Re: [PATCH] mm: Re-allow pinning of zero pfns

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Jun 11, 2022 at 08:29:47PM +0200, David Hildenbrand wrote:
> On 11.06.22 00:35, Alex Williamson wrote:
> > The commit referenced below subtly and inadvertently changed the logic
> > to disallow pinning of zero pfns.  This breaks device assignment with
> > vfio and potentially various other users of gup.  Exclude the zero page
> > test from the negation.
> 
> I wonder which setups can reliably work with a long-term pin on a shared
> zeropage. In a MAP_PRIVATE mapping, any write access via the page tables
> will end up replacing the shared zeropage with an anonymous page.
> Something similar should apply in MAP_SHARED mappings, when lazily
> allocating disk blocks.
> 
> In the future, we might trigger unsharing when taking a R/O pin for the
> shared zeropage, just like we do as of now upstream for shared anonymous
> pages (!PageAnonExclusive). And something similar could then be done
> when finding a !anon page in a MAP_SHARED mapping.

I'm also confused how qemu is hitting this and it isn't already a bug?

It is arising because vfio doesn't use FOLL_FORCE|FOLL_WRITE to move
away the zero page in most cases.

And why does Yishai say it causes an infinite loop in the kernel?

Jason