fsdax: set a CoW flag when associate reflink mappings

Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
mappings.  In this case, since the dax-rmap has already took 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.

Link: https://lkml.kernel.org/r/20220603053738.1218681-8-ruansy.fnst@fujitsu.comSigned-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dan Williams <dan.j.wiliams@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.de>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/dax.c

@@ -334,13 +334,35 @@ 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)
+{
+	return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
+}
+
+/*
+ * Set the page->mapping with FS_DAX_MAPPING_COW flag, increase the refcount.
+ */
+static inline void dax_mapping_set_cow(struct page *page)
+{
+	if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
+		/*
+		 * 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->index++;
+}
+
 /*
- * TODO: for reflink+dax we need a way to associate a single page with
- * multiple address_space instances at different linear_page_index()
- * offsets.
+ * 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
+ * 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;
@@ -352,10 +374,14 @@ 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);
+		} else {
 			WARN_ON_ONCE(page->mapping);
 			page->mapping = mapping;
 			page->index = index + i++;
 		}
+	}
 }
 
 static void dax_disassociate_entry(void *entry, struct address_space *mapping,
@@ -370,6 +396,11 @@ 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)
+				continue;
+		} else
 			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
 		page->mapping = NULL;
 		page->index = 0;
@@ -830,7 +861,8 @@ 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

include/linux/page-flags.h

@@ -661,6 +661,12 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
 #define PAGE_MAPPING_KSM	(PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
 #define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
 
+/*
+ * 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
+
 static __always_inline bool folio_mapping_flags(struct folio *folio)
 {
 	return ((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) != 0;