Re: [PATCH v3 01/37] mm:gup/writeback: add callbacks for inaccessible pages

[David Hildenbrand <david@redhat.com>]

On 20.02.20 11:39, Christian Borntraeger wrote:
> From: Claudio Imbrenda <imbrenda@linux.ibm.com>
> 
> With the introduction of protected KVM guests on s390 there is now a
> concept of inaccessible pages. These pages need to be made accessible
> before the host can access them.
> 
> While cpu accesses will trigger a fault that can be resolved, I/O
> accesses will just fail.  We need to add a callback into architecture
> code for places that will do I/O, namely when writeback is started or
> when a page reference is taken.
> 
> This is not only to enable paging, file backing etc, it is also
> necessary to protect the host against a malicious user space. For
> example a bad QEMU could simply start direct I/O on such protected
> memory.  We do not want userspace to be able to trigger I/O errors and
> thus we the logic is "whenever somebody accesses that page (gup) or
> doing I/O, make sure that this page can be accessed. When the guest
> tries to access that page we will wait in the page fault handler for
> writeback to have finished and for the page_ref to be the expected
> value.


Subject: "mm: gup..."

And I'd probably add that on s390x, it's unlikely to ever return !0.
(why the WARN_ON is okay and has to be tackled once relevant)

> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Acked-by: Will Deacon <will@kernel.org>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb