Re: [Question] get_vma_policy() isn't compatible with {pin, get}_user_pages_remote

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2025/7/8 11:05, Huang, Ying 写道:
> Jinjiang Tu <tujinjiang@huawei.com> writes:
>
>> 在 2025/7/8 9:53, Huang, Ying 写道:
>>> Hi, Jinjiang,
>>>
>>> Jinjiang Tu <tujinjiang@huawei.com> writes:
>>>
>>>> get_vma_policy() returns the mempolicy for the vma. If the vma has set
>>>> mempolicy, the policy is returned. Otherwise,
>>>> call get_task_policy(current) to get the mempolicy of current
>>>> task. However, it isn't reasonable for
>>>> pin_user_pages_remote() and get_user_pages_remote() cases.
>>>>
>>>> Assume task A calls pin_user_pages_remote() to pin user pages from
>>>> task B. If the [start, start + nr_pages) isn't
>>>> populated with pages, handle_mm_fault() will be called by task
>>>> A. However, if the vma doesn't set memory policy,
>>>> the mempolicy of task A instead of task B is used to allocate. It
>>>> seems to be unreasonable. See
>>>> dequeue_hugetlb_folio_vma()->huge_node().
>>>>
>>>> We can only obtain mm in get_vma_policy(), but we couldn't get the
>>>> task, since a mm can be associated with multiple
>>>> tasks(threads) and the task mempolicy is at thread granularity.
>>>>
>>>> Is this situation reasonable? And if not, how could we fix it?
>>> Yes.  This sounds like an issue in theory and it's hard to be resolved
>>> if possible.  Please take a look at get_user_pages_remote() usage in
>>> exec().
>> IIUC, exec() replaces