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Message-ID: <6f76a497-248b-4f92-9448-755006c732c8@vivo.com> Date: Mon, 7 Apr 2025 11:21:51 +0800 From: Huan Yang <link@...o.com> To: Muchun Song <muchun.song@...ux.dev> Cc: bingbu.cao@...ux.intel.com, Christoph Hellwig <hch@....de>, Matthew Wilcox <willy@...radead.org>, Gerd Hoffmann <kraxel@...hat.com>, Vivek Kasireddy <vivek.kasireddy@...el.com>, Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, Andrew Morton <akpm@...ux-foundation.org>, Uladzislau Rezki <urezki@...il.com>, Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org, linux-kselftest@...r.kernel.org, opensource.kernel@...o.com Subject: Re: CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is broken, was Re: [RFC PATCH 0/6] Deep talk about folio vmap 在 2025/4/7 10:57, Muchun Song 写道: > >> On Apr 7, 2025, at 09:59, Huan Yang <link@...o.com> wrote: >> >> >> 在 2025/4/4 18:07, Muchun Song 写道: >>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@...ux.dev> wrote: >>>> >>>> >>>> >>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@....de> 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? > The answer is you can. It is essential to distinguish between virtual Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's 2-7 page: struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); for (i = 2; i < 8; ++i) pages[i] = folio_page(folio, i); //set 2-7 range page into pages, void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just got the head page? and how vmap can correctly map each page? Please correct me. :) Thanks, Huan Yang > address (VA) and physical address (PA). The VAs of tail struct pages > aren't freed but remapped to the physical page mapped by the VA of the > head struct page (since contents of those tail physical pages are the > same). Thus, the freed pages are the physical pages mapped by original > tail struct pages, not their virtual addresses. Moreover, while it > is possible to read the virtual addresses of these tail struct pages, > any write operations are prohibited since it is within the realm of > acceptability that the kernel is expected to perform write operations > solely on the head struct page of a compound head and conduct read > operations only on the tail struct pages. BTW, folio infrastructure > is also based on this assumption. > > Thanks, > Muchun. > >> 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. >>>> >
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