linux-kernel - Re: [PATCH 2/3] x86/mm: Sync also unmappings in vmalloc_sync

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Message-ID: <alpine.DEB.2.21.1907192309190.1782@nanos.tec.linutronix.de>
Date:   Fri, 19 Jul 2019 23:10:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joerg Roedel <jroedel@...e.de>
cc:     Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] x86/mm: Sync also unmappings in vmalloc_sync_one()

On Fri, 19 Jul 2019, Joerg Roedel wrote:
> On Thu, Jul 18, 2019 at 11:04:57AM +0200, Thomas Gleixner wrote:
> > Joerg,
> > 
> > On Thu, 18 Jul 2019, Joerg Roedel wrote:
> > > On Wed, Jul 17, 2019 at 11:43:43PM +0200, Thomas Gleixner wrote:
> > > > On Wed, 17 Jul 2019, Joerg Roedel wrote:
> > > > > +
> > > > > +	if (!pmd_present(*pmd_k))
> > > > > +		return NULL;
> > > > >  	else
> > > > >  		BUG_ON(pmd_pfn(*pmd) != pmd_pfn(*pmd_k));
> > > > 
> > > > So in case of unmap, this updates only the first entry in the pgd_list
> > > > because vmalloc_sync_all() will break out of the iteration over pgd_list
> > > > when NULL is returned from vmalloc_sync_one().
> > > > 
> > > > I'm surely missing something, but how is that supposed to sync _all_ page
> > > > tables on unmap as the changelog claims?
> > > 
> > > No, you are right, I missed that. It is a bug in this patch, the code
> > > that breaks out of the loop in vmalloc_sync_all() needs to be removed as
> > > well. Will do that in the next version.
> > 
> > I assume that p4d/pud do not need the pmd treatment, but a comment
> > explaining why would be appreciated.
> 
> Actually there is already a comment in this function explaining why p4d
> and pud don't need any treatment:
> 
>         /*
>          * set_pgd(pgd, *pgd_k); here would be useless on PAE
>          * and redundant with the set_pmd() on non-PAE. As would
>          * set_p4d/set_pud.
>          */ 

Indeed. Why did I think there was none?

> I couldn't say it with less words :)

It's perfectly fine.

Thanks,

	tglx