Re: [PATCH 1/9] mm/migrate.c: Always allow device private pages to migrate

[Jason Gunthorpe]

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