linux-kernel - Re: [PATCH v3 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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Message-ID: <08e7cb79-d175-448e-9b6f-07e9f07ba042@linux.intel.com>
Date: Wed, 27 Aug 2025 14:34:56 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>, "Tian, Kevin"
 <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>, Jann Horn <jannh@...gle.com>,
 Vasant Hegde <vasant.hegde@....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>, "Lai, Yi1" <yi1.lai@...el.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "security@...nel.org" <security@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] iommu/sva: Invalidate KVA range on kernel TLB
 flush

On 8/27/25 00:21, Dave Hansen wrote:
> On 8/25/25 19:49, Baolu Lu wrote:
>> What's strange is that order is almost always 0, except in the path of
>> remove_pmd_table() -> free_hugepage_table(), where order can be greater
>> than 0. However, in this context path, free_hugepage_table() isn't used
>> to free a page table page itself. Instead, it's used to free the actual
>> pages that a leaf PMD is pointing to.
> When I first read this, I thought you meant that remove_pmd_table() was
> trying to free a high-order page composed of multiple pte pages. I don't
> think it is doing that.
> 
> Just to be clear: remove_pmd_table() has two modes:
> 
> 	1. The pmd_leaf() code that calls free_hugepage_table(). It is
>             removing the memory pointed to by a PMD*entry*. It is*NOT*
> 	   removing page tables themselves.
> 	2. The !pmd_leaf() code that does remove the pointers to
> 	   individual pte pages and frees them via free_pte_table().
> 
> *Neither* of these is freeing high-order page tables pages. It either
> frees plain old kernel data pages or it frees an order-0 page table page.
> 
> In other words, the pmd_leaf() (mode #1) code is irrelevant to us. Those
> entries are all _PAGE_KERNEL so IOMMU accesses with user privilege can't
> do anything with them.

I have the same understanding, so case #3 (higher-order non-compound
page table pages) doesn't exist. Page table pages always have an order
equal to 0.

I will head in the direction you pointed out in your previous reply and
post the change later for further review. Thank you for the insights.

> 
> Or have I just horribly confused myself?

Thanks,
baolu