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

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Message-ID: <ZjoUSjWhEbohMfX0@arm.com>
Date: Tue, 7 May 2024 12:45:14 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, 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 Tue, May 07, 2024 at 12:17:18PM +0100, Ryan Roberts wrote:
> On 07/05/2024 12:07, Will Deacon wrote:
> > 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 ;-)

Fine by me as well to go in 6.10. Will is taking the blame if it all
falls apart ;).

-- 
Catalin