Re: CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP is broken, was Re: [RFC PATCH 0/6] Deep talk about folio vmap

[Huan Yang <link@vivo.com>]

在 2025/4/7 10:57, Muchun Song 写道:
>
>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote:
>>
>>
>> 在 2025/4/4 18:07, Muchun Song 写道:
>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote:
>>>>
>>>>
>>>>
>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.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.
>>>>
>