linux-kernel - Re: [PATCH v6 3/7] x86/mm: Use 'ptdesc' when freeing PMD pages

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Message-ID: <54a30819-25e2-4835-80c7-5631237b8e9a@redhat.com>
Date: Thu, 16 Oct 2025 21:33:43 +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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>,
 Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...nel.org>,
 Matthew Wilcox <willy@...radead.org>
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 v6 3/7] x86/mm: Use 'ptdesc' when freeing PMD pages

On 14.10.25 15:04, 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>
> ---
>   include/linux/mm.h    |  6 ++++--
>   arch/x86/mm/pgtable.c | 12 ++++++------
>   2 files changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 15ce0c415d36..94e2ec6c5685 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3203,8 +3203,7 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
>   	((unlikely(pmd_none(*(pmd))) && __pte_alloc_kernel(pmd))? \
>   		NULL: pte_offset_kernel(pmd, address))
>   
> -#if defined(CONFIG_SPLIT_PMD_PTLOCKS)
> -
> +#if defined(CONFIG_SPLIT_PMD_PTLOCKS) || defined(CONFIG_X86_64)

Yeah, that is weird. I'd have thought we can simply move this out of
the ifdef? The CONFIG_X86_64 stuff certainly has to go one way or the other.

As PTE tables always fit in a single page, pgtable_page(pte) is sufficient.

PMD tables can exceed a single page on some archs, so we have to find 
the first page first that we can then cast.

Can't immediately see why that causes compile issues.

>   static inline struct page *pmd_pgtable_page(pmd_t *pmd)
>   {
>   	unsigned long mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
> @@ -3215,6 +3214,9 @@ static inline struct ptdesc *pmd_ptdesc(pmd_t *pmd)
>   {
>   	return page_ptdesc(pmd_pgtable_page(pmd));
>   }
> +#endif

-- 
Cheers

David / dhildenb