linux-kernel - Re: [PATCH v5 3/8] x86/mm: Use 'ptdesc' when freeing PMD pages

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Message-ID: <9648e990-18e0-4aed-8110-b02f4b45aa47@redhat.com>
Date: Thu, 9 Oct 2025 21:25:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>,
 Jann Horn <jannh@...gle.com>, Vasant Hegde <vasant.hegde@....com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>,
 Alistair Popple <apopple@...dia.com>, Peter Zijlstra <peterz@...radead.org>,
 Uladzislau Rezki <urezki@...il.com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Andy Lutomirski <luto@...nel.org>, Yi Lai <yi1.lai@...el.com>
Cc: iommu@...ts.linux.dev, security@...nel.org, x86@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v5 3/8] x86/mm: Use 'ptdesc' when freeing PMD pages

On 19.09.25 07:40, Lu Baolu wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> There are a billion ways to refer to a physical memory address.
> One of the x86 PMD freeing code location chooses to use a 'pte_t *' to
> point to a PMD page and then call a PTE-specific freeing function for
> it.  That's a bit wonky.
> 
> Just use a 'struct ptdesc *' instead. Its entire purpose is to refer
> to page table pages. It also means being able to remove an explicit
> cast.
> 
> Right now, pte_free_kernel() is a one-liner that calls
> pagetable_dtor_free(). Effectively, all this patch does is
> remove one superfluous __pa(__va(paddr)) conversion and then
> call pagetable_dtor_free() directly instead of through a helper.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> ---
>   arch/x86/mm/pgtable.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index ddf248c3ee7d..2e5ecfdce73c 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -729,7 +729,7 @@ int pmd_clear_huge(pmd_t *pmd)
>   int pud_free_pmd_page(pud_t *pud, unsigned long addr)
>   {
>   	pmd_t *pmd, *pmd_sv;
> -	pte_t *pte;
> +	struct ptdesc *pt;
>   	int i;
>   
>   	pmd = pud_pgtable(*pud);
> @@ -750,8 +750,8 @@ int pud_free_pmd_page(pud_t *pud, unsigned long addr)
>   
>   	for (i = 0; i < PTRS_PER_PMD; i++) {
>   		if (!pmd_none(pmd_sv[i])) {
> -			pte = (pte_t *)pmd_page_vaddr(pmd_sv[i]);
> -			pte_free_kernel(&init_mm, pte);
> +			pt = page_ptdesc(pmd_page(pmd_sv[i]));
> +			pagetable_dtor_free(pt);

There is pmd_ptdesc() which does

	page_ptdesc(pmd_pgtable_page(pmd));

It's buried in a

	#if defined(CONFIG_SPLIT_PMD_PTLOCKS)

Can't we just make that always available so we can use it here?

-- 
Cheers

David / dhildenb