Re: [PATCH V2 1/7] dax: fix misleading comment about share/index union in dax_folio_reset_order()

[Dave Jiang <dave.jiang@intel.com>]

On 5/22/26 12:18 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> 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
> 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")
> 
> Reviewed-by: Jonathan Cameron <jic23@kernel.org>
> Signed-off-by: John Groves <john@groves.net>
> ---
>  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.

In the old comments, there is the pre-condition that "callers ensure share has been decremented to zero before calling here." Is this precondition remain true? Maybe should leave that comment in if that is the case?

 
>  	 */
>  	folio->mapping = NULL;
>  	folio->share = 0;