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[2003:cb:c706:d800:568a:6ea7:5272:797c]) by smtp.gmail.com with ESMTPSA id w5-20020a5d4045000000b00346ceb9e060sm11638411wrp.103.2024.04.15.09.02.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Apr 2024 09:02:28 -0700 (PDT) Message-ID: Date: Mon, 15 Apr 2024 18:02:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64 To: Ryan Roberts , Mark Rutland , Catalin Marinas , Will Deacon , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Andrew Morton , Muchun Song Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240215121756.2734131-1-ryan.roberts@arm.com> <8bd9e136-8575-4c40-bae2-9b015d823916@redhat.com> <86680856-2532-495b-951a-ea7b2b93872f@arm.com> <35236bbf-3d9a-40e9-84b5-e10e10295c0c@redhat.com> <4fba71aa-8a63-4a27-8eaf-92a69b2cff0d@arm.com> <5a23518b-7974-4b03-bd6e-80ecf6c39484@redhat.com> <81aa23ca-18b1-4430-9ad1-00a2c5af8fc2@arm.com> <70a36403-aefd-4311-b612-84e602465689@redhat.com> <3e50030d-2289-4470-a727-a293baa21618@redhat.com> <772de69a-27fa-4d39-a75d-54600d767ad1@arm.com> <969dc6c3-2764-4a35-9fa6-7596832fb2a3@redhat.com> <11b1c25b-3e20-4acf-9be5-57b508266c5b@redhat.com> <89e04df9-6a2f-409c-ae7d-af1f91d0131e@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: A478112003A X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: q5maxxs59w6oi7dznbinq7qcu7yj6xy4 X-HE-Tag: 1713196952-383209 X-HE-Meta: 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 2chptBct Weyraca6AJwqVlV1VGhNy3Y+CVe4T6SyLVgdiFj54sQsXC9fJJQxp7thWh71D813RXHcj1Kcw6GU/n0XJ0A2Venh7MrTHfB52Ds8K+98D+gqCNvkiqK90xy3hZRRCntP6erSjTd4UhpHZ9MXzvk+cWJ8jid1psKdFck3UDgPC/Q84TMviTouI3QXJ6lOM+dFQWipK7eXfkjf0WV6VctvrdqrinmUZh0c0K6gZZPuNQBfZUaya1HY7uk7GPWkEGvmbenKO2sBCsAjDkyiUXvQnkOxR2wjxH5ScXj65DMKYaQq/ADhW7zHfpD1w8fm2nsP5R0aBUBL7UCxHl59za2fipIJDtngjn91tQin/OO2gffA8pQCyilYPuF6DZyq4OJUXw7v17Jzfr9igLsl6YQZENFOIscG1KzWtXuZNNFA1OBf0awo6809KuiI49QwQxopDtEeeLQGv/J92186TBzKjnrKboQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> 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