linux-kernel - Re: [PATCH v4 0/4] arm64/mm: Enable userfaultfd write-protect

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Message-ID: <3764382b-9fa6-4e94-ad5b-2e22e3e7c71a@arm.com>
Date: Tue, 7 May 2024 12:17:18 +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 v4 0/4] arm64/mm: Enable userfaultfd write-protect

On 07/05/2024 12:07, Will Deacon wrote:
> Hi Ryan,
> 
> On Fri, May 03, 2024 at 03:45:58PM +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 4 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 [4] - or some other usage).
>>
>> ---
>>
>> This applies on top of v6.9-rc5.
> 
> I chucked this into the CI on Friday and it looks to have survived the
> long weekend, so I've gone ahead and merged it into for-next/core. Short
> of any last minute failures (touch wood), this should land in 6.10.

Oh great - thanks!

Catalin was previously proposing to hold this until 6.11 - I'll leave you two to
fight it out in case that's still his preference ;-)

> 
> Thanks!
> 
> Will