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

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Date:   Tue, 4 Sep 2018 06:32:59 +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 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;
        }


> 
> Thanks,
> 
> 	tglx