[PATCH v2.2 1/8] fsdax: introduce page->share for fsdax in reflink mode

[Shiyang Ruan]

fsdax page is used not only when CoW, but also mapread. To make the it
easily understood, use 'share' to indicate that the dax page is shared
by more than one extent.  And add helper functions to use it.

Also, the flag needs to be renamed to PAGE_MAPPING_DAX_SHARED.

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
---
 fs/dax.c                   | 38 ++++++++++++++++++++++----------------
 include/linux/mm_types.h   |  5 ++++-
 include/linux/page-flags.h |  2 +-
 3 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 1c6867810cbd..84fadea08705 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -334,35 +334,41 @@ static unsigned long dax_end_pfn(void *entry)
 	for (pfn = dax_to_pfn(entry); \
 			pfn < dax_end_pfn(entry); pfn++)
 
-static inline bool dax_mapping_is_cow(struct address_space *mapping)
+static inline bool dax_page_is_shared(struct page *page)
 {
-	return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
+	return page->mapping == PAGE_MAPPING_DAX_SHARED;
 }
 
 /*
- * Set the page->mapping with FS_DAX_MAPPING_COW flag, increase the refcount.
+ * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
+ * refcount.
  */
-static inline void dax_mapping_set_cow(struct page *page)
+static inline void dax_page_share_get(struct page *page)
 {
-	if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
+	if (page->mapping != PAGE_MAPPING_DAX_SHARED) {
 		/*
 		 * Reset the index if the page was already mapped
 		 * regularly before.
 		 */
 		if (page->mapping)
-			page->index = 1;
-		page->mapping = (void *)PAGE_MAPPING_DAX_COW;
+			page->share = 1;
+		page->mapping = PAGE_MAPPING_DAX_SHARED;
 	}
-	page->index++;
+	page->share++;
+}
+
+static inline unsigned long dax_page_share_put(struct page *page)
+{
+	return --page->share;
 }
 
 /*
- * When it is called in dax_insert_entry(), the cow flag will indicate that
+ * When it is called in dax_insert_entry(), the shared flag will indicate that
  * whether this entry is shared by multiple files.  If so, set the page->mapping
- * FS_DAX_MAPPING_COW, and use page->index as refcount.
+ * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
  */
 static void dax_associate_entry(void *entry, struct address_space *mapping,
-		struct vm_area_struct *vma, unsigned long address, bool cow)
+		struct vm_area_struct *vma, unsigned long address, bool shared)
 {
 	unsigned long size = dax_entry_size(entry), pfn, index;
 	int i = 0;
@@ -374,8 +380,8 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		if (cow) {
-			dax_mapping_set_cow(page);
+		if (shared) {
+			dax_page_share_get(page);
 		} else {
 			WARN_ON_ONCE(page->mapping);
 			page->mapping = mapping;
@@ -396,9 +402,9 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 		struct page *page = pfn_to_page(pfn);
 
 		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
-		if (dax_mapping_is_cow(page->mapping)) {
-			/* keep the CoW flag if this page is still shared */
-			if (page->index-- > 0)
+		if (dax_page_is_shared(page)) {
+			/* keep the shared flag if this page is still shared */
+			if (dax_page_share_put(page) > 0)
 				continue;
 		} else
 			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 500e536796ca..f46cac3657ad 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -103,7 +103,10 @@ struct page {
 			};
 			/* See page-flags.h for PAGE_MAPPING_FLAGS */
 			struct address_space *mapping;
-			pgoff_t index;		/* Our offset within mapping. */
+			union {
+				pgoff_t index;		/* Our offset within mapping. */
+				unsigned long share;	/* share count for fsdax */
+			};
 			/**
 			 * @private: Mapping-private opaque data.
 			 * Usually used for buffer_heads if PagePrivate.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 0b0ae5084e60..d8e94f2f704a 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -641,7 +641,7 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
  * Different with flags above, this flag is used only for fsdax mode.  It
  * indicates that this page->mapping is now under reflink case.
  */
-#define PAGE_MAPPING_DAX_COW	0x1
+#define PAGE_MAPPING_DAX_SHARED	((void *)0x1)
 
 static __always_inline bool folio_mapping_flags(struct folio *folio)
 {
-- 
2.38.1