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

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Date: Fri, 3 May 2024 13:53:59 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Will Deacon <will@...nel.org>
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

On 03/05/2024 13:01, Will Deacon wrote:
> 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.

Yep, I'll try to get that out this afternoon. Thanks!

> 
> Will