On Wed, 18 Mar 2026 20:28:20 -0500 John Groves <[email protected]> wrote:
> From: John Groves <[email protected]> > > 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 <[email protected]> > Reviewed-by: Ira Weiny <[email protected]> > Reviewed-by: Dave Jiang <[email protected]> > Signed-off-by: John Groves <[email protected]> Comment below. I may well be needing more coffee, or failing wrt to background knowledge as I only occasionally dip into dax. > --- > 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; This is different from the code you are replacing.. Just above the call to this in dax_folio_put() if (!dax_folio_is_shared(folio)) // in here is the interesting bit... ref = 0; else //this is fine because either it's still > 0 and we return //or it is zero and you are writing that again. ref = --folio->share; if (ref) return ref; So the path that bothers me is if !dax_folio_is_shared() can return false with shared != 0 /* * A DAX folio is considered shared if it has no mapping set and ->share (which * shares the ->index field) is non-zero. Note this may return false even if the * page is shared between multiple files but has not yet actually been mapped * into multiple address spaces. */ static inline bool dax_folio_is_shared(struct folio *folio) { return !folio->mapping && folio->share; } So it can if !folio->mapping is false (i.e. folio->mapping is set) Now I have zero idea of whether this is a real path and have a long review queue so not looking into it for now. However if it's not then I'd expect some commentary in the patch description to say why it's not a problem. Maybe even a precursor patch adding the folio->share so there is a place to state clearly that it doesn't matter and why. > + > + if (!order) { > + folio->pgmap = pgmap; This is also different... > + return 0; > + } > + > + folio_reset_order(folio); > + > + for (i = 0; i < (1UL << order); i++) { I'd take advantage of evolving conventions and do for (int i = 0; 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;
