linux-kernel - Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD

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Message-ID: <e9b314d03804024d1de09e1ad83eda9f54964828.camel@fb.com>
Date:   Tue, 20 Aug 2019 13:21:53 +0000
From:   Rik van Riel <riel@...com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Song Liu <songliubraving@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by
 PUD_SIZE

On Tue, 2019-08-20 at 13:16 +0200, Thomas Gleixner wrote:
> On Tue, 20 Aug 2019, Peter Zijlstra wrote:
> > What that code wants to do is skip to the end of the pud, a
> > pmd_size
> > increase will not do that. And right below this, there's a second
> > instance of this exact pattern.
> > 
> > Did I get the below right?
> > 
> > ---
> > diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> > index b196524759ec..32b20b3cb227 100644
> > --- a/arch/x86/mm/pti.c
> > +++ b/arch/x86/mm/pti.c
> > @@ -330,12 +330,14 @@ pti_clone_pgtable(unsigned long start,
> > unsigned long end,
> >  
> >  		pud = pud_offset(p4d, addr);
> >  		if (pud_none(*pud)) {
> > +			addr &= PUD_MASK;
> >  			addr += PUD_SIZE;
> 
> 			round_up(addr, PUD_SIZE);
> 
> perhaps?

Won't that keep returning the same address forever
if addr & PUD_MASK == 0?