[PATCH v5 08/15] mm: Add unsafe_follow_pfn

[Jason Gunthorpe]

On Mon, Nov 02, 2020 at 02:23:58PM +0100, Daniel Vetter wrote:
> On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe <jgg at ziepe.ca> wrote:
> >
> > On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote:
> > > On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig <hch at infradead.org> wrote:
> > > >
> > > > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote:
> > > > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> > > > > that by drivers/modules is together with an mmu_notifier, and that's
> > > > > all _GPL stuff.
> > > >
> > > > I also think it also needs to be renamed to explicitly break any existing
> > > > users out of tree or int the submission queue.
> > >
> > > Ok I looked at the mmu notifier locking again and noticed that
> > > mm->subscriptions has its own spinlock. Since there usually shouldn't
> > > be a huge pile of these I think it's feasible to check for the mmu
> > > notifier in follow_pfn. And that would stuff this gap for good. I'll
> > > throw that on top as a final patch and see what people think.
> >
> > Probably the simplest is to just check mm_has_notifiers() when in
> > lockdep or something very simple like that
> 
> lockdep feels wrong, was locking more at CONFIG_DEBUG_VM. And since
> generally you only have 1 mmu notifier (especially for kvm) I think we
> can also pay the 2nd cacheline miss and actually check the right mmu
> notifier is registered.

Need to hold the lock to check that and there are two ways to register
notifiers these days, so it feels to expensive to me.

CH's 'export symbol only for kvm' really does seem the most robust way
to handle this though.

Jason