[PATCH v5 08/25] fs/dax: Remove PAGE_MAPPING_DAX_SHARED mapping flag

[Alistair Popple <apopple@nvidia.com>]

PAGE_MAPPING_DAX_SHARED is the same as PAGE_MAPPING_ANON. This isn't
currently a problem because FS DAX pages are treated
specially. However a future change will make FS DAX pages more like
normal pages, so folio_test_anon() must not return true for a FS DAX
page.

We could explicitly test for a FS DAX page in folio_test_anon(),
etc. however the PAGE_MAPPING_DAX_SHARED flag isn't actually
needed. Instead we can use the page->mapping field to implicitly track
the first mapping of a page. If page->mapping is non-NULL it implies
the page is associated with a single mapping at page->index. If the
page is associated with a second mapping clear page->mapping and set
page->share to 1.

This is possible because a shared mapping implies the file-system
implements dax_holder_operations which makes the ->mapping and
->index, which is a union with ->share, unused.

The page is considered shared when page->mapping == NULL and
page->share > 0 or page->mapping != NULL, implying it is present in at
least one address space. This also makes it easier for a future change
to detect when a page is first mapped into an address space which
requires special handling.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
 fs/dax.c                   | 45 +++++++++++++++++++++++++--------------
 include/linux/page-flags.h |  6 +-----
 2 files changed, 29 insertions(+), 22 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 4e49cc4..3346658 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -351,38 +351,41 @@ static unsigned long dax_end_pfn(void *entry)
 	for (pfn = dax_to_pfn(entry); \
 			pfn < dax_end_pfn(entry); pfn++)
 
+/*
+ * A DAX page is considered shared if it has no mapping set and ->share (which
+ * shares the ->index field) is non-zero. Note this may return false even if the
+ * page if shared between multiple files but has not yet actually been mapped
+ * into multiple address spaces.
+ */
 static inline bool dax_page_is_shared(struct page *page)
 {
-	return page->mapping == PAGE_MAPPING_DAX_SHARED;
+	return !page->mapping && page->share;
 }
 
 /*
- * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
- * refcount.
+ * Increase the page share refcount, warning if the page is not marked as shared.
  */
 static inline void dax_page_share_get(struct page *page)
 {
-	if (page->mapping != PAGE_MAPPING_DAX_SHARED) {
-		/*
-		 * Reset the index if the page was already mapped
-		 * regularly before.
-		 */
-		if (page->mapping)
-			page->share = 1;
-		page->mapping = PAGE_MAPPING_DAX_SHARED;
-	}
+	WARN_ON_ONCE(!page->share);
+	WARN_ON_ONCE(page->mapping);
 	page->share++;
 }
 
 static inline unsigned long dax_page_share_put(struct page *page)
 {
+	WARN_ON_ONCE(!page->share);
 	return --page->share;
 }
 
 /*
- * 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
- * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
+ * When it is called in dax_insert_entry(), the shared flag will indicate
+ * whether this entry is shared by multiple files. If the page has not
+ * previously been associated with any mappings the ->mapping and ->index
+ * fields will be set. If it has already been associated with a mapping
+ * the mapping will be cleared and the share count set. It's then up the
+ * file-system to track which mappings contain which pages, ie. by implementing
+ * dax_holder_operations.
  */
 static void dax_associate_entry(void *entry, struct address_space *mapping,
 		struct vm_area_struct *vma, unsigned long address, bool shared)
@@ -397,7 +400,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		if (shared) {
+		if (shared && page->mapping && page->share) {
+			if (page->mapping) {
+				page->mapping = NULL;
+
+				/*
+				 * Page has already been mapped into one address
+				 * space so set the share count.
+				 */
+				page->share = 1;
+			}
+
 			dax_page_share_get(page);
 		} else {
 			WARN_ON_ONCE(page->mapping);
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 691506b..598334e 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -668,12 +668,6 @@ 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_SHARED	((void *)0x1)
-
 static __always_inline bool folio_mapping_flags(const struct folio *folio)
 {
 	return ((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) != 0;
-- 
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