Re: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell]

On 9/15/20 10:36 PM, Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 09:39:47AM -0700, Ralph Campbell wrote:
> > > I don't think any of the three ->page_free instances even cares about
> > > the page refcount.
> > Not true. The page_free() callback records the page is free by setting
> > a bit or putting the page on a free list but when it allocates a free
> > device private struct page to be used with migrate_vma_setup(), it needs to
> > increment the refcount.
> >
> > For the ZONE_DEVICE MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_PCI_P2PDMA
> > struct pages, I think you are correct because they don't define page_free()
> > and from what I can see, don't decrement the page refcount to zero.
>
> Umm, the whole point of ZONE_DEVICE is to have a struct page for
> something that is not system memory.  For both the ppc kvm case (magic
> hypervisor pool) and Noveau (device internal) memory that clear is the
> case.  But looks like test_hmm uses normal pages to fake this up, so
> I was wrong about the third caller.  But I think we can just call
> set_page_count just before freeing the page there with a comment
> explaining what is goin on.

Dan Williams thought that having the ZONE_DEVICE struct pages
be on a free list with a refcount of one was a bit strange and
that the driver should handle the zero to one transition.
But, that would mean a bit more invasive change to the 3 drivers
to set the reference count to zero after calling memremap_pages()
and setting the reference count to one when allocating a struct
page. What you are suggesting is what I also proposed in v1.