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

From: Ralph Campbell
Date: Wed Sep 16 2020 - 20:34:43 EST

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.

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.