linux-kernel - Re: [PATCH v6 6/7] mm: Introduce deferred freeing for kernel page tables

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Message-ID: <e62bad6a-bd49-48d9-aa2d-9e2022f829b3@redhat.com>
Date: Thu, 16 Oct 2025 21:35:12 +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 6/7] mm: Introduce deferred freeing for kernel page
 tables

On 14.10.25 15:04, Lu Baolu wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> This introduces a conditional asynchronous mechanism, enabled by
> CONFIG_ASYNC_KERNEL_PGTABLE_FREE. When enabled, this mechanism defers the
> freeing of pages that are used as page tables for kernel address mappings.
> These pages are now queued to a work struct instead of being freed
> immediately.
> 
> This deferred freeing allows for batch-freeing of page tables, providing
> a safe context for performing a single expensive operation (TLB flush)
> for a batch of kernel page tables instead of performing that expensive
> operation for each page table.
> 
> On x86, CONFIG_ASYNC_KERNEL_PGTABLE_FREE is selected if CONFIG_IOMMU_SVA
> is enabled, because both Intel and AMD IOMMU architectures could
> potentially cache kernel page table entries in their paging structure
> cache, regardless of the permission.

See below, I assume this is patch #7 material.

> 
> 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/Kconfig     |  1 +
>   mm/Kconfig           |  3 +++
>   include/linux/mm.h   | 16 +++++++++++++---
>   mm/pgtable-generic.c | 37 +++++++++++++++++++++++++++++++++++++
>   4 files changed, 54 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index fa3b616af03a..ded29ee848fd 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -279,6 +279,7 @@ config X86
>   	select HAVE_PCI
>   	select HAVE_PERF_REGS
>   	select HAVE_PERF_USER_STACK_DUMP
> +	select ASYNC_KERNEL_PGTABLE_FREE 	if IOMMU_SVA

That should belong into patch #7, no?

-- 
Cheers

David / dhildenb