dax: Factor out dax_folio_reset_order() helper

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

...

This commit is contained in:

John Groves 2026-03-27 21:04:22 +00:00 • committed by Ira Weiny

parent a73cc506ad

commit 59eb73b98a

2 changed files with 56 additions and 18 deletions

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73

fs/dax.c

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1

include/linux/dax.h

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