[RFC v2 PATCH 17/17] mm: Batch-zap large anonymous folio PTE mappings

[Ryan Roberts <ryan.roberts@arm.com>]

This allows batching the rmap removal with folio_remove_rmap_range(),
which means we avoid spuriously adding a partially unmapped folio to the
deferrred split queue in the common case, which reduces split queue lock
contention.

Previously each page was removed from the rmap individually with
page_remove_rmap(). If the first page belonged to a large folio, this
would cause page_remove_rmap() to conclude that the folio was now
partially mapped and add the folio to the deferred split queue. But
subsequent calls would cause the folio to become fully unmapped, meaning
there is no value to adding it to the split queue.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 mm/memory.c | 139 ++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 119 insertions(+), 20 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index fac686e9f895..e1cb4bf6fd5d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1351,6 +1351,95 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
 	pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
 }

+static inline unsigned long page_addr(struct page *page,
+				struct page *anchor, unsigned long anchor_addr)
+{
+	unsigned long offset;
+	unsigned long addr;
+
+	offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
+	addr = anchor_addr + offset;
+
+	if (anchor > page) {
+		if (addr > anchor_addr)
+			return 0;
+	} else {
+		if (addr < anchor_addr)
+			return ULONG_MAX;
+	}
+
+	return addr;
+}
+
+static int calc_anon_folio_map_pgcount(struct folio *folio,
+				       struct page *page, pte_t *pte,
+				       unsigned long addr, unsigned long end)
+{
+	pte_t ptent;
+	int floops;
+	int i;
+	unsigned long pfn;
+
+	end = min(page_addr(&folio->page + folio_nr_pages(folio), page, addr),
+		  end);
+	floops = (end - addr) >> PAGE_SHIFT;
+	pfn = page_to_pfn(page);
+	pfn++;
+	pte++;
+
+	for (i = 1; i < floops; i++) {
+		ptent = *pte;
+
+		if (!pte_present(ptent) ||
+		    pte_pfn(ptent) != pfn) {
+			return i;
+		}
+
+		pfn++;
+		pte++;
+	}
+
+	return floops;
+}
+
+static unsigned long zap_anon_pte_range(struct mmu_gather *tlb,
+					struct vm_area_struct *vma,
+					struct page *page, pte_t *pte,
+					unsigned long addr, unsigned long end,
+					bool *full_out)
+{
+	struct folio *folio = page_folio(page);
+	struct mm_struct *mm = tlb->mm;
+	pte_t ptent;
+	int pgcount;
+	int i;
+	bool full;
+
+	pgcount = calc_anon_folio_map_pgcount(folio, page, pte, addr, end);
+
+	for (i = 0; i < pgcount;) {
+		ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
+		tlb_remove_tlb_entry(tlb, pte, addr);
+		full = __tlb_remove_page(tlb, page, 0);
+
+		if (unlikely(page_mapcount(page) < 1))
+			print_bad_pte(vma, addr, ptent, page);
+
+		i++;
+		page++;
+		pte++;
+		addr += PAGE_SIZE;
+
+		if (unlikely(full))
+			break;
+	}
+
+	folio_remove_rmap_range(folio, page - i, i, vma);
+
+	*full_out = full;
+	return i;
+}
+
 static unsigned long zap_pte_range(struct mmu_gather *tlb,
 				struct vm_area_struct *vma, pmd_t *pmd,
 				unsigned long addr, unsigned long end,
@@ -1387,6 +1476,36 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			page = vm_normal_page(vma, addr, ptent);
 			if (unlikely(!should_zap_page(details, page)))
 				continue;
+
+			/*
+			 * Batch zap large anonymous folio mappings. This allows
+			 * batching the rmap removal, which means we avoid
+			 * spuriously adding a partially unmapped folio to the
+			 * deferrred split queue in the common case, which
+			 * reduces split queue lock contention. Require the VMA
+			 * to be anonymous to ensure that none of the PTEs in
+			 * the range require zap_install_uffd_wp_if_needed().
+			 */
+			if (page && PageAnon(page) && vma_is_anonymous(vma)) {
+				bool full;
+				int pgcount;
+
+				pgcount = zap_anon_pte_range(tlb, vma,
+						page, pte, addr, end, &full);
+
+				rss[mm_counter(page)] -= pgcount;
+				pgcount--;
+				pte += pgcount;
+				addr += pgcount << PAGE_SHIFT;
+
+				if (unlikely(full)) {
+					force_flush = 1;
+					addr += PAGE_SIZE;
+					break;
+				}
+				continue;
+			}
+
 			ptent = ptep_get_and_clear_full(mm, addr, pte,
 							tlb->fullmm);
 			tlb_remove_tlb_entry(tlb, pte, addr);
@@ -3051,26 +3170,6 @@ struct anon_folio_range {
 	bool exclusive;
 };

-static inline unsigned long page_addr(struct page *page,
-				struct page *anchor, unsigned long anchor_addr)
-{
-	unsigned long offset;
-	unsigned long addr;
-
-	offset = (page_to_pfn(page) - page_to_pfn(anchor)) << PAGE_SHIFT;
-	addr = anchor_addr + offset;
-
-	if (anchor > page) {
-		if (addr > anchor_addr)
-			return 0;
-	} else {
-		if (addr < anchor_addr)
-			return ULONG_MAX;
-	}
-
-	return addr;
-}
-
 /*
  * Returns index of first pte that is not none, or nr if all are none.
  */
--
2.25.1