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

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.

On Wed, Feb 09, 2022 at 07:23:45AM +0100, Christoph Hellwig
wrote:
> 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.
It is nice, but the other series are still impacted by the fsdax mess
- they still stuff pages into ptes without proper refcounts and have
to carry nonsense to dance around this problem.

I certainly would be unhappy if the amd driver, for instance, gained
the fsdax problem as well and started pushing 4k pages into PMDs.

Jason