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

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Message-ID: <710f6a6fcc00f836eb78952741b79969f159beb9.camel@intel.com>
Date:   Fri, 7 Sep 2018 02:12:23 +0000
From:   "Yang, Bin" <bin.yang@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Gross, Mark" <mark.gross@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH v3 5/5] x86/mm: add WARN_ON_ONCE() for wrong large page
 mapping

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