Re: [PATCH v2 01/42] mm:gup/writeback: add callbacks for inaccessible pages

[David Hildenbrand <david@redhat.com>]

On 14.02.20 23:26, 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.
> 
> If wanted by others, the callbacks can be extended with error handlin
> and a parameter from where this is called.

s/handlin/handling/

One last question from my side:

Why is it OK to ignore errors here. IOW, why not squash "[PATCH v2
39/42] example for future extension: mm:gup/writeback: add callbacks for
inaccessible pages: error cases" into this patch.

I can see in patch "[PATCH v2 05/42] s390/mm: provide memory management
functions for protected KVM guests", that the call can fail for various
reasons. That puzzles me a bit - what would happen if any of that fails?
Or will it actually never fail for s390x (and all that error handling in
arch_make_page_accessible() is essentially dead code in real life?)

-- 
Thanks,

David / dhildenb