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CH3PR12MB8909.namprd12.prod.outlook.com (2603:10b6:610:179::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9769.18; Fri, 3 Apr 2026 11:08:45 +0000 Received: from BN9PR12MB5259.namprd12.prod.outlook.com ([fe80::122c:cca7:c2b3:90ed]) by BN9PR12MB5259.namprd12.prod.outlook.com ([fe80::122c:cca7:c2b3:90ed%3]) with mapi id 15.20.9769.018; Fri, 3 Apr 2026 11:08:45 +0000 Message-ID: <3e1f520f-e3d8-4ef2-b474-ca4e988bbf2d@amd.com> Date: Fri, 3 Apr 2026 16:38:32 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 2/6] mm/migrate: skip data copy for already-copied folios To: "Huang, Ying" Cc: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, 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, apopple@nvidia.com, 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S0Mpahsc0pSQkA3jWZlBWuE1zrwa1dFb33m9u Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/24/2026 1:52 PM, Huang, Ying wrote: > Shivank Garg writes: > >> static int move_to_new_folio(struct folio *dst, struct folio *src, >> - enum migrate_mode mode) >> + enum migrate_mode mode, bool already_copied) >> { >> struct address_space *mapping = folio_mapping(src); >> int rc = -EAGAIN; >> @@ -1096,6 +1114,9 @@ static int move_to_new_folio(struct folio *dst, struct folio *src, >> VM_BUG_ON_FOLIO(!folio_test_locked(src), src); >> VM_BUG_ON_FOLIO(!folio_test_locked(dst), dst); >> >> + if (already_copied) >> + dst->private = (void *)(unsigned long)PAGE_ALREADY_COPIED; >> + > > IMHO, this appears to be an unusual way to pass arguments to a function. > Why not adjust the parameters of migrate_folio()? How about turning enum > migrate_mode into a bitmask (migrate_flags)? > Using folio->private, keeps the change self-contained in migrate.c David suggested adding a dedicated unsigned long migrate_info field in the folio union. I'll switch to that, as this is cleaner and avoid hacky use of ->private. https://lore.kernel.org/linux-mm/27b1b602-129f-4bc5-a553-386e8d1f5d90@kernel.org Changing the migrate_folio() a_ops signature would touch nearly every filesystem for something that only core migration cares about, and does not look practical. We can't add to migrate_mode enum either as currently those values are mutually exclusive and ordered levels of increasing synchrony (ASYNC < SYNC_LIGHT < SYNC) And there are checks like this (cc->mode < MIGRATE_SYNC) or mode != MIGRATE_SYNC This could break it. Best regards, Shivank