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

[David Hildenbrand <david@redhat.com>]

On 15.04.24 17:17, Ryan Roberts wrote:
> On 15/04/2024 15:58, David Hildenbrand wrote:
>> 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.
> 
> I don't think the "problematic" thing is actually a problem; set_ptes() will
> always set the dirty bit to the same value for all ptes it covers (and if you do
> set_ptes() on a partial contpte block, it will be unfolded first). Although I
> suspect I've misunderstood what you meant there...

It's more code like that following that I am concerned about.

if (pte_dirty()) {
	/* Great, nothing to do */
} else
	mte_mkdirty();
	set_ptes();
	...
}

> 
> The potential problem I see with this is that the Arm ARM doesn't specify which
> PTE of a contpte block the HW stores a/d in. So the HW _could_ update them
> randomly and this could spuriously increase your check failure rate. In reality
> I believe most implementations will update the PTE for the address that caused
> the TLB to be populated. But in some cases, you could have eviction (due to
> pressure or explicit invalidation) followed by re-population due to faulting on
> a different page of the contpte block. In this case you would see this type of
> problem too.
> 
> But ultimately, isn't this basically equivalent to ptep_get_lockless() returning
> potentially false-negatives for access and dirty? Just with a much higher chance
> of getting a false-negative. How is this helping?

You are performing an atomic read like GUP-fast wants you to. So there 
are no races to worry about like on other architectures: HW might *set* 
the dirty bit concurrently, but that's just fine.

The whole races you describe with concurrent folding/unfolding/ ... are 
irrelevant.

To me that sounds ... much simpler ;) But again, just something I've 
been thinking about.

The reuse of pte_get_lockless() outside GUP code might not have been the 
wisest choice.

-- 
Cheers,

David / dhildenb


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