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

[David Hildenbrand <david@redhat.com>]

On 15.04.24 16:34, Ryan Roberts wrote:
> 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.

Okay ...

I agree that the semantics are a bit weird, but if we could get rid of 
ptep_get_lockless() on arm64, that would also be nice.


Something I've been thinking of ... just to share what I've had in mind. 
The two types of users we currently have are:

(1) ptep_get_lockless() followed by ptep_get() check under PTL.

(2) ptep_get_lockless() followed by ptep_get() check without PTL.

What if we had the following instead:

(1) ptep_get_lockless() followed by ptep_get() check under PTL.

(2) ptep_get_gup_fast() followed by ptep_get_gup_fast() check without
     PTL.

And on arm64 let

(1) ptep_get_lockless() be ptep_get()

(2) ptep_get_gup_fast() be __ptep_get().

That would mean, that (2) would not care if another cont-pte is dirty, 
because we don't collect access+dirty bits. That way, we avoid any races 
with concurrent unfolding etc. The only "problamtic" thing is that 
pte_mkdirty() -> set_ptes() would have to set all cont-PTEs dirty, even 
if any of these already is dirty.

-- 
Cheers,

David / dhildenb