[PATCH v1 10/11] mm: thp: introduce and use page_trans_huge_anon_shared()

[David Hildenbrand]

Let's add an optimized way to check "page_trans_huge_mapcount() > 1"
that is allowed to break the loop early.

This commit is based on a prototype patch by Andrea.

Co-developed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/huge_mm.h |  7 +++++++
 mm/gup.c                |  2 +-
 mm/huge_memory.c        | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 44e02d47c65a..3a9d8cf64219 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -361,6 +361,8 @@ static inline void thp_mapcount_unlock(struct page *page,
 	local_irq_restore(irq_flags);
 }
 
+extern bool page_trans_huge_anon_shared(struct page *page);
+
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
 #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
@@ -532,6 +534,11 @@ static inline void thp_mapcount_unlock(struct page *page,
 {
 }
 
+static inline bool page_trans_huge_anon_shared(struct page *page)
+{
+	return false;
+}
+
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 /**
diff --git a/mm/gup.c b/mm/gup.c
index 35d1b28e3829..496575ff9ac8 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -70,7 +70,7 @@ bool gup_must_unshare(unsigned int flags, struct page *page, bool is_head)
 		return __page_mapcount(page) > 1;
 	if (is_head) {
 		VM_BUG_ON(!PageTransHuge(page));
-		return page_trans_huge_mapcount(page, NULL) > 1;
+		return page_trans_huge_anon_shared(page);
 	}
 	return page_mapcount(page) > 1;
 }
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 57842e8b13d4..dced82274f1d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1281,6 +1281,40 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
 	spin_unlock(vmf->ptl);
 }
 
+
+static bool __page_trans_huge_anon_shared(struct page *page)
+{
+	int i, mapcount;
+
+	mapcount = head_compound_mapcount(page);
+	if (mapcount > 1)
+		return true;
+	if (PageDoubleMap(page))
+		mapcount -= 1;
+	for (i = 0; i < thp_nr_pages(page); i++) {
+		if (atomic_read(&page[i]._mapcount) + mapcount + 1 > 1)
+			return true;
+	}
+	return false;
+}
+
+/* A lightweight check corresponding to "page_trans_huge_mapcount() > 1". */
+bool page_trans_huge_anon_shared(struct page *page)
+{
+	unsigned int seqcount;
+	bool shared;
+
+	VM_BUG_ON_PAGE(PageHuge(page) || PageTail(page), page);
+	VM_BUG_ON_PAGE(!PageAnon(page) || !PageTransHuge(page), page);
+
+	do {
+		seqcount = thp_mapcount_read_begin(page);
+		shared = __page_trans_huge_anon_shared(page);
+	} while (thp_mapcount_read_retry(page, seqcount));
+
+	return shared;
+}
+
 vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
-- 
2.31.1