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

[Jann Horn]

On Tue, Oct 25, 2022 at 5:06 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Oct 25, 2022 at 04:18:20PM +0200, Jann Horn wrote:
> > 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.
>
> Sure (not updated).
>
> > 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).
>
> cmpxchg8b; but no, I think we want to fix MADV_DONTNEED, incoherent TLB
> states are a pain nobody needs.
>
> Something like so should force TLB flushes before dropping pte_lock (not
> looked at the various pmd level things yet).
[...]
>  #endif /* _LINUX_MM_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index f88c351aecd4..9bb63b3fbee1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1440,6 +1440,11 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>                         tlb_remove_tlb_entry(tlb, pte, addr);
>                         zap_install_uffd_wp_if_needed(vma, addr, pte, details,
>                                                       ptent);
> +
> +                       if (!force_flush && !tlb->fullmm && details &&
> +                           details->zap_flags & ZAP_FLAG_FORCE_FLUSH)
> +                               force_flush = 1;
> +

Hmm... I guess that might work, assuming that there is no other
codepath we might race with that first turns the present PTE into a
non-present PTE but keeps the flush queued for later. At least
codepaths that use the tlb_batched infrastructure are unproblematic...