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

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Message-ID: <Zi-sReFGhSKmHWNh@arm.com>
Date: Mon, 29 Apr 2024 15:18:45 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....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 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).

-- 
Catalin