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

[Daniel Vetter]

On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe <jgg@xxxxxxxx> 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@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

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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch