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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-51c109d374fsm20272631cf.21.2026.06.30.08.29.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 30 Jun 2026 08:29:40 -0700 (PDT) Date: Tue, 30 Jun 2026 11:29:36 -0400 From: Gregory Price To: "Huang, Ying" Cc: Johannes Weiner , Andrew Morton , David Hildenbrand , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Neha Gholkar Subject: Re: [PATCH] mm: mempolicy: fix automatic numa balancing for shmem Message-ID: References: <20260629163337.1264881-1-hannes@cmpxchg.org> <87h5mkz33h.fsf@DESKTOP-5N7EMDA> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87h5mkz33h.fsf@DESKTOP-5N7EMDA> On Tue, Jun 30, 2026 at 07:20:50PM +0800, Huang, Ying wrote: > Gregory Price writes: > > [snip] > > > Demotions don't care about mempolicy, so opting shmem out of NUMA > > balancing and mbind'ing on a tiered system is just full sadness. > > > > This is all just more evidence that demotion needs to be completely > > redone, it's creating a mess of undefined behavior for memory placement. > > It's hard to respect mempolicy during demotion in the current > implementation. Do you have any ideas on how to improve this? > I think it's feasible we could respect per-vma mempolicies, but not per-task. That would at least make this particular interaction less painful and mbind() would do what you'd expect. It is a bit racy, but with MPOL_MF_MOVE_ALL the user can get what they actually want. I think task-wide mempolicy is problematic and generally a bad idea on tiered systems, maybe it's ok if we simply document task policies are not respected on tiered systems? ~Gregory