On 26/05/19 12:34PM, Jonathan Cameron wrote:
> 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
Will do, thanks
>
> > 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;
>
