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Tue, 11 Aug 2026 10:26:28 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1wtqF9-00000002gAk-2Giq; Tue, 11 Aug 2026 14:26:27 -0300 Date: Tue, 11 Aug 2026 14:26:27 -0300 From: Jason Gunthorpe To: David Woodhouse Cc: akpm@linux-foundation.org, david@kernel.org, mhocko@suse.com, rostedt@goodmis.org, bigeasy@linutronix.de, simona.vetter@ffwll.ch, jglisse@redhat.com, christian.koenig@amd.com, paulmck@kernel.org, seanjc@google.com, pbonzini@redhat.com, linux-mm@kvack.org, kvm@vger.kernel.org, linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation Message-ID: <20260811172627.GN544626@ziepe.ca> References: <20260811162423.GL544626@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 1D3ABA0017 X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: yq5mc5y3a8zuni68q8czwfpfx1xqaw4p X-HE-Tag: 1786469189-290914 X-HE-Meta: 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 8iR6NoJM i0m+SZFm4ULvv2bAa5VmT/eHktS5CAwtTW1rIAjHIW59Tbo7gdXFusPKLkL4Jb6x3ndIDaurRIpLyg+XC1FCh45hwravW02T3tcOcgOqAb2r8RW/tMNzELPmGkZUKUviRSvCyTNghzd5HC22v5ynQAbU39WqJVQQq27kktguP62Xvmtp3a+Wvql+l0DfEsaOGCFxdFW+98dTBig/rTw91vJ+CI0gSe5rELMaWiRs0qw3TeaYDjWz9OiBBU0AqPLdvbLMi2R+m0RhQ0AX9W0jovTs64tIY5NRdhIssaia8/h7Et9o9f8ocr7C1lUCfGLVqt4B0UofG2ukuRIg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 11, 2026 at 06:22:12PM +0100, David Woodhouse wrote: > On Tue, 2026-08-11 at 13:24 -0300, Jason Gunthorpe wrote: > > To be clear you should not be using any synchronize_[s]rcu() primitive > > inside the invalidation callbacks. These are well known to have > > multi-second delays on loaded systems which are a completely > > inappropriate performance characteristic for these mm callbacks. > > > > This statement has nothing to do with deadlock. > > > > RCU is always a trade off, you can make the read side run really fast > > and the write side is ghastly slow. If you can't handle the slow write > > you shouldn't use RCU techniques. > > The multi-second horror stories are about the *global* RCU/SRCU > domains, where the grace period has to wait out arbitrary readers all > over the kernel. > > This is not that. It is a dedicated srcu_struct, private to one VM, > and its entire reader population is a handful of KVM fast paths that > until now were under irqsave rwlocks. Are you sure? I've never heard that srcu has those kinds of properties. If its so fast you should just propose a non-sleeping version and leave the notifiers out of it Jason