Re: [RFC PATCH v3 2/2] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell]

On 10/1/20 10:59 PM, Christoph Hellwig wrote:
> On Thu, Oct 01, 2020 at 11:17:15AM -0700, Ralph Campbell wrote:
> > ZONE_DEVICE struct pages have an extra reference count that complicates the
> > code for put_page() and several places in the kernel that need to check the
> > reference count to see that a page is not being used (gup, compaction,
> > migration, etc.). Clean up the code so the reference count doesn't need to
> > be treated specially for ZONE_DEVICE.
> >
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
>
> Looks good,
>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>

Thanks for the review.

I still have reservations about making this an official patch.
Did you see the updated cover letter?
Basically, I'm concerned about ZONE_DEVICE struct pages being inserted into
the process page table with a zero reference count with vmf_insert_mixed().
If it is to be a non-zero reference count, then DAX, pmem, and other uses
of ZONE_DEVICE pages need to be changed (or vmf_insert_mixed()) to
inc/dec in appropriate places but I don't feel I know that code well enough
to make those changes.