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[RFC PATCH 0/6] Deep talk about folio vmap To: Muchun Song Cc: bingbu.cao@linux.intel.com, Christoph Hellwig , Matthew Wilcox , Gerd Hoffmann , Vivek Kasireddy , Sumit Semwal , =?UTF-8?Q?Christian_K=C3=B6nig?= , Andrew Morton , Uladzislau Rezki , Shuah Khan , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, opensource.kernel@vivo.com References: <20250327092922.536-1-link@vivo.com> <20250404090111.GB11105@lst.de> <9A899641-BDED-4773-B349-56AF1DD58B21@linux.dev> <43DD699A-5C5D-429B-A2B5-61FBEAE2E252@linux.dev> From: Huan Yang In-Reply-To: <43DD699A-5C5D-429B-A2B5-61FBEAE2E252@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI2PR01CA0047.apcprd01.prod.exchangelabs.com (2603:1096:4:193::10) To PUZPR06MB5676.apcprd06.prod.outlook.com (2603:1096:301:f8::10) MIME-Version: 1.0 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 9lYRxYlf 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 在 2025/4/4 18:07, Muchun Song 写道: > >> On Apr 4, 2025, at 17:38, Muchun Song wrote: >> >> >> >>> On Apr 4, 2025, at 17:01, Christoph Hellwig wrote: >>> >>> After the btrfs compressed bio discussion I think the hugetlb changes that >>> skip the tail pages are fundamentally unsafe in the current kernel. >>> >>> That is because the bio_vec representation assumes tail pages do exist, so >>> as soon as you are doing direct I/O that generates a bvec starting beyond >>> the present head page things will blow up. Other users of bio_vecs might >>> do the same, but the way the block bio_vecs are generated are very suspect >>> to that. So we'll first need to sort that out and a few other things >>> before we can even think of enabling such a feature. >>> >> I would like to express my gratitude to Christoph for including me in the >> thread. I have carefully read the cover letter in [1], which indicates >> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >> wondering if we could consider using `vmap()` instead. In the HVO scenario, >> the tail struct pages do **exist**, but they are read-only. I've examined >> the code of `vmap()`, and it appears that it only reads the struct page. >> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >> udmabuf.). Right? > I believe my stance is correct. I've also reviewed another thread in [2]. > Allow me to clarify and correct the viewpoints you presented. You stated: > " > So by HVO, it also not backed by pages, only contains folio head, each > tail pfn's page struct go away. > " > This statement is entirely inaccurate. The tail pages do not cease to exist; > rather, they are read-only. For your specific use-case, please use `vmap()` > to resolve the issue at hand. If you wish to gain a comprehensive understanding I see the document give a simple graph to point:  +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+  |           |                                     |     0     | -------------> |     0     |  |           | +-----------+                +-----------+  |           |                                      |     1     | -------------> |     1     |  |           | +-----------+                +-----------+  |           |                                      |     2     | ----------------^ ^ ^ ^ ^ ^  |           | +-----------+                      | | | | |  |           |                                      |     3     | ------------------+ | | | |  |           | +-----------+                        | | | |  |           |                                      |     4     | --------------------+ | | |  |    PMD    | +-----------+                          | | |  |   level   |                                   |     5     | ----------------------+ | |  |  mapping  | +-----------+                             | |  |           |                                     |     6     | ------------------------+ |  |           | +-----------+                              |  |           |                                     |     7     | --------------------------+  |           |                                    +-----------+  |           |  |           |  |           |  +-----------+ If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do something correctly? Or something I still misunderstand, please correct me. Thanks, Huan Yang > of the fundamentals of HVO, I kindly suggest a thorough review of the document > in [3]. > > [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t > [3] Documentation/mm/vmemmap_dedup.rst > >> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >> >> Thanks, >> Muchun. >> >