Re: [PATCH v7 19/20] fs/dax: Properly refcount fs dax pages

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Feb 05, 2025 at 09:48:16AM +1100, Alistair Popple wrote:
> Currently fs dax pages are considered free when the refcount drops to
> one and their refcounts are not increased when mapped via PTEs or
> decreased when unmapped. This requires special logic in mm paths to
> detect that these pages should not be properly refcounted, and to
> detect when the refcount drops to one instead of zero.
> 
> On the other hand get_user_pages(), etc. will properly refcount fs dax
> pages by taking a reference and dropping it when the page is
> unpinned.
> 
> Tracking this special behaviour requires extra PTE bits
> (eg. pte_devmap) and introduces rules that are potentially confusing
> and specific to FS DAX pages. To fix this, and to possibly allow
> removal of the special PTE bits in future, convert the fs dax page
> refcounts to be zero based and instead take a reference on the page
> each time it is mapped as is currently the case for normal pages.
> 
> This may also allow a future clean-up to remove the pgmap refcounting
> that is currently done in mm/gup.c.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> 
> ---
...
> -static inline unsigned long dax_page_share_put(struct page *page)
> +static inline unsigned long dax_folio_put(struct folio *folio)
>  {
> -	WARN_ON_ONCE(!page->share);
> -	return --page->share;
> +	unsigned long ref;
> +
> +	if (!dax_folio_is_shared(folio))
> +		ref = 0;
> +	else
> +		ref = --folio->share;
> +
> +	WARN_ON_ONCE(ref < 0);

Kind of unlikely for an unsigned long to ever be < 0.

[ thanks to coverity for noticing ]

Guenter