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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id af79cd13be357-930f687466esm570122185a.4.2026.07.23.20.55.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Jul 2026 20:55:51 -0700 (PDT) Date: Thu, 23 Jul 2026 23:55:46 -0400 From: Gregory Price To: Andrew Morton Cc: Johannes Weiner , Sean Christopherson , Yan Zhao , pbonzini@redhat.com, David Hildenbrand , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Ying Huang , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Neha Gholkar , kvm@vger.kernel.org, rick.p.edgecombe@intel.com, vishal.l.verma@intel.com Subject: Re: [PATCH] mm: mempolicy: fix automatic numa balancing for shmem Message-ID: References: <20260629163337.1264881-1-hannes@cmpxchg.org> <20260723171515.b5ddbc59c5c36ad01ef8f7fb@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260723171515.b5ddbc59c5c36ad01ef8f7fb@linux-foundation.org> X-Stat-Signature: qpowyataaepbsgcz5tbztmjcibpt1g4n X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 26AA840002 X-HE-Tag: 1784865353-253663 X-HE-Meta: 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 I2gTJpgD 2tyjAOUe0f7zquFPX0By8DscZRI+/v3MTxA4oUB40LcHiuKACmNsLBe+PUbZQmkmP6RIaBepyrxs34YCntOZn6MKnzA+RYUFi+7lUTjcwGIvxhvbw16GznnZaY1OHOXg1flvkMd976AeDc+cWg0Fkl4B4PdK5Vset2/rgW1R1SFG8JSpyYE0brdo/LF0p8WSkwIi1tPb5yYfx7KU9GP/pqXVmg6Zz763RaDkOfQpt/q8X08v+jdYv0Kt1KlIYfZ6YbCIBVY38vEYFqMtRqG+5f4uef5wzmoSHX9LztssODwoMbdmwAphwdADMaMX4Y8N7nP5utjF0nBMTTcbVmP06xjC66Fbd4Sdfvck9zq4Cfe9KH6q11A70AUMoUSmNiL1iiKZh5a6pQNX4abPc26mGfzDSywwnf61+v1ufv0tzPkgke+xpsK/Y4NvmzWqcA8IWkSkgFq4mVfEgnaoa0Rsrx7EyjQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jul 23, 2026 at 05:15:15PM -0700, Andrew Morton wrote: > On Thu, 23 Jul 2026 10:54:06 -0400 Johannes Weiner wrote: > > > > But in this case, IIUC, this change will "silently" enable NUMA balancing for > > > shmem-based KVM setups where it was previously disabled (albeit unintentionally). > > > > > > I'm not fundamentally opposed to the change, but I do worry that downstream KVM > > > users could be in for a nasty surprise. > > It is a bit concerning. > > > This should be very visible in early kernel validation after an > > upgrade. VM hosts tend to not do much else, with VM memory dominating > > the host. You'd expect a significant uptick in numa_pte_updates, > > numa_hint_faults, and a change in per-node nr_shmem stats. And the > > patch subject makes it trivial to find in a commit delta scan. > > Is there anything we can do to make this process less painful? > Documentation of course, but what about detecting the situation and > emitting a once-off warning? Maybe a rate-limited or one-time pr_info/warn on the first PROT_NONE fault affecting KVM in the painful zap path? Non-invasive, targeted, and helpful. ~Gregory