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

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Date:   Mon, 10 Aug 2020 07:27:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>, x86@...nel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
        Mike Rapoport <rppt@...ux.ibm.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

... 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?

		/*
		 * 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;
>  	}