Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes]

On Mon, May 15, 2023 at 09:12:49AM -0300, Jason Gunthorpe wrote:
> On Mon, May 15, 2023 at 12:16:21PM +0100, Lorenzo Stoakes wrote:
> > > One thing that came to mind is KVM with "qemu -object memory-backend-file,share=on..."
> > > It is mostly used for pmem emulation.
> > >
> > > Do we have plan B?
> >
> > Yes, we can make it opt-in or opt-out via a FOLL_FLAG. This would be easy
> > to implement in the event of any issues arising.
>
> I'm becoming less keen on the idea of a per-subsystem opt out. I think
> we should make a kernel wide opt out. I like the idea of using lower
> lockdown levels. Lots of things become unavaiable in the uAPI when the
> lockdown level increases already.

This would be the 'safest' in the sense that a user can't be surprised by
higher lockdown = access modes disallowed, however we'd _definitely_ need
to have an opt-in in that instance so io_uring can make use of this
regardless. That's easy to add however.

If we do go down that road, we can be even stricter/vary what we do at
different levels right?

>
> > Jason will have some thoughts on this I'm sure. I guess the key question
> > here is - is it actually feasible for this to work at all? Once we
> > establish that, the rest are details :)
>
> Surely it is, but like Ted said, the FS folks are not interested and
> they are at least half the solution..

:'(

>
> The FS also has to actively not write out the page while it cannot be
> write protected unless it copies the data to a stable page. The block
> stack needs the source data to be stable to do checksum/parity/etc
> stuff. It is a complicated subject.

Yes my sense was that being able to write arbitrarily to these pages _at
all_ was a big issue, not only the dirty tracking aspect.

I guess at some level letting filesystems have such total flexibility as to
how they implement things leaves us in a difficult position.

>
> Jason