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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id af79cd13be357-930f6a03834sm441323085a.30.2026.07.23.08.58.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Jul 2026 08:58:33 -0700 (PDT) Date: Thu, 23 Jul 2026 11:58:27 -0400 From: Gregory Price To: Johannes Weiner Cc: Sean Christopherson , Yan Zhao , pbonzini@redhat.com, Andrew Morton , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 135134000A X-Rspam-User: X-Stat-Signature: 9abb1pa3arb3739stnsf5td8dgjsqnqw X-Rspamd-Server: rspam04 X-HE-Tag: 1784822314-459550 X-HE-Meta: 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 ANJJxLiX 1wQztjRwrl6RFYueLJPaMkD8Gf98HXJUnkmDhE1GP654mdn2Cy2SRZ4wCHTp4g/a1ugh1lqLfTnleMHFDxuAMooVKogy01zfy7ILJSmDeZho4htFftnBewnMT+rkiS/vbG7e/PCBn5+DxgAHmOpMStKq4cXAEzl6sCAjbJfZfX+3voBcC2aqbtdlGrDgYEqdrCyKMa2kRKX906TAu969/9QE9tbA+rJF+1x7jUkZlaEBaJIr7ZS48zu+kgBOc+5XgJsIFNpvfnxCKNm5OlORwunaW4VBI3Oe5YgijI6OXh/LYe3d24OmMEuB3zfV9uGkobOmSk8kg8u9NKAUv8Q3cbwC3GQBwd1Xu8HL59tLs6ceaYavYZ1sJlcJsZfw7MVADUULCQqcxj0yGPtRZbm1HonJ3JQ== 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 10:54:06AM -0400, Johannes Weiner wrote: > > > > My stance is that using NUMA balancing with KVM guests is a terrible idea, and > > that anyone that insists on using such a setup gets to suffer the consequences. > > I agree. Surely we cannot leave shmem balancing broken due to > that. It's also not clear to me how many people even enable numa > balancing. > I lean towards Johannes' stance here that consistency is better, and if KVM has a known-poor interaction with numa balancing, maybe KVM should simply set a default policy (when none are present) to avoid this. That said, this is painful because MPOL_F_MOF / MPOL_F_MORON are already a silently inherited policy on boot (added during init). This will be confusing for users. This is not documented anywhere. Maybe this change should come with a docs addition that says the default global policy is ACTUALLY: MPOL_LOCAL + (MPOL_F_MOF | MPOL_F_MORON) Instead of just MPOL_LOCAL and letting that silent implementation detail bite people. ~Gregory