Merge tag 'libnvdimm-for-7.1' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm

Pull dax updates from Ira Weiny:
 "The series adds DAX support required for the upcoming fuse/famfs file
  system.[1] The support here is required because famfs is backed by
  devdax rather than pmem. This all lays the groundwork for using shared
  memory as a file system"

Link: https://lore.kernel.org/all/0100019d43e5f632-f5862a3e-361c-4b54-a9a6-96c242a8f17a-000000@email.amazonses.com/ [1]

* tag 'libnvdimm-for-7.1' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm:
  dax/fsdev: fix uninitialized kaddr in fsdev_dax_zero_page_range()
  dax: export dax_dev_get()
  dax: Add fs_dax_get() func to prepare dax for fs-dax usage
  dax: Add dax_set_ops() for setting dax_operations at bind time
  dax: Add dax_operations for use by fs-dax on fsdev dax
  dax: Save the kva from memremap
  dax: add fsdev.c driver for fs-dax on character dax
  dax: Factor out dax_folio_reset_order() helper
  dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

1 files changed, 56 insertions, 18 deletions

diff --git a/fs/dax.c b/fs/dax.c
index a5237169b467..6d175cd47a99 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -377,6 +377,59 @@ static void dax_folio_make_shared(struct folio *folio)
 	folio->share = 1;
 }
 
+/**
+ * dax_folio_reset_order - Reset a compound DAX folio to order-0 pages
+ * @folio: The folio to reset
+ *
+ * Splits a compound folio back into individual order-0 pages,
+ * clearing compound state and restoring pgmap pointers.
+ *
+ * Returns: the original folio order (0 if already order-0)
+ */
+int dax_folio_reset_order(struct folio *folio)
+{
+	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
+	int order = folio_order(folio);
+
+	/*
+	 * DAX maintains the invariant that folio->share != 0 only when
+	 * folio->mapping == NULL (enforced by dax_folio_make_shared()).
+	 * Equivalently: folio->mapping != NULL implies folio->share == 0.
+	 * Callers ensure share has been decremented to zero before
+	 * calling here, so unconditionally clearing both fields is
+	 * correct.
+	 */
+	folio->mapping = NULL;
+	folio->share = 0;
+
+	if (!order) {
+		/*
+		 * Restore pgmap explicitly even for order-0 folios. For the
+		 * dax_folio_put() caller this is a no-op (same value), but
+		 * fsdev_clear_folio_state() may call this on folios that
+		 * were previously compound and need pgmap re-established.
+		 */
+		folio->pgmap = pgmap;
+		return 0;
+	}
+
+	folio_reset_order(folio);
+
+	for (int i = 0; i < (1UL << order); i++) {
+		struct page *page = folio_page(folio, i);
+		struct folio *f = (struct folio *)page;
+
+		ClearPageHead(page);
+		clear_compound_head(page);
+		f->mapping = NULL;
+		f->share = 0;
+		f->pgmap = pgmap;
+	}
+
+	return order;
+}
+EXPORT_SYMBOL_GPL(dax_folio_reset_order);
+
 static inline unsigned long dax_folio_put(struct folio *folio)
 {
 	unsigned long ref;
@@ -390,28 +443,13 @@ static inline unsigned long dax_folio_put(struct folio *folio)
 	if (ref)
 		return ref;
 
-	folio->mapping = NULL;
-	order = folio_order(folio);
-	if (!order)
-		return 0;
-	folio_reset_order(folio);
+	order = dax_folio_reset_order(folio);
 
+	/* Debug check: verify refcounts are zero for all sub-folios */
 	for (i = 0; i < (1UL << order); i++) {
-		struct dev_pagemap *pgmap = page_pgmap(&folio->page);
 		struct page *page = folio_page(folio, i);
-		struct folio *new_folio = (struct folio *)page;
-
-		ClearPageHead(page);
-		clear_compound_head(page);
 
-		new_folio->mapping = NULL;
-		/*
-		 * Reset pgmap which was over-written by
-		 * prep_compound_page().
-		 */
-		new_folio->pgmap = pgmap;
-		new_folio->share = 0;
-		WARN_ON_ONCE(folio_ref_count(new_folio));
+		WARN_ON_ONCE(folio_ref_count((struct folio *)page));
 	}
 
 	return ref;