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b=QZDSnRYopXKAJMRYUmq5QTkslx6Kh9jmh3HWibjIAp+ME/RfgEQBlX9t25t75zs0k 0F2rSaC1mFzk4YlMjOSHAJ6mC1v/YxZ6J6YUTP/SNlMo7E9J/gWzbDhDwoeoSzEkn8 xwxRkzO1dZQfsB92x8fYnXHh6cOrQ8glU21g9Z+4= Date: Thu, 23 Jul 2026 17:15:15 -0700 From: Andrew Morton To: Johannes Weiner Cc: Sean Christopherson , Yan Zhao , pbonzini@redhat.com, David Hildenbrand , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , 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: <20260723171515.b5ddbc59c5c36ad01ef8f7fb@linux-foundation.org> In-Reply-To: References: <20260629163337.1264881-1-hannes@cmpxchg.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: EB76E180005 X-Rspam-User: X-Stat-Signature: f4mqqrzxyj7xism1inixka1c341yfpau X-HE-Tag: 1784852117-237890 X-HE-Meta: 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 V8LNBocp +FigZguToOPdx7qypKYgDEc4adWWcnaDoexmTbZm9TkLEK0A71RDg8cjSVpRzNXA/Kt0+C3VD5zb1rEVf2w7sEsnEydKi4puI1d/dFjQQY2QkmZm/QU/hKgXxidL9FauYDplZVm6GnFVCY8WJGeCIlZuiLUqzifXwoFEqcFi23/3f7Im2NuqUSC+qdSvzfwUvanec1yIYE/5Ft6mnCvFyQymK0cg7dUdfh6D4CpLQYRdC33ocgdetR5/mFJK8S8vNsbYYfMrqi0kYiKvM76PWlptDXvQ//1/h++F1DSDpM7Ymq/V67jokiTRidfKR9hkO7AhpxzDZ42hHdMx+IKnuPfrH6paPtMAUNKY74Md/8A2cSJxxZJKTo9AEDAU+srL9d0pQ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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?