Nouveau - Feb 2022 - [PATCH 7/8] mm: remove the extra ZONE_DEVICE struct page refcount

On Sun, Feb 6, 2022 at 10:33 PM Christoph Hellwig <hch at lst.de> wrote:
[..]
> @@ -500,28 +482,27 @@ void free_devmap_managed_page(struct page *page)
>          */
>         page->mapping = NULL;
>         page->pgmap->ops->page_free(page);
> +
> +       /*
> +        * Reset the page count to 1 to prepare for handing out the page
again.
> +        */
> +       set_page_count(page, 1);
Interesting. I had expected that to really fix the refcount problem
that fs/dax.c would need to start taking real page references as pages
were added to a mapping, just like page cache.

This looks ok to me, and passes my tests. So given I'm still working
my way back to fixing the references properly I'm ok for this hack to
replace the more broken hack that is there presently.

Reviewed-by: Dan Williams <dan.j.williams at intel.com>

On Tue, Feb 08, 2022 at 07:30:11PM -0800, Dan Williams
wrote:
> Interesting. I had expected that to really fix the refcount problem
> that fs/dax.c would need to start taking real page references as pages
> were added to a mapping, just like page cache.
I think we should do that eventually.  But I think this series that
just attacks the device private type and extends to the device coherent
and p2p enhacements is a good first step to stop the proliferation of
the one off refcount and to allow to deal with the fsdax pages in another
more focuessed series.