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Received: from DESKTOP-5N7EMDA(mailfrom:ying.huang@linux.alibaba.com fp:SMTPD_---0X0azT3w_1775544748 cluster:ay36) by smtp.aliyun-inc.com; Tue, 07 Apr 2026 14:52:37 +0800 From: "Huang, Ying" To: "Garg, Shivank" 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, dave@stgolabs.net, Jonathan.Cameron@huawei.com, rkodsara@amd.com, vkoul@kernel.org, bharata@amd.com, sj@kernel.org, weixugc@google.com, dan.j.williams@intel.com, 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, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v4 2/6] mm/migrate: skip data copy for already-copied folios In-Reply-To: <3e1f520f-e3d8-4ef2-b474-ca4e988bbf2d@amd.com> (Shivank Garg's message of "Fri, 3 Apr 2026 16:38:32 +0530") References: <20260309120725.308854-3-shivankg@amd.com> <20260309120725.308854-8-shivankg@amd.com> <87wlz1zlf8.fsf@DESKTOP-5N7EMDA> <3e1f520f-e3d8-4ef2-b474-ca4e988bbf2d@amd.com> Date: Tue, 07 Apr 2026 14:52:30 +0800 Message-ID: <87pl4bfej5.fsf@DESKTOP-5N7EMDA> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9444A40010 X-Stat-Signature: 5szof9mynsddpuzbd3b1yhz7crnr7xa9 X-Rspam-User: X-HE-Tag: 1775544762-826847 X-HE-Meta: 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 Ri7yEQ95 +Py0lkgJxQj8/0CtSyido3bobmF3um06ONud6Tbnn5Y/UVNsDRnxXsSDA06Wh+aVnmPil7joXGVtwx+WFoJtwTRy36XFiOzOGwpbsSaTKnG9p3Fly/RjHJzQFlXXPR9K7WsYqSl4czHaHCbNvXlh50QiSO3xugmuF9RiU+b9zthLw7yxQFhuPfnw5cZ8O03sqDBzCz/69zL5gvRE+V4bixNCRnysH1q5WoiWTy7PLaQWpbIFw1yS8T6aoehvesROyspp27faquTrP0S14B3gMMptkMSTI3aNEedNb4MCyFVWVwpVy8kFOe4sNiOu6LlCTFpLya0qLZlI//qe782ueyt7dQ7iiWwCIMa3PnNWU0HJAugGsG0p+hmI4Vy2XJ9Pxsiu1bgZ+6aojb2c= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: "Garg, Shivank" writes: > 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 That's good for the original usage of folio->private. That is, to record migration related information for a list of folios. > > 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. IMHO, code readability is more important than limiting the scope of changes. The migrate_folio callback of most file systems shares a few common implementations in migrate.c. So, I think it is doable. --- Best Regards, Huang, Ying