Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

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

On 15/04/2024 15:23, David Hildenbrand wrote:
> On 15.04.24 15:30, Ryan Roberts wrote:
>> On 15/04/2024 11:57, David Hildenbrand wrote:
>>> On 15.04.24 11:28, Ryan Roberts wrote:
>>>> On 12/04/2024 21:16, David Hildenbrand wrote:
>>>>>>
>>>>>> Yes agreed - 2 types; "lockless walkers that later recheck under PTL" and
>>>>>> "lockless walkers that never take the PTL".
>>>>>>
>>>>>> Detail: the part about disabling interrupts and TLB flush syncing is
>>>>>> arch-specifc. That's not how arm64 does it (the hw broadcasts the TLBIs). But
>>>>>> you make that clear further down.
>>>>>
>>>>> Yes, but disabling interrupts is also required for RCU-freeing of page tables
>>>>> such that they can be walked safely. The TLB flush IPI is arch-specific and
>>>>> indeed to sync against PTE invalidation (before generic GUP-fast).
>>>>> [...]
>>>>>
>>>>>>>>
>>>>>>>> Could it be this easy? My head is hurting...
>>>>>>>
>>>>>>> I think what has to happen is:
>>>>>>>
>>>>>>> (1) pte_get_lockless() must return the same value as ptep_get() as long as
>>>>>>> there
>>>>>>> are no races. No removal/addition of access/dirty bits etc.
>>>>>>
>>>>>> Today's arm64 ptep_get() guarantees this.
>>>>>>
>>>>>>>
>>>>>>> (2) Lockless page table walkers that later verify under the PTL can handle
>>>>>>> serious "garbage PTEs". This is our page fault handler.
>>>>>>
>>>>>> This isn't really a property of a ptep_get_lockless(); its a statement
>>>>>> about a
>>>>>> class of users. I agree with the statement.
>>>>>
>>>>> Yes. That's a requirement for the user of ptep_get_lockless(), such as page
>>>>> fault handlers. Well, mostly "not GUP".
>>>>>
>>>>>>
>>>>>>>
>>>>>>> (3) Lockless page table walkers that cannot verify under PTL cannot handle
>>>>>>> arbitrary garbage PTEs. This is GUP-fast. Two options:
>>>>>>>
>>>>>>> (3a) pte_get_lockless() can atomically read the PTE: We re-check later if
>>>>>>> the
>>>>>>> atomically-read PTE is still unchanged (without PTL). No IPI for TLB flushes
>>>>>>> required. This is the common case. HW might concurrently set access/dirty
>>>>>>> bits,
>>>>>>> so we can race with that. But we don't read garbage.
>>>>>>
>>>>>> Today's arm64 ptep_get() cannot garantee that the access/dirty bits are
>>>>>> consistent for contpte ptes. That's the bit that complicates the current
>>>>>> ptep_get_lockless() implementation.
>>>>>>
>>>>>> But the point I was trying to make is that GUP-fast does not actually care
>>>>>> about
>>>>>> *all* the fields being consistent (e.g. access/dirty). So we could spec
>>>>>> pte_get_lockless() to say that "all fields in the returned pte are
>>>>>> guarranteed
>>>>>> to be self-consistent except for access and dirty information, which may be
>>>>>> inconsistent if a racing modification occured".
>>>>>
>>>>> We *might* have KVM in the future want to check that a PTE is dirty, such that
>>>>> we can only allow dirty PTEs to be writable in a secondary MMU. That's not
>>>>> there
>>>>> yet, but one thing I was discussing on the list recently. Burried in:
>>>>>
>>>>> https://lkml.kernel.org/r/20240320005024.3216282-1-seanjc@google.com
>>>>>
>>>>> We wouldn't care about racing modifications, as long as MMU notifiers will
>>>>> properly notify us when the PTE would lose its dirty bits.
>>>>>
>>>>> But getting false-positive dirty bits would be problematic.
>>>>>
>>>>>>
>>>>>> This could mean that the access/dirty state *does* change for a given page
>>>>>> while
>>>>>> GUP-fast is walking it, but GUP-fast *doesn't* detect that change. I *think*
>>>>>> that failing to detect this is benign.
>>>>>
>>>>> I mean, HW could just set the dirty/access bit immediately after the check. So
>>>>> if HW concurrently sets the bit and we don't observe that change when we
>>>>> recheck, I think that would be perfectly fine.
>>>>
>>>> Yes indeed; that's my point - GUP-fast doesn't care about access/dirty (or
>>>> soft-dirty or uffd-wp).
>>>>
>>>> But if you don't want to change the ptep_get_lockless() spec to explicitly
>>>> allow
>>>> this (because you have the KVM use case where false-positive dirty is
>>>> problematic), then I think we are stuck with ptep_get_lockless() as implemented
>>>> for arm64 today.
>>>
>>> At least regarding the dirty bit, we'd have to guarantee that if
>>> ptep_get_lockless() returns a false-positive dirty bit, that the PTE recheck
>>> would be able to catch that.
>>>
>>> Would that be possible?
>>
>> Hmm maybe. My head hurts. Let me try to work through some examples...
>>
>>
>> Let's imagine for this example, a contpte block is 4 PTEs. Lat's say PTEs 0, 1,
