[PATCH v8 08/20] fs/dax: Remove PAGE_MAPPING_DAX_SHARED mapping flag

[Alistair Popple <apopple@nvidia.com>]

The page ->mapping pointer can have magic values like
PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON for page owner specific
usage. Currently PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON alias to the
same value. This isn't a problem because FS DAX pages are never seen by the
anonymous mapping code and vice versa.

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>

---

Changes for v8:

 - Rebased on mm-unstable which includes Matthew Wilcox's "dax: use
   folios more widely within DAX"

Changes for v7:

 - Fix for checking when creating a shared mapping in dax_associate_entry.
 - Remove dax_page_share_get().
 - Add dax_page_make_shared().
---
 fs/dax.c                   | 55 +++++++++++++++++++++++----------------
 include/linux/page-flags.h |  6 +----
 2 files changed, 33 insertions(+), 28 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index bc538ba..6674540 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -351,27 +351,40 @@ static unsigned long dax_end_pfn(void *entry)
 	for (pfn = dax_to_pfn(entry); \
 			pfn < dax_end_pfn(entry); pfn++)
 
+/*
+ * A DAX folio 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 is shared between multiple files but has not yet actually been mapped
+ * into multiple address spaces.
+ */
 static inline bool dax_folio_is_shared(struct folio *folio)
 {
-	return folio->mapping == PAGE_MAPPING_DAX_SHARED;
+	return !folio->mapping && folio->page.share;
 }
 
 /*
- * Set the folio->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
- * refcount.
+ * When it is called by 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 to
+ * reverse map users like memory_failure() to call back into the filesystem to
+ * recover ->mapping and ->index information. For example by implementing
+ * dax_holder_operations.
  */
-static inline void dax_folio_share_get(struct folio *folio)
+static void dax_folio_make_shared(struct folio *folio)
 {
-	if (folio->mapping != PAGE_MAPPING_DAX_SHARED) {
-		/*
-		 * Reset the index if the page was already mapped
-		 * regularly before.
-		 */
-		if (folio->mapping)
-			folio->page.share = 1;
-		folio->mapping = PAGE_MAPPING_DAX_SHARED;
-	}
-	folio->page.share++;
+	/*
+	 * folio is not currently shared so mark it as shared by clearing
+	 * folio->mapping.
+	 */
+	folio->mapping = NULL;
+
+	/*
+	 * folio has previously been mapped into one address space so set the
+	 * share count.
+	 */
+	folio->page.share = 1;
 }
 
 static inline unsigned long dax_folio_share_put(struct folio *folio)
@@ -379,12 +392,6 @@ static inline unsigned long dax_folio_share_put(struct folio *folio)
 	return --folio->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 folio->mapping 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 shared)
 {
@@ -398,8 +405,12 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct folio *folio = pfn_folio(pfn);
 
-		if (shared) {
-			dax_folio_share_get(folio);
+		if (shared && (folio->mapping || folio->page.share)) {
+			if (folio->mapping)
+				dax_folio_make_shared(folio);
+
+			WARN_ON_ONCE(!folio->page.share);
+			folio->page.share++;
 		} else {
 			WARN_ON_ONCE(folio->mapping);
 			folio->mapping = mapping;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 3f6a64f..30fe3eb 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -710,12 +710,6 @@ 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;
-- 
git-series 0.9.1