[PATCH v8 9/9] fsdax: set a CoW flag when associate reflink mappings

[Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>]

Introduce a FS_DAX_MAPPING_COW flag to support association with CoW file
mappings.  In this case, the dax-RMAP already takes the responsibility
to look up for shared files by given dax page.  The page->mapping is no
longer to used for rmap but for marking that this dax page is shared.
And to make sure disassociation works fine, we use page->index as
refcount, and clear page->mapping to the initial state when page->index
is decreased to 0.

With the help of this new flag, it is able to distinguish normal case
and CoW case, and keep the warning in normal case.

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
---
 fs/dax.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 57 insertions(+), 9 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 66366ba83ffc..18823f2c2385 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -335,12 +335,46 @@ static unsigned long dax_end_pfn(void *entry)
 			pfn < dax_end_pfn(entry); pfn++)
 
 /*
- * TODO: for reflink+dax we need a way to associate a single page with
- * multiple address_space instances at different linear_page_index()
- * offsets.
+ * Set FS_DAX_MAPPING_COW flag on the last bit of page->mapping to indicate that
+ * this is a reflink case.  In this case, we associate this page->mapping with
+ * file mapping at the first time and only once.
+ */
+#define FS_DAX_MAPPING_COW	1UL
+
+#define MAPPING_SET_COW(m)	(m = (struct address_space *)FS_DAX_MAPPING_COW)
+#define MAPPING_TEST_COW(m)	(((unsigned long)m & FS_DAX_MAPPING_COW) == \
+					FS_DAX_MAPPING_COW)
+
+/*
+ * Set or Update the page->mapping with FS_DAX_MAPPING_COW flag.
+ * Return true if it is an Update.
+ */
+static inline bool dax_mapping_set_cow(struct page *page)
+{
+	if (page->mapping) {
+		/* flag already set  */
+		if (MAPPING_TEST_COW(page->mapping))
+			return false;
+
+		/*
+		 * This page has been mapped even before it is shared, just
+		 * need to set this FS_DAX_MAPPING_COW flag.
+		 */
+		MAPPING_SET_COW(page->mapping);
+		return true;
+	}
+	/* Newly associate CoW mapping */
+	MAPPING_SET_COW(page->mapping);
+	return false;
+}
+
+/*
+ * When it is called in dax_insert_entry(), the cow flag will indicate that
+ * whether this entry is shared by multiple files.  If so, set the page->mapping
+ * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
  */
 static void dax_associate_entry(void *entry, struct address_space *mapping,
-		struct vm_area_struct *vma, unsigned long address)
+		struct vm_area_struct *vma, unsigned long address, bool cow)
 {
 	unsigned long size = dax_entry_size(entry), pfn, index;
 	int i = 0;
@@ -352,9 +386,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		WARN_ON_ONCE(page->mapping);
-		page->mapping = mapping;
-		page->index = index + i++;
+		if (cow) {
+			if (dax_mapping_set_cow(page)) {
+				/* Was normal, now updated to CoW */
+				page->index = 2;
+			} else
+				page->index++;
+		} else {
+			WARN_ON_ONCE(page->mapping);
+			page->mapping = mapping;
+			page->index = index + i++;
+		}
 	}
 }
 
@@ -370,7 +412,12 @@ 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);
-		WARN_ON_ONCE(page->mapping && page->mapping != mapping);
+		if (!MAPPING_TEST_COW(page->mapping)) {
+			/* keep the CoW flag if this page is still shared */
+			if (page->index-- > 0)
+				continue;
+		} else
+			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
 		page->mapping = NULL;
 		page->index = 0;
 	}
@@ -829,7 +876,8 @@ static void *dax_insert_entry(struct xa_state *xas,
 		void *old;
 
 		dax_disassociate_entry(entry, mapping, false);
-		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
+		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
+				false);
 		/*
 		 * Only swap our new entry into the page cache if the current
 		 * entry is a zero page or an empty entry.  If a normal PTE or
-- 
2.34.0