linux-kernel - Re: [PATCH v3 0/3] arm64/mm: Enable userfaultfd write-protect

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Date: Fri, 3 May 2024 13:01:04 +0100
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Joey Gouly <joey.gouly@....com>, Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>,
	Mike Rapoport <rppt@...ux.ibm.com>,
	Shivansh Vij <shivanshvij@...look.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] arm64/mm: Enable userfaultfd write-protect

Hi Ryan,

On Wed, May 01, 2024 at 03:54:16PM +0100, Ryan Roberts wrote:
> This series adds uffd write-protect support for arm64.
> 
> Previous attempts to add uffd-wp (and soft-dirty) have failed because of a
> perceived lack of available PTE SW bits. However it actually turns out that
> there are 2 available but they are hidden. PTE_PROT_NONE was previously
> occupying a SW bit, but can be moved, freeing up the SW bit. Bit 63 is marked as
> "IGNORED" in the Arm ARM, but it does not currently indicate "reserved for SW
> use" like it does for the other SW bits. I've confirmed with the spec owner that
> this is an oversight; the bit is intended to be reserved for SW use and the spec
> will clarify this in a future update.
> 
> So now we have two spare bits; patch 3 enables uffd-wp on arm64, using the SW
> bit freed up by moving PTE_PROT_NONE. This leaves bit 63 spare for future use
> (e.g. soft-dirty - see RFC at [3] - or some other usage).

Thanks, I think this series looks really good now.

>From your discussion with Anshuman, it sounds like you're going to post
a new version with the first patch split up so I'll wait for that.

Will