Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

[Jann Horn]

On Tue, Oct 25, 2022 at 4:02 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Oct 24, 2022 at 09:58:07PM +0200, Jann Horn wrote:
>
> > Unless I'm completely misunderstanding what's going on here, the whole
> > "remove_table" thing only happens when you "remove a table", meaning
> > you free an entire *pagetable*. Just zapping PTEs doesn't trigger that
> > logic.
>
> Aah; yes true. OTOH even it that were not so, I think it would still be
> broken because the current code relies on the TLB flush to have
> completed, whereas the RCU scheme is effectively async and can be
> considered pending until the callback runs.
>
> Hurmph... easiest fix is probably to dis-allow kvm_flush_tlb_multi()
> for i386-pae builds.
>
> Something like so... nobody in his right mind should care about i386-pae
> virt performance much.

I think Xen and HyperV have similar codepaths.
hyperv_flush_tlb_multi() looks like it uses remote flush hypercalls,
xen_flush_tlb_multi() too.

On top of that, I think that theoretically, Linux doesn't even ensure
that you have a TLB flush in between tearing down one PTE and
installing another PTE (see
https://lore.kernel.org/all/CAG48ez1Oz4tT-N2Y=Zs6jumu=zOp7SQRZ=V2c+b5bT9P4retJA@xxxxxxxxxxxxxx/),
but I haven't tested that, and if it is true, I'm also not entirely
sure if it's correct (in the sense that it only creates incoherent-TLB
states when userspace is doing something stupid like racing
MADV_DONTNEED and page faults on the same region).

I think the more clearly correct fix would be to get rid of the split
loads and use CMPXCHG16B instead (probably destroying the performance
of GUP-fast completely), but that's complicated because some of the
architectures that use the split loads path don't have cmpxchg_double
(or at least don't have it wired up).