Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Tue, May 02, 2023 at 09:33:45PM +0200, David Hildenbrand wrote:
> On 02.05.23 21:25, Lorenzo Stoakes wrote:
> > On Tue, May 02, 2023 at 04:07:50PM -0300, Jason Gunthorpe wrote:
> > > On Tue, May 02, 2023 at 07:17:14PM +0100, Lorenzo Stoakes wrote:
> > >
> > > > On a specific point - if mapping turns out to be NULL after we confirm
> > > > stable PTE, I'd be inclined to reject and let the slow path take care of
> > > > it, would you agree that that's the correct approach?
> > >
> > > I think in general if GUP fast detects any kind of race it should bail
> > > to the slow path.
> > >
> > > The races it tries to resolve itself should have really safe and
> > > obvious solutions.
> > >
> > > I think this comment is misleading:
> > >
> > > > +	/*
> > > > +	 * GUP-fast disables IRQs - this prevents IPIs from causing page tables
> > > > +	 * to disappear from under us, as well as preventing RCU grace periods
> > > > +	 * from making progress (i.e. implying rcu_read_lock()).
> > >
> > > True, but that is not important here since we are not reading page
> > > tables
> > >
> > > > +	 * This means we can rely on the folio remaining stable for all
> > > > +	 * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
> > > > +	 * and those that do not.
> > >
> > > Not really clear. We have a valid folio refcount here, that is all.
> >
> > Some of this is a product of mixed signals from different commenters and
> > my being perhaps a little _too_ willing to just go with the flow.
> >
> > With interrupts disabled and IPI blocked, plus the assurances that
> > interrupts being disabled implied the RCU version of page table
> > manipulation is also blocked, my understanding was that remapping in this
> > process to another page could not occur.
> >
> > Of course the folio is 'stable' in the sense we have a refcount on it, but
> > it is unlocked so things can change.
> >
> > I'm guessing the RCU guarantees in the TLB logic are not as solid as IPI,
> > because in the IPI case it seems to me you couldn't even clear the PTE
> > entry before getting to the page table case.
> >
> > Otherwise, I'm a bit uncertain actually as to how we can get to the point
> > where the folio->mapping is being manipulated. Is this why?
>
> I'll just stress again that I think there are cases where we unmap and free
> a page without synchronizing against GUP-fast using an IPI or RCU.

OK that explains why we need to be careful. Don't worry I am going to move the
check after we confirm PTE entry hasn't changed.

>
> That's one of the reasons why we recheck if the PTE changed to back off, so
> I've been told.
>
> I'm happy if someone proves me wrong and a page we just (temporarily) pinned
> cannot have been freed+reused in the meantime.

Let's play it safe for now :)

>
> --
> Thanks,
>
> David / dhildenb
>