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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>