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MN2PR12MB4045.namprd12.prod.outlook.com (2603:10b6:208:1d6::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.48.16; Wed, 20 May 2026 15:35:44 +0000 Received: from BN9PR12MB5259.namprd12.prod.outlook.com ([fe80::122c:cca7:c2b3:90ed]) by BN9PR12MB5259.namprd12.prod.outlook.com ([fe80::122c:cca7:c2b3:90ed%4]) with mapi id 15.21.0048.016; Wed, 20 May 2026 15:35:43 +0000 Message-ID: Date: Wed, 20 May 2026 21:05:30 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/7] Accelerate page migration with batch copying and hardware offload To: "David Hildenbrand (Arm)" , akpm@linux-foundation.org Cc: kinseyho@google.com, weixugc@google.com, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, willy@infradead.org, 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, dave@stgolabs.net, Jonathan.Cameron@huawei.com, rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, rientjes@google.com, 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 References: <20260428155043.39251-2-shivankg@amd.com> <98a16642-35b7-4cf9-9ee2-8de15e877920@kernel.org> Content-Language: en-US From: "Garg, Shivank" In-Reply-To: <98a16642-35b7-4cf9-9ee2-8de15e877920@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: PN5PR01CA0025.INDPRD01.PROD.OUTLOOK.COM (2603:1096:c01:25e::14) To BN9PR12MB5259.namprd12.prod.outlook.com (2603:10b6:408:100::17) MIME-Version: 1.0 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/2026 9:23 PM, David Hildenbrand (Arm) wrote: > On 4/28/26 17:50, Shivank Garg wrote: >> This is the fifth RFC of the patchset to enhance page migration by > > Ah, this is an RFC ... > > ... I suggest b4 for patch series management :P > > That also explains why patch #7 is still in there. > yes, started using it :) Patch 7 is for testing only but I need to think on optimum batch-size for offload which depends on HW, or have a callback as per Huang Ying's suggestion. >> batching folio-copy operations and enabling acceleration via DMA offload. >> >> Single-threaded, folio-by-folio copying bottlenecks page migration in >> modern systems with deep memory hierarchies, especially for large folios >> where copy overhead dominates, leaving significant hardware potential >> untapped. >> >> By batching the copy phase, we create an opportunity for hardware >> acceleration. This series builds the framework and provides a DMA >> offload driver (dcbm) as a reference implementation, targeting bulk >> migration workloads where offloading the copy improves throughput >> and latency while freeing the CPU cycles. >> >> See the RFC V3 cover letter [2] for motivation. >> >> Changelog since V4: >> ------------------- >> >> 1. Renamed PAGE_* migration state flags to FOLIO_*. (David) >> 2. Use the new folio->migrate_info field instead of folio->private >> for migration state. (David) >> 3. Fold folios_mc_copy patch in batch-copy implementation patch. (David) >> 3. Renamed migrate_offload_start()/stop() to register()/unregister(). >> (Huang, Ying) >> 4. Dropped should_batch() callback from struct migrator. Reason-based >> policy now lives in migrate_pages_batch(). Migrators can still skip >> a batch they don't want (size based policy). (Huang, Ying) >> 5. CONFIG_MIGRATION_COPY_OFFLOAD is now hidden and selected by the >> migrator driver. CONFIG_DCBM_DMA is tristate. (Huang Ying, Gregory Price). >> 6. Wrapped the SRCU + static_call dispatch in a small helper. (Huang, Ying) >> 7. Requir m->owner in migrate_offload_register(), SRCU sync at >> unregister relies on it. Counters are atomic_long_t to avoid lock-order >> issue. >> 9. Moved DCBM sysfs from /sys/kernel/dcbm to /sys/module/dcbm (Huang, Ying) >> 10. Rebased on v7.1-rc1. >> > > [...] > >> >> OPEN QUESTIONS: >> --------------- >> >> 1. Should the batch path run without a registered migrator? Patches 1-4 >> are self-contained and use folios_mc_copy() (CPU). I have several >> options like making batch path always-on for eligible folios, or >> giving admin an option to flip the static branch, or keep the gate. >> I'm leaning toward always-on. > > Hiding that detail from migrate.c sounds interesting. > Yes, will do that. >> 2. Carrying already_copied via folio->migrate_info vs changing the >> migrate_folio() callback signature (Huang, Ying). I went with the >> field for now to avoid touching every fs callback before the design >> settles. Happy to revisit. >> >> 3. Per-caller offload selection: Today eligibility is by migrate_reason >> only. Some are latency-tolerant, others may be not. Is reason the >> right granularity, or do we want a per-caller hint? > > Isn't it sufficient to just do it based on the #folios or sth like that? > > If someone migrates a handful of folios, latency is likely more important (and > batching less beneficial). > > I'd assume when migrating many folios, batching could just always be done. Or > what's the concern? > It could be a requirement for some users who want only specific use cases to go through DMA offload. I agree with your point, and will discuss more on it. >> >> 4. Cgroup integration: How should per-cgroup be accounted for different >> migrators (e.g.: any accounting for DMA-busy time)? > > Oh. Do we even have to mess with that? Probably not for the intial series. Will drop this question. >> >> 5. Tuning migrate_pages callers for offloading. For instance, in >> compaction COMPACT_CLUSTER_MAX = 32 caps DMA's payoff for compaction >> (V4 experiment). > > Is that HW dependent? > >> >> 6. Where do batch-size thresholds live, and how are they tuned? Per >> Huang Ying's split, that policy lives in the migrator. DCBM has no >> threshold today. Open whether it should later be a per-migrator >> sysfs knob or hard-coded; probably clearer once a second migrator >> (SDXI, mtcopy) shows the trade-off. > > Again, sounds like being HW dependent, no? Yes, both are HW dependent. Batch-size gating fits naturally in the migrator. For something like COMPACT_CLUSTER_MAX, would a callback from compaction to registered migrator is right thought? or do you have something else in mind? For initial series, I think I need not mess with it. Thanks, Shivank