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Date: Mon, 12 Feb 2024 20:38:59 +0000
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Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
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From: Ryan Roberts <ryan.roberts@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
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 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
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[...]

>>>> +static inline bool mm_is_user(struct mm_struct *mm)
>>>> +{
>>>> +	/*
>>>> +	 * Don't attempt to apply the contig bit to kernel mappings, because
>>>> +	 * dynamically adding/removing the contig bit can cause page faults.
>>>> +	 * These racing faults are ok for user space, since they get serialized
>>>> +	 * on the PTL. But kernel mappings can't tolerate faults.
>>>> +	 */
>>>> +	return mm != &init_mm;
>>>> +}
>>>
>>> We also have the efi_mm as a non-user mm, though I don't think we manipulate
>>> that while it is live, and I'm not sure if that needs any special handling.
>>
>> Well we never need this function in the hot (order-0 folio) path, so I think I
>> could add a check for efi_mm here with performance implication. It's probably
>> safest to explicitly exclude it? What do you think?
> 
> Oops: This should have read "I think I could add a check for efi_mm here
> *without* performance implication"

It turns out that efi_mm is only defined when CONFIG_EFI is enabled. I can do this:

return mm != &init_mm && (!IS_ENABLED(CONFIG_EFI) || mm != &efi_mm);

Is that acceptable? This is my preference, but nothing else outside of efi
references this symbol currently.

Or perhaps I can convince myself that its safe to treat efi_mm like userspace.
There are a couple of things that need to be garanteed for it to be safe:

  - The PFNs of present ptes either need to have an associated struct page or
    need to have the PTE_SPECIAL bit set (either pte_mkspecial() or
    pte_mkdevmap())

  - Live mappings must either be static (no changes that could cause fold/unfold
    while live) or the system must be able to tolerate a temporary fault

Mark suggests efi_mm is not manipulated while live, so that meets the latter
requirement, but I'm not sure about the former?

Thanks,
Ryan