linux-kernel - Re: [PATCH v1] x86/mm/pat: fix VM

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Date: Tue, 12 Mar 2024 19:22:56 +0000
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	Wupeng Ma <mawupeng1@...wei.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings

On Tue, Mar 12, 2024 at 07:11:18PM +0100, David Hildenbrand wrote:
> PAT handling won't do the right thing in COW mappings: the first PTE
> (or, in fact, all PTEs) can be replaced during write faults to point at
> anon folios. Reliably recovering the correct PFN and cachemode using
> follow_phys() from PTEs will not work in COW mappings.

I guess the first question is: Why do we want to support COW mappings
of VM_PAT areas?  What breaks if we just disallow it?