>> 2 and 3 initially contpte-map order-2 mTHP, FolioA. The dirty state is stored in
>> PTE0 for the contpte block, and it is dirty.
>>
>> Now let's say there are 2 racing threads:
>>
>>    - ThreadA is doing a GUP-fast for PTE3
>>    - ThreadB is remapping order-0 FolioB at PTE0
>>
>> (ptep_get_lockless() below is actaully arm64's ptep_get() for the sake of the
>> example - today's arm64 ptep_get_lockless() can handle the below correctly).
>>
>> ThreadA                    ThreadB
>> =======                    =======
>>
>> gup_pte_range()
>>    pte1 = ptep_get_lockless(PTE3)
>>      READ_ONCE(PTE3)
>>                     mmap(PTE0)
>>                       clear_pte(PTE0)
>>                         unfold(PTE0 - PTE3)
>>                           WRITE_ONCE(PTE0, 0)
>>                           WRITE_ONCE(PTE1, 0)
>>                           WRITE_ONCE(PTE2, 0)
>>      READ_ONCE(PTE0) (for a/d) << CLEAN!!
>>      READ_ONCE(PTE1) (for a/d)
>>      READ_ONCE(PTE2) (for a/d)
>>      READ_ONCE(PTE3) (for a/d)
>>    <do speculative work with pte1 content>
>>    pte2 = ptep_get_lockless(PTE3)
>>      READ_ONCE(PTE3)
>>      READ_ONCE(PTE0) (for a/d)
>>      READ_ONCE(PTE1) (for a/d)
>>      READ_ONCE(PTE2) (for a/d)
>>      READ_ONCE(PTE3) (for a/d)
>>    true = pte_same(pte1, pte2)
>>                           WRITE_ONCE(PTE3, 0)
>>                           TLBI
>>                           WRITE_ONCE(PTE0, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE1, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE2, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE3, <orig & ~CONT>)
>>                         WRITE_ONCE(PTE0, 0)
>>                       set_pte_at(PTE0, <new>)
>>
>> This example shows how a *false-negative* can be returned for the dirty state,
>> which isn't detected by the check.
>>
>> I've been unable to come up with an example where a *false-positive* can be
>> returned for dirty state without the second ptep_get_lockless() noticing. In
>> this second example, let's assume everything is the same execpt FolioA is
>> initially clean:
>>
>> ThreadA                    ThreadB
>> =======                    =======
>>
>> gup_pte_range()
>>    pte1 = ptep_get_lockless(PTE3)
>>      READ_ONCE(PTE3)
>>                     mmap(PTE0)
>>                       clear_pte(PTE0)
>>                         unfold(PTE0 - PTE3)
>>                           WRITE_ONCE(PTE0, 0)
>>                           WRITE_ONCE(PTE1, 0)
>>                           WRITE_ONCE(PTE2, 0)
>>                           WRITE_ONCE(PTE3, 0)
>>                           TLBI
>>                           WRITE_ONCE(PTE0, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE1, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE2, <orig & ~CONT>)
>>                           WRITE_ONCE(PTE3, <orig & ~CONT>)
>>                         WRITE_ONCE(PTE0, 0)
>>                       set_pte_at(PTE0, <new>)
>>                     write to FolioB - HW sets PTE0's dirty
>>      READ_ONCE(PTE0) (for a/d) << DIRTY!!
>>      READ_ONCE(PTE1) (for a/d)
>>      READ_ONCE(PTE2) (for a/d)
>>      READ_ONCE(PTE3) (for a/d)
>>    <do speculative work with pte1 content>
>>    pte2 = ptep_get_lockless(PTE3)
>>      READ_ONCE(PTE3)           << BUT THIS IS FOR FolioB
>>      READ_ONCE(PTE0) (for a/d)
>>      READ_ONCE(PTE1) (for a/d)
>>      READ_ONCE(PTE2) (for a/d)
>>      READ_ONCE(PTE3) (for a/d)
>>    false = pte_same(pte1, pte2) << So this fails
>>
>> The only way I can see false-positive not being caught in the second example is
>> if ThreadB subseuently remaps the original folio, so you have an ABA scenario.
>> But these lockless table walkers are already suseptible to that.
>>
>> I think all the same arguments can be extended to the access bit.
>>
>>
>> For me this is all getting rather subtle and difficult to reason about and even
>> harder to spec in a comprehensible way. The best I could come up with is:
>>
>> "All fields in the returned pte are guarranteed to be self-consistent except for
>> access and dirty information, which may be inconsistent if a racing modification
>> occured. Additionally it is guranteed that false-positive access and/or dirty
>> information is not possible if 2 calls are made and both ptes are the same. Only
>> false-negative access and/or dirty information is possible in this scenario."
>>
>> which is starting to sound bonkers. Personally I think we are better off at this
>> point, just keeping today's arm64 ptep_get_lockless().
> 
> Remind me again, does arm64 perform an IPI broadcast during a TLB flush that
> would sync against GUP-fast?

No, the broadcast is in HW. There is no IPI.



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