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

[David Hildenbrand]

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?

-- 
Cheers,

David / dhildenb