap(folio, page, 1, PGTABLE_LEVEL_PTE);
 }
 
+/**
+ * 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 first mapped exclusive page of the batch
+ * @nr_pages:	The number of pages to share (batch size)
+ *
+ * See folio_try_share_anon_rmap_pte for full description.
+ *
+ * Context: The caller needs to hold the page table lock and has to have the
+ * page table entries cleared/invalidated. Those PTEs used to map consecutive
+ * pages of the folio passed here. The PTEs are all in the same PMD and VMA.
+ */
+static inline int folio_try_share_anon_rmap_ptes(struct folio *folio,
+		struct page *page, unsigned int nr)
+{
+	return __folio_try_share_anon_rmap(folio, page, nr, PGTABLE_LEVEL_PTE);
+}
+
 /**
  * folio_try_share_anon_rmap_pmd - try marking an exclusive anonymous page
  *				   range mapped by a PMD possibly shared to
diff --git a/mm/rmap.c b/mm/rmap.c
index 42f6b00cced01..bba5b571946d8 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2300,7 +2300,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 
 			/* See folio_try_share_anon_rmap(): clear PTE first. */
 			if (anon_exclusive &&
-			    folio_try_share_anon_rmap_pte(folio, subpage)) {
+			    folio_try_share_anon_rmap_ptes(folio, subpage, 1)) {
 				folio_put_swap(folio, subpage, 1);
 				set_pte_at(mm, address, pvmw.pte, pteval);
 				goto walk_abort;
-- 
2.34.1