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

[Dan Williams]

On Wed, Sep 16, 2020 at 5:29 PM Ralph Campbell <rcampbell@xxxxxxxxxx> wrote:
>
>
> 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.

IIUC, isn't what Christoph recommending is that drivers handle
set_page_count() directly rather than the core since some are prepared
for it to be zero on entry?