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Thu, 23 Feb 2023 11:58:20 -0800 (PST) Date: Thu, 23 Feb 2023 11:58:19 -0800 In-Reply-To: Mime-Version: 1.0 References: <20230217041230.2417228-1-yuzhao@google.com> <20230217041230.2417228-6-yuzhao@google.com> Message-ID: Subject: Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear_young() From: Sean Christopherson To: Yu Zhao Cc: Johannes Weiner , Andrew Morton , Paolo Bonzini , Jonathan Corbet , Michael Larabel , kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-mm@google.com Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 56A7714000F X-Stat-Signature: x1ata3ze1gq87qr364gx78ygai1ebtu7 X-HE-Tag: 1677182302-346711 X-HE-Meta: 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 vl0zRoQG 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 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: On Thu, Feb 23, 2023, Yu Zhao wrote: > On Thu, Feb 23, 2023 at 12:11=E2=80=AFPM Sean Christopherson wrote: > > > > On Thu, Feb 23, 2023, Yu Zhao wrote: > > > > As alluded to in patch 1, unless batching the walks even if KVM doe= s _not_ support > > > > a lockless walk is somehow _worse_ than using the existing mmu_noti= fier_clear_flush_young(), > > > > I think batching the calls should be conditional only on LRU_GEN_SP= TE_WALK. Or > > > > if we want to avoid batching when there are no mmu_notifier listene= rs, probe > > > > mmu_notifiers. But don't call into KVM directly. > > > > > > I'm not sure I fully understand. Let's present the problem on the MM > > > side: assuming KVM supports lockless walks, batching can still be > > > worse (very unlikely), because GFNs can exhibit no memory locality at > > > all. So this option allows userspace to disable batching. > > > > I'm asking the opposite. Is there a scenario where batching+lock is wo= rse than > > !batching+lock? If not, then don't make batching depend on lockless wa= lks. >=20 > Yes, absolutely. batching+lock means we take/release mmu_lock for > every single PTE in the entire VA space -- each small batch contains > 64 PTEs but the entire batch is the whole KVM. Who is "we"? I don't see anything in the kernel that triggers walking the = whole VMA, e.g. lru_gen_look_around() limits the walk to a single PMD. I feel li= ke I'm missing something...