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

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Message-ID: <d887e9e228440097b666bcd316aabc9827a4b01e.camel@fb.com>
Date:   Tue, 20 Aug 2019 13:55:57 +0000
From:   Rik van Riel <riel@...com>
To:     Song Liu <songliubraving@...com>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Peter Zijlstra <peterz@...radead.org>,
        "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 09:21 -0400, Song Liu wrote:
> > On Aug 20, 2019, at 4:16 AM, Thomas Gleixner <tglx@...utronix.de>
> > 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);
> 
> I guess we need "round_up(addr + PMD_SIZE, PUD_SIZE)". 

What does that do if start is less than PMD_SIZE
away from the next PUD_SIZE boundary?

How about:   round_up(addr + 1, PUD_SIZE)  ?