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

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Date:   Fri, 19 Jul 2019 16:01:22 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Thomas Gleixner <tglx@...utronix.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 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.
         */ 

I couldn't say it with less words :)


Regards,

	Joerg