linux-kernel - Re: [PATCH v1 1/2] arm64/mm: Move PTE_PROT_NONE and PMD_PRESENT

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Message-ID: <5ea44a93-08a8-4385-b684-bf6fcd007bfb@arm.com>
Date: Mon, 29 Apr 2024 16:04:39 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: David Hildenbrand <david@...hat.com>, 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>, 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 v1 1/2] arm64/mm: Move PTE_PROT_NONE and
 PMD_PRESENT_INVALID

On 29/04/2024 15:18, Catalin Marinas wrote:
> On Mon, Apr 29, 2024 at 02:23:35PM +0100, Ryan Roberts wrote:
>> On 29/04/2024 14:01, Ryan Roberts wrote:
>>> On 29/04/2024 13:38, Catalin Marinas wrote:
>>>> On Mon, Apr 29, 2024 at 11:04:53AM +0100, Ryan Roberts wrote:
>>>>> On 26/04/2024 15:48, Catalin Marinas wrote:
>>>>>> On Thu, Apr 25, 2024 at 11:37:42AM +0100, Ryan Roberts wrote:
>>>>>>> Also, IMHO we shouldn't really need to reserve PMD_PRESENT_INVALID for swap
>>>>>>> ptes; it would be cleaner to have one bit that defines "present" when valid is
>>>>>>> clear (similar to PTE_PROT_NONE today) then another bit which is only defined
>>>>>>> when "present && !valid" which tells us if this is PTE_PROT_NONE or
>>>>>>> PMD_PRESENT_INVALID (I don't think you can ever have both at the same time?).
>>>>>>
>>>>>> I think this make sense, maybe rename the above to PTE_PRESENT_INVALID
>>>>>> and use it for both ptes and pmds.
>>>>>
>>>>> Yep, sounds good. I've already got a patch to do this, but it's exposed a bug in
>>>>> core-mm so will now fix that before I can validate my change. see
>>>>> https://lore.kernel.org/linux-arm-kernel/ZiuyGXt0XWwRgFh9@x1n/
>>>>>
>>>>> With this in place, I'm proposing to remove PTE_PROT_NONE entirely and instead
>>>>> represent PROT_NONE as a present but invalid pte (PTE_VALID=0, PTE_INVALID=1)
>>>>> with both PTE_WRITE=0 and PTE_RDONLY=0.
>>>>>
>>>>> While the HW would interpret PTE_WRITE=0/PTE_RDONLY=0 as "RW without dirty bit
>>>>> modification", this is not a problem as the pte is invalid, so the HW doesn't
>>>>> interpret it. And SW always uses the PTE_WRITE bit to interpret the writability
>>>>> of the pte. So PTE_WRITE=0/PTE_RDONLY=0 was previously an unused combination
>>>>> that we now repurpose for PROT_NONE.
>>>>
>>>> Why not just keep the bits currently in PAGE_NONE (PTE_RDONLY would be
>>>> set) and check PTE_USER|PTE_UXN == 0b01 which is a unique combination
>>>> for PAGE_NONE (bar the kernel mappings).
>>>
>>> Yes I guess that works. I personally prefer my proposal because it is more
>>> intuitive; you have an R bit and a W bit, and you encode RO, WR, and NONE. But
>>> if you think reusing the kernel mapping check (PTE_USER|PTE_UXN == 0b01) is
>>> preferable, then I'll go with that.
>>
>> Ignore this - I looked at your proposed approach and agree it's better. I'll use
>> `PTE_USER|PTE_UXN==0b01`. Posting shortly...
> 
> You nearly convinced me until I read your second reply ;). The
> PTE_WRITE|PTE_RDONLY == 0b00 still has the mkwrite problem if we care
> about (I don't think it can happen though).

Yes, just to clearly enumerate the reasons I prefer your approach:

 - PTE_RDONLY is also used for HW dirty bit. I had to add a conditional to
   pte_mkclean() for my scheme to prevent pte_mkclean() on a PROT_NONE pte
   eroneously making it RO. No such problem with your scheme.

 - With my scheme, we have the mkwrite problem, as you call it. Although, as I
   said some arches already have this semantic, so I don't think its a problem.
   But with your scheme we keep the existing arm64 semantics so it reduces risk
   of a problem in a corner I overlooked.

Anyway, I've posted the v2. Take a look when you get time - perhaps we can get
it into v6.10?