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bh=Kt0LuE28XbF55ehOiA2DPZrjnmRa799DCvT54SE1CAI=; b=BALx+mjCpZ9MRlyfEvhHPl+45PHhb/m2w7VIZ5MYQwHcRbUEqkUEa0kKp5ybLBD4bu IlF/apY9WlWVrJz4X0e9CJxvCMa4x2tNBxZmkE3OQ4VIVAb7OkKTGgLJPHp5VMqJA2xU UY1+M7e83uFn0pjsAchrGYBNkdW1NbMvvmBFrU5UCHoOQzoGoi1SzVEp3vbW5qhAZxH1 HF4ILQfY9h4LrhukBpc51SY3T9bDf//j77xKOo9557uffU/sT3RMAzozAikXjqoyiMm4 GukY43lS07L8CKzWj7dF2FgqFoTEyibNas00OfopYVYpnFpmVJ8hFZXBrfANSjK8/kuU MPpQ== X-Forwarded-Encrypted: i=1; AKwUvBzZ7lT+YFcgSmM8hnmK3bY15IHu9B4j1BTbgAIDGnJy4pOKxhRGZ5UU/oRwpEBhf3IscyaLGwjU/A==@kvack.org X-Gm-Message-State: AFuF++meHcZJDVQ8I+wMxgwrxD1ouaUriJyGx7xMnHyIuAfZGkbDUdMo j/Us7nms6gAwdo1EOgxXTJ7Huj41gqFInXS8CVnm4sFYmTY6SzNs172xjxTME6P0ba6q+fs++/c yXfl4hg== X-Received: from pjtn13.prod.google.com ([2002:a17:90a:c68d:b0:39a:e7d1:b083]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2d06:b0:38e:5964:97a8 with SMTP id 98e67ed59e1d1-39ae83ec5demr364047a91.16.1788289972383; Tue, 01 Sep 2026 12:12:52 -0700 (PDT) Date: Tue, 1 Sep 2026 12:12:51 -0700 In-Reply-To: <8cd4aa65b1b499d4c614aa4cba445b6aeb89acf7.camel@infradead.org> Mime-Version: 1.0 References: <333c4cdb-8f34-4e3f-a47f-961f47e089a0@paulmck-laptop> <11dcf5a87125451897982f728c1f65a5151987e4.camel@infradead.org> <69567965-48fd-48c5-abb6-0699d7f5bd16@paulmck-laptop> <124af87fb49c267eeb26b8b4d952b9b1a5b3fb68.camel@infradead.org> <9427a8e0-3ba6-4f31-a35d-54429bd7e071@paulmck-laptop> <3d0463b6099d4fcde9a025a2f9e231d75303b61c.camel@infradead.org> <90d91673-a912-439f-98ee-e43285294486@paulmck-laptop> <8cd4aa65b1b499d4c614aa4cba445b6aeb89acf7.camel@infradead.org> Message-ID: Subject: Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation From: Sean Christopherson To: David Woodhouse Cc: paulmck@kernel.org, Jason Gunthorpe , Michal Hocko , Steven Rostedt , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Sebastian Andrzej Siewior , Clark Williams , Simona Vetter , Jerome Glisse , Christian Koenig , Paolo Bonzini , linux-mm@kvack.org, kvm@vger.kernel.org, linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: sdyatao6d8hhyii9uezhfasqgo7e7nmu X-Rspamd-Queue-Id: EC50C4000E X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788289973-116760 X-HE-Meta: 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 pFWMT/KK 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 01, 2026, David Woodhouse wrote: > On Mon, 2026-08-31 at 17:58 -0700, Paul E. McKenney wrote: > > On Fri, Aug 28, 2026 at 12:25:01AM +0100, David Woodhouse wrote: > >=20 > >=20 > > > =C2=A0 * 23f10e07aba3 srcu: Add try_synchronize_srcu() for callers wh= ich can prove readers absent > >=20 > > I do not intend to provide a separate API for this given the possibilit= y > > of indefinite postponement. >=20 > Makes sense. In that case, calling it internally from the 'right' > places becomes important, as the callers who care can't do so for > themselves. >=20 > We discussed the fact that synchronize_srcu_expedited() will also want > to use the same fast path. >=20 > I'm also looking back to Sean's call_srcu_expedited() patch from March: > https://lore.kernel.org/all/20260309193059.2244645-1-seanjc@google.com/ >=20 >=20 > | Due to differences in how VMMs manage guest devices, and in the > | architecture being emulated by userspace, some updates trigger call_src= u() > | with concurrent readers (i.e. while the VM is active), while others occ= ur > | without readers, e.g. when configuring devices during a pre-boot setup. > | For the later case (no concurrent readers), using the vanilla call_srcu= () > | is problematic, as it can kick off a normal grace period (totally fine = for > | freeing the object) and effectively transfer the non-expedited grace pe= riod > | to the upcoming synchronize_srcu_expedited(). >=20 > So the offending path uses call_srcu() and triggers a normal GP, while > the victim calls synchronize_srcu_expedited() and gets stuck behind > that non-expedited GP. >=20 > Sean, if the victim is the "no concurrent readers" code path, as you > said above, do you think the fast path in the victim should suffice, > without the cost of an expedited GP for every bus registration? IIUC, you're asking if being able to use try_synchronize_srcu() for the fas= t/happy of synchronize_srcu_expedited() (i.e. for kvm_swap_active_memslots()()), ev= en if there's an in-flight GP, would suffice for a fix of the regression introduc= ed by commit 7d9a0273c459 ("KVM: Avoid synchronize_srcu() in kvm_io_bus_register_= dev()"). If my understanding is correct, then yes, that should work, and presumably = would be a notable improvement overall.