Re: [PATCH v1 1/2] arm64/mm: Move PTE_PROT_NONE and PMD_PRESENT_INVALID

[Ryan Roberts <ryan.roberts@arm.com>]

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...

> 
>>
>> For ptes, it doesn't matter, we can assume that PTE_PRESENT_INVALID
>> means pte_protnone(). For pmds, however, we can end up with
>> pmd_protnone(pmd_mkinvalid(pmd)) == true for any of the PAGE_*
>> permissions encoded into a valid pmd. That's where a dedicated
>> PTE_PROT_NONE bit helped.
> 
> Yes agreed.
> 
>>
>> Let's say a CPU starts splitting a pmd and does a pmdp_invalidate*()
>> first to set PTE_PRESENT_INVALID. A different CPU gets a fault and since
>> the pmd is present, it goes and checks pmd_protnone() which returns
>> true, ending up on do_huge_pmd_numa_page() path. Maybe some locks help
>> but it looks fragile to rely on them.
>>
>> So I think for protnone we need to check some other bits (like USER and
>> UXN) in addition to PTE_PRESENT_INVALID.
> 
> Yes 100% agree. But using PTE_WRITE|PTE_RDONLY==0b00 is just as valid for that
> purpose, I think?
> 
>>
>>> This will subtly change behaviour in an edge case though. Imagine:
>>>
>>> pte_t pte;
>>>
>>> pte = pte_modify(pte, PAGE_NONE);
>>> pte = pte_mkwrite_novma(pte);
>>> WARN_ON(pte_protnone(pte));
>>>
>>> Should that warning fire or not? Previously, because we had a dedicated bit for
>>> PTE_PROT_NONE it would fire. With my proposed change it will not fire. To me
>>> it's more intuitive if it doesn't fire. Regardless there is no core code that
>>> ever does this. Once you have a protnone pte, its terminal - nothing ever
>>> modifies it with these helpers AFAICS.
>>
>> I don't think any core code should try to make page a PAGE_NONE pte
>> writeable.
> 
> I looked at some other arches; some (at least alpha and hexagon) will not fire
> this warning because they have R and W bits and 0b00 means NONE. Others (x86)
> will fire it because they have an explicit NONE bit and don't remove it on
> permission change. So I conclude its UB and fine to do either.
> 
>>
>>> Personally I think this is a nice tidy up that saves a SW bit in both present
>>> and swap ptes. What do you think? (I'll just post the series if its easier to
>>> provide feedback in that context).
>>
>> It would be nice to tidy this up and get rid of PTE_PROT_NONE as long as
>> it doesn't affect the pmd case I mentioned above.
>>
>>>>> 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().
>>>
>>> I've posted a fix here:
>>> https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.roberts@arm.com/
>>>
>>> My position is that you shouldn't be calling pmd_mkinvalid() on a non-present pmd.
>>
>> I agree, thanks.
>>
>