linux-kernel - Re: [PATCH v3 1/3] x86/mm/pat: Convert pte code to use ptdescs

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Message-ID: <e029f69b-6d99-4029-94f5-caafb5eb767c@intel.com>
Date: Tue, 3 Feb 2026 09:23:47 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 "Mike Rapoport (Microsoft)" <rppt@...nel.org>
Cc: akpm@...ux-foundation.org, "Matthew Wilcox (Oracle)"
 <willy@...radead.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/3] x86/mm/pat: Convert pte code to use ptdescs

On 2/2/26 09:20, Vishal Moola (Oracle) wrote:
> In order to separately allocate ptdescs from pages, we need all allocation
> and free sites to use the appropriate functions. Convert these pte
> allocation/free sites to use ptdescs.

Imperative voice, please.

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 6c6eb486f7a6..f9f9d4ca8e71 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1408,7 +1408,7 @@ static bool try_to_free_pte_page(pte_t *pte)
>  		if (!pte_none(pte[i]))
>  			return false;
>  
> -	free_page((unsigned long)pte);
> +	pagetable_free(virt_to_ptdesc((void *)pte));
>  	return true;
>  }

This looks wrong to me, or at least that the API needs improvement. Most
callers are going to have a pointer that they've been modifying. They're
not going to have a ptdesc handy.

So I think this needs to look like:

	pagetable_free(pte);

You can convert to ptdescs internally or do whatever you want with
ptdesc sanity checks, but the API needs to be on writeable pointers. If
the API takes a const pointer that requires callers to cast it, I think
the API is broken.

> @@ -1537,12 +1537,15 @@ static void unmap_pud_range(p4d_t *p4d, unsigned long start, unsigned long end)
>  	 */
>  }
>  
> -static int alloc_pte_page(pmd_t *pmd)
> +static int alloc_pte_ptdesc(pmd_t *pmd)

Why change the name? Nobody cares what this is doing internally.

>  {
> -	pte_t *pte = (pte_t *)get_zeroed_page(GFP_KERNEL);
> -	if (!pte)
> +	pte_t *pte;
> +	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL | __GFP_ZERO, 0);
> +
> +	if (!ptdesc)
>  		return -1;

This also looks wrong.

What kind of maniac is ever going to allocate page tables without
__GFP_ZERO? __GFP_ZERO really should be a part of pagetable_alloc(),
don't you think?

> +	pte = (pte_t *) ptdesc_address(ptdesc);
>  	set_pmd(pmd, __pmd(__pa(pte) | _KERNPG_TABLE));
>  	return 0;
>  }

Why is there a cast here? ptdesc_address() returns void*, no?

Also, if there a ptdesc_pa(), this could be:

static int alloc_pte_ptdesc(pmd_t *pmd)
{
	struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL, 0);

	if (!ptdesc)
 		return -1;

 	set_pmd(pmd, __pmd(ptdesc_pa(ptdesc) | _KERNPG_TABLE));
 	return 0;
}

This *should* be a very common pattern. After you allocate a page table
page, you almost always need its physical address because it's going to
get pointed to by other page table or hardware register.

To me, it doesn't look like the ptdesc API is very mature yet, or at
least hasn't been expanded for ease for actual users. I don't want to
grow its use in arch/x86 until it's a wee bit more mature.