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

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Date: Fri, 26 Apr 2024 15:48:15 +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 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.

> But there is a problem with this: __split_huge_pmd_locked() calls
> pmdp_invalidate() for a pmd before it determines that it is pmd_present(). So
> the PMD_PRESENT_INVALID can be set in a swap pte today. That feels wrong to me,
> but was trying to avoid the whole thing unravelling so didn't persue.

Maybe what's wrong is the arm64 implementation setting this bit on a
swap/migration pmd (though we could handle this in the core code as
well, it depends what the other architectures do). The only check for
the PMD_PRESENT_INVALID bit is in the arm64 code and it can be absorbed
into the pmd_present() check. I think it is currently broken as
pmd_present() can return true for a swap pmd after pmd_mkinvalid().

So I don't think we lose anything if pmd_mkinvalid() skips any bit
setting when !PTE_VALID. Maybe it even fixes some corner case we never
hit yet (like pmd_present() on a swap/migration+invalid pmd).

-- 
Catalin