On Tue, Feb 09, 2021 at 12:07:14PM +1100, Alistair Popple wrote:

Device private pages are used to represent device memory that is not
directly accessible from the CPU. Extra references to a device private
page are only used to ensure the struct page itself remains valid whilst
waiting for migration entries. Therefore extra references should not
prevent device private page migration as this can lead to failures to
migrate pages back to the CPU which are fatal to the user process.

This should identify the extra references in expected_count, just
disabling this protection seems unsafe, ZONE_DEVICE is not so special
that the refcount means nothing

Is this a side effect of the extra refcounts that Ralph was trying to
get rid of? I'd rather see that work finished :)

Jason