linux-kernel - Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries

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Date:   Mon, 10 Aug 2020 18:53:59 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
        Joerg Roedel <jroedel@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason@...c4.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Shutemov, Kirill" <kirill.shutemov@...el.com>
Subject: Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries

On Mon, Aug 10, 2020 at 07:27:33AM -0700, Dave Hansen wrote:
> ... adding Kirill
> 
> On 8/7/20 1:40 AM, Joerg Roedel wrote:
> > +		lvl = "p4d";
> > +		p4d = p4d_alloc(&init_mm, pgd, addr);
> > +		if (!p4d)
> > +			goto failed;
> >  
> > +		/*
> > +		 * With 5-level paging the P4D level is not folded. So the PGDs
> > +		 * are now populated and there is no need to walk down to the
> > +		 * PUD level.
> > +		 */
> >  		if (pgtable_l5_enabled())
> >  			continue;
> 
> It's early and I'm a coffee or two short of awake, but I had to stare at
> the comment for a but to make sense of it.
> 
> It feels wrong, I think, because the 5-level code usually ends up doing
> *more* allocations and in this case, it is _appearing_ to do fewer.
> Would something like this make sense?

Unless I miss something, with 5 levels vmalloc mappings are shared at
p4d level, so allocating a p4d page would be enough. With 4 levels,
p4d_alloc() is a nop and pud is the first actually populated level below
pgd.

> 		/*
> 		 * The goal here is to allocate all possibly required
> 		 * hardware page tables pointed to by the top hardware
> 		 * level.
> 		 *
> 		 * On 4-level systems, the p4d layer is folded away and
> 		 * the above code does no preallocation.  Below, go down
> 		 * to the pud _software_ level to ensure the second
> 		 * hardware level is allocated.
> 		 */
> 
> 
> > -		pud = pud_offset(p4d, addr);
> > -		if (pud_none(*pud)) {
> > -			/* Ends up here only with 4-level paging */
> > -			pud = pud_alloc(&init_mm, p4d, addr);
> > -			if (!pud) {
> > -				lvl = "pud";
> > -				goto failed;
> > -			}
> > -		}
> > +		lvl = "pud";
> > +		pud = pud_alloc(&init_mm, p4d, addr);
> > +		if (!pud)
> > +			goto failed;
> >  	}

-- 
Sincerely yours,
Mike.