On Mon, 18 May 2026 21:35:56 +0000 John Groves <[email protected]> wrote:
> From: John Groves <[email protected]> > > The comment in dax_folio_reset_order() claims that DAX maintains an > invariant where folio->share != 0 only when folio->mapping == NULL, > implying folio->share is zero whenever mapping is non-NULL. This is > misleading because folio->share and folio->index are a union -- for > non-shared folios with mapping != NULL, reading folio->share returns Maybe for consistency refer to that as folio->mapping != NULL > the file page offset (folio->index), which is typically non-zero. > > Reword the comment to accurately describe the union aliasing: the > assignment clears whichever interpretation of the union word is active > (index for non-shared folios, share for shared folios), which is correct > because the folio is being released in either case. > > No functional change -- the code was already correct, only the > justification was wrong. > > Fixes: 59eb73b98ae0b ("dax: Factor out dax_folio_reset_order() helper") > Signed-off-by: John Groves <[email protected]> Reviewed-by: Jonathan Cameron <[email protected]> > --- > fs/dax.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 6d175cd47a99b..df19c9317d10e 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -392,12 +392,12 @@ int dax_folio_reset_order(struct folio *folio) > 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. > + * Clear the mapping and the index/share union word. folio->share > + * and folio->index occupy the same union in struct folio. For > + * non-shared folios (mapping != NULL), the union holds folio->index > + * (file page offset); for shared folios (mapping == NULL), it holds > + * folio->share (reference count). Either way, we are releasing the > + * folio and both fields should be zeroed. > */ > folio->mapping = NULL; > folio->share = 0;
