Re: [PATCH] vfio/type1: Unpin zero pages

[David Hildenbrand <david@redhat.com>]

On 07.09.22 01:30, Jason Gunthorpe wrote:
> On Fri, Sep 02, 2022 at 10:32:01AM +0200, David Hildenbrand wrote:
> 
>>> So I wonder instead of continuing to fix trickiness around the zero
>>> page whether it is a better idea to pursue allocating a normal
>>> page from the beginning for pinned RO mappings?
>>
>> That's precisely what I am working. For example, that's required to get
>> rid of FOLL_FORCE|FOLL_WRITE for taking a R/O pin as done by RDMA:
> 
> And all these issues are exactly why RDMA uses FOLL_FORCE and it is,
> IMHO, a simple bug that VFIO does not.

I consider the BUG that our longterm page pinning behaves the way it 
currently does, not that we're not using the FOLL_FORCE flag here.

But it doesn't matter, I'm working on fixing/cleaning it up.

> 
>> I do wonder if that's a real issue, though. One approach would be to
>> warn the VFIO users and allow for slightly exceeding the MEMLOCK limit
>> for a while. Of course, that only works if we assume that such pinned
>> zeropages are only extremely rarely longterm-pinned for a single VM
>> instance by VFIO.
> 
> I'm confused, doesn't vfio increment the memlock for every page of VA
> it pins? Why would it matter if the page was COW'd or not? It is
> already accounted for today as though it was a unique page.
> 
> IOW if we add FOLL_FORCE it won't change the value of the memlock.

I only briefly skimmed over the code Alex might be able to provide more 
details and correct me if I'm wrong:

vfio_pin_pages_remote() contains a comment:

"Reserved pages aren't counted against the user, externally pinned pages 
are already counted against the user."

is_invalid_reserved_pfn() should return "true" for the shared zeropage 
and prevent us from accounting it via vfio_lock_acct(). Otherwise, 
vfio_find_vpfn() seems to be in place to avoid double-accounting pages.

-- 
Thanks,

David / dhildenb