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

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Date: Wed, 8 May 2024 11:14:24 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 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>, 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 08/05/2024 11:00, Anshuman Khandual wrote:
> 
> 
> On 5/7/24 16:37, 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.
> 
> It would be great to have some memory migration tests (including THP and HugeTLB)
> thrown at this series, which should test the mapped, migration entry transitions
> etc. But not sure if there are any such tests off the shelf and readily available
> in the CI system.

The "private_anon_thp" migration test in mm selftests is doing that for THP. and
invoking pmd_mkinvalid() as I recall; that's what originally led to me finding
the pmd_mkinvalid()-on-a-swap-pmd bug. There is nothing in that suite for
HugeTLB though - happy to run if someone can recommend anything.