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Sun, 24 May 2026 19:19:16 -0700 (PDT) Received: from [2a00:79e0:2eb0:8:6235:ad35:aed6:8a48] ([2a00:79e0:2eb0:8:6235:ad35:aed6:8a48]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2beb56bc46esm83987925ad.25.2026.05.24.19.19.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 24 May 2026 19:19:16 -0700 (PDT) Date: Sun, 24 May 2026 19:19:15 -0700 (PDT) From: David Rientjes To: "Garg, Shivank" cc: "David Hildenbrand (Arm)" , Andrew Morton , kinseyho@google.com, weixugc@google.com, ljs@kernel.org, "Liam R. Howlett" , vbabka@kernel.org, Matthew Wilcox , rppt@kernel.org, surenb@google.com, mhocko@suse.com, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, Davidlohr Bueso , Jonathan Cameron , rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, xuezhengchu@huawei.com, yiannis@zptcorp.com, dave.hansen@intel.com, hannes@cmpxchg.org, jhubbard@nvidia.com, peterx@redhat.com, riel@surriel.com, shakeel.butt@linux.dev, stalexan@redhat.com, tj@kernel.org, nifan.cxl@gmail.com, jic23@kernel.org, aneesh.kumar@kernel.org, nathan.lynch@amd.com, Frank.li@nxp.com, djbw@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 5/7] mm/migrate: add copy offload registration infrastructure In-Reply-To: Message-ID: <468db563-73b0-3f77-cf42-bb24f744c7d1@google.com> References: <20260428155043.39251-2-shivankg@amd.com> <20260428155043.39251-12-shivankg@amd.com> <9145ae5b-a33e-4092-8cb5-efa18ae61287@kernel.org> <70ee3800-0469-4c3d-8a1c-bb6594c64f42@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Stat-Signature: 65e5sfc16a3crj6rz9kezy3mfi8zwzwq X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: BFB1480003 X-HE-Tag: 1779675558-294711 X-HE-Meta: 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 yxU18uCW 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Received a bounce for Jonathan, so adding in his other email address. On Sun, 24 May 2026, David Rientjes wrote: > On Wed, 20 May 2026, Garg, Shivank wrote: > > > >> +static bool migrate_offload_do_batch(int reason) > > >> +{ > > >> + if (!static_branch_unlikely(&migrate_offload_enabled)) > > >> + return false; > > >> + > > >> + switch (reason) { > > >> + case MR_COMPACTION: > > >> + case MR_SYSCALL: > > >> + case MR_DEMOTION: > > >> + case MR_NUMA_MISPLACED: > > >> + return true; > > >> + default: > > >> + return false; > > > > > > > > > What's the exact reason we don't do this for hotunplug etc? IOW, why do we make > > > this depend on a reason? > > > > Reason-based filtering could be a requirement for some users who want only specific > > use cases to go through DMA offload. > > > > +1, I think this makes a lot of sense; not all DMA offloads are created > equally and we may prefer to unburden them from being contended by > migrations that they are not intending to accelerate. > > > For the RFC, I introduced a placeholder to enable further discussion on which use cases > > should allow migration offload and whether offload users actually need this control? > > > > Your other point also makes sense: "If someone migrates a handful of folios, latency is > > likely more important (and batching less beneficial)." > > Based on this, we could either fully rely on batch size. I'll think more about this. > > > > There are, or will be, some offloads that must be used for certain types > of page migrations, like Confidential Computing. That's for functional > reasons, not a heuristic. > > We want to use certain hardware assists solely for promotion and demotion > of memory for tiering. We certainly wouldn't want those hardware assists > to be inundated by users doing tons of move_pages(2) on their own or by > best effort memory compaction in the kernel. > > I think the use cases should be configurable by the user if at all > possible so we can control what has access to being offloaded. These are > often shared system resources and can be contended like any other > resource, so configuring which migrations can use them vs not use them > seems important. >