Re: [PATCH v3 5/5] x86/mm: add WARN_ON_ONCE() for wrong large page mapping

Yang, Bin
2018-09-07T02:12:32Z

On Tue, 2018-09-04 at 18:52 +0200, Thomas Gleixner wrote:
> On Tue, 4 Sep 2018, Thomas Gleixner wrote:
> > On Tue, 4 Sep 2018, Yang, Bin wrote:
> > > On Tue, 2018-09-04 at 00:27 +0200, Thomas Gleixner wrote:
> > > > On Tue, 21 Aug 2018, Bin Yang wrote:
> > > > > @@ -625,6 +625,7 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
> > > > >
> > > > > psize = page_level_size(level);
> > > > > pmask = page_level_mask(level);
> > > > > + addr = address & pmask;
> > > > >
> > > > > /*
> > > > > * Calculate the number of pages, which fit into this large
> > > > > @@ -636,6 +637,12 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
> > > > > cpa->numpages = numpages;
> > > > >
> > > > > /*
> > > > > + * The old pgprot should not have any protection bit. Otherwise,
> > > > > + * the existing mapping is wrong already.
> > > > > + */
> > > > > + WARN_ON_ONCE(needs_static_protections(old_prot, addr, psize, old_pfn));
> > > >
> > > > The check itself is fine, but it just emits a warning and goes on as if
> > > > nothing happened.
> > > >
> > > > We really want to think about a proper way to fix that up without overhead
> > > > for the sane case.
> > >
> > > could we change it as below? I think it should be safe to split large
> > > page if current mapping is wrong already.
> > >
> > > if (needs_static_protections(old_prot, addr, psize, old_pfn)) {
> > > WARN_ON_ONCE(1);
> > > goto out_unlock;
> > > }
> >
> > Sure, but what enforces the static protections on the pages which are not
> > in the modified range of the current CPA call? Nothing.
>
> I looked deeper into that. For the PMD split it's rather trivial to do, but
> a PUD split would require a horrible pile of changes as we'd have to remove
> the protections from the new PMD first, go all the way back and rescan the
> new PMDs whether they need to be split up further. But that needs a lot of
> refactoring and I'm not sure if it's worth the trouble right now.
>
> As we haven't cared about that since CPA got introduced, I think we just do
> the consistency check and warn. That's better what we have now and when it
> ever triggers revisit it.

Is below check enough?

if ((pgprot_val(old_prot) & _PAGE_PRESENT) &&
needs_static_protections(old_prot, addr, psize, old_pfn)) {
WARN_ON_ONCE(1);
}

>
> Thanks,
>
> tglx