Re: [PATCH v5 08/15] mm: Add unsafe_follow_pfn

[Jason Gunthorpe]

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@xxxxxxxxxxxxx> 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

Jason