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Date: Tue, 13 Feb 2024 11:01:09 +0100
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Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
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To: Ryan Roberts <ryan.roberts@arm.com>, 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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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <64395ae4-3a7d-45dd-8f1d-ea6b232829c5@arm.com>
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On 12.02.24 21:38, Ryan Roberts wrote:
> [...]
> 
>>>>> +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);

Please use all the lines you need ;)

if (IS_ENABLED(CONFIG_EFI) && unlikely(mm == &efi_mm))
	return false;
return mm != &init_mm;

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

We could also mark MMs in some way to be special.

return mm->is_user;

Then it's easy to extend.

-- 
Cheers,

David / dhildenb