linux-kernel - Re: [PATCH 08/15] x86: Physical address comparisons in fill

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Message-ID: <c344dfaa-7e79-498f-89d7-44631140d0f4@intel.com>
Date: Wed, 5 Feb 2025 16:57:15 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, luto@...nel.org,
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Subject: Re: [PATCH 08/15] x86: Physical address comparisons in fill_p*d/pte

On 2/4/25 09:33, Maciej Wieczor-Retman wrote:
> @@ -287,7 +287,7 @@ static pte_t *fill_pte(pmd_t *pmd, unsigned long vaddr)
>  	if (pmd_none(*pmd)) {
>  		pte_t *pte = (pte_t *) spp_getpage();
>  		pmd_populate_kernel(&init_mm, pmd, pte);
> -		if (pte != pte_offset_kernel(pmd, 0))
> +		if (__pa(pte) != __pa(pte_offset_kernel(pmd, 0)))
>  			printk(KERN_ERR "PAGETABLE BUG #03!\n");
>  	}
>  	return pte_offset_kernel(pmd, vaddr);

Maciej, could you do a quick check on this and make sure that it doesn't
hurt code generation on current kernels?

pte_offset_kernel() has an internal __va() so this ends up logically
being something like:

-	if (     pte  !=      __va(pmd))
+	if (__pa(pte) != __pa(__va(pmd)))

The __pa() and __va() obviously logically cancel each other out in the
new version. But if the compiler for whatever reason can't figure this
out we might end up with worse code.

If it generates crummy code we might want to do this differently like
avoiding pte_offset_kernel() and adding some other helper that's more
direct and to the point.