Re: [PATCH V7 02/19] dax: Factor out dax_folio_reset_order() helper

On 26/02/23 07:00PM, Ackerley Tng wrote:

John Groves [off-list ref] writes:

quoted

From: John Groves <John@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.cameron@huawei.com>
Signed-off-by: John Groves <john@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;
+}
+

I'm implementing something similar for guest_memfd and was going to
reuse __split_folio_to_order(). Would you consider using the
__split_folio_to_order() function?

I see that dax_folio_reset_order() needs to set f->share to 0 though,
which is a union with index, and __split_folio_to_order() sets non-0
indices.

Also, __split_folio_to_order() doesn't handle f->pgmap (or f->lru).

Could these two steps be added to a separate loop after
__split_folio_to_order()?

Does dax_folio_reset_order() need to handle any of the folio flags that
__split_folio_to_order() handles?

Sorry to reply slowly; this took some thought.

I'm nervous about sharing folio initialization code between the page cache
and dax. Might this be something we could unify after the fact - if it
passes muster? 

Unifying paths like this could be regression-prone (page cache changes
breaking dax or vice versa) unless it's really well conceived...

quoted

 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().
-		 */

Actually, where's the call to prep_compound_page()? Was that in
dax_folio_init()? Is this comment still valid and does pgmap have to be
reset?

Yep, in dax_folio_init()...


Thanks,
John

[snip]