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Date: Mon, 15 Apr 2024 18:02:26 +0200
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Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64
To: Ryan Roberts <ryan.roberts@arm.com>, Mark Rutland <mark.rutland@arm.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Alexander Shishkin <alexander.shishkin@linux.intel.com>,
 Jiri Olsa <jolsa@kernel.org>, Ian Rogers <irogers@google.com>,
 Adrian Hunter <adrian.hunter@intel.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Muchun Song <muchun.song@linux.dev>
Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240215121756.2734131-1-ryan.roberts@arm.com>
 <a41b0534-b841-42c2-8c06-41337c35347d@arm.com>
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 <70a36403-aefd-4311-b612-84e602465689@redhat.com>
 <f13d1e4d-1eea-4379-b683-4d736ad99c2c@arm.com>
 <3e50030d-2289-4470-a727-a293baa21618@redhat.com>
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <c880ba19-93ab-492b-a720-7272a1f8756d@arm.com>
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>>> 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.
> 
> But you can still see false-negatives for access and dirty...

Yes.

> 
>>
>> The whole races you describe with concurrent folding/unfolding/ ... are irrelevant.
> 
> And I think I convinced myself that you will only see false-negatives with
> today's arm64 ptep_get(). But an order or magnitude fewer than with your
> proposal (assuming 16 ptes per contpte block, and the a/d bits are in one of those).
> 
>>
>> To me that sounds ... much simpler ;) But again, just something I've been
>> thinking about.
> 
> OK so this approach upgrades my "I'm fairly sure we never see false-positives"
> to "we definitely never see false-positives". But it certainly increases the
> quantity of false-negatives.

Yes.

> 
>>
>> The reuse of pte_get_lockless() outside GUP code might not have been the wisest
>> choice.
>>
> 
> If you want to go down the ptep_get_gup_fast() route, you've still got to be
> able to spec it, and I think it will land pretty close to my most recent stab at
> respec'ing ptep_get_lockless() a couple of replies up on this thread.
> 
> Where would your proposal leave the KVM use case? If you call it
> ptep_get_gup_fast() presumably you wouldn't want to use it for KVM? So it would
> be left with ptep_get()...

It's using GUP-fast.

> 
> Sorry this thread is getting so long. Just to summarise, I think there are
> currently 3 solutions on the table:
> 
>    - ptep_get_lockless() remains as is
>    - ptep_get_lockless() wraps ptep_get()
>    - ptep_get_lockless() wraps __ptep_get() (and gets a gup_fast rename)
> 
> Based on discussion so far, that's also the order of my preference.

(1) seems like the easiest thing to do.

> 
> Perhaps its useful to enumerate why we dislike the current ptep_get_lockless()?

Well, you sent that patch series with "that aims to reduce the cost and 
complexity of ptep_get_lockless() for arm64". (2) and (3) would achieve 
that. :)

-- 
Cheers,

David / dhildenb