Commit 59eb73b9 authored by John Groves's avatar John Groves Committed by Ira Weiny
Browse files

dax: Factor out dax_folio_reset_order() helper 



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)

Two intentional differences from the original dax_folio_put() logic:

1. folio->share is cleared unconditionally. This is correct because the DAX
   subsystem maintains the invariant that share != 0 only when
   mapping == NULL (enforced by dax_folio_make_shared()). dax_folio_put()
   ensures share has reached zero before calling this helper, so the
   unconditional clear is safe.

2. folio->pgmap is now explicitly restored for order-0 folios. For the
   dax_folio_put() caller this is a no-op (reads and writes back the same
   field). It is intentional for the upcoming fsdev_clear_folio_state()
   caller, which converts previously-compound folios and needs pgmap
   re-established for all pages regardless of order.

This simplifies fsdev_clear_folio_state() from ~50 lines to ~15 lines.

Suggested-by: default avatarJonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: default avatarIra Weiny <ira.weiny@intel.com>
Reviewed-by: default avatarDave Jiang <dave.jiang@intel.com>
Reviewed-by: default avatarJonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: default avatarJohn Groves <john@groves.net>
Link: https://patch.msgid.link/0100019d311cc6b9-5be7428a-7f16-4774-8f90-a44b88ac5660-000000@email.amazonses.com


Signed-off-by: default avatarIra Weiny <ira.weiny@intel.com>
parent a73cc506

fs/dax.c

+55 −18
Original line number Diff line number Diff line
@@ -378,6 +378,58 @@ 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;

}



static inline unsigned long dax_folio_put(struct folio *folio)

{

	unsigned long ref;
@@ -391,28 +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;

include/linux/dax.h

+1 −0
Original line number Diff line number Diff line
@@ -153,6 +153,7 @@ static inline void fs_put_dax(struct dax_device *dax_dev, void *holder) 
#if IS_ENABLED(CONFIG_FS_DAX)

int dax_writeback_mapping_range(struct address_space *mapping,

		struct dax_device *dax_dev, struct writeback_control *wbc);

int dax_folio_reset_order(struct folio *folio);



struct page *dax_layout_busy_page(struct address_space *mapping);

struct page *dax_layout_busy_page_range(struct address_space *mapping, loff_t start, loff_t end);