Re: [PATCH 7/8] mm: remove the extra ZONE_DEVICE struct page refcount

[Jason Gunthorpe]

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