[PATCH V5 02/19] dax: Factor out dax_folio_reset_order() helper

[John Groves]

From: John Groves <j...@groves.net>

Both fs/dax.c:dax_folio_put() and drivers/dax/fsdev.c:
fsdev_clear_folio_state() (the latter coming in the next commit after this
one) contain nearly identical code to reset a compound DAX folio back to
order-0 pages. Factor this out into a shared helper function.

The new dax_folio_reset_order() function:
- Clears the folio's mapping and share count
- Resets compound folio state via folio_reset_order()
- Clears PageHead and compound_head for each sub-page
- Restores the pgmap pointer for each resulting order-0 folio
- Returns the original folio order (for callers that need to advance by
  that many pages)

This simplifies fsdev_clear_folio_state() from ~50 lines to ~15 lines while
maintaining the same functionality in both call sites.

Suggested-by: Jonathan Cameron <jonathan.came...@huawei.com>
Signed-off-by: John Groves <j...@groves.net>
---
 fs/dax.c | 60 +++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 42 insertions(+), 18 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 289e6254aa30..7d7bbfb32c41 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -378,6 +378,45 @@ 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);
+       int i;
+
+       folio->mapping = NULL;
+       folio->share = 0;
+
+       if (!order) {
+               folio->pgmap = pgmap;
+               return 0;
+       }
+
+       folio_reset_order(folio);
+
+       for (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;
+}
+
 static inline unsigned long dax_folio_put(struct folio *folio)
 {
        unsigned long ref;
@@ -391,28 +430,13 @@ 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;
-- 
2.52.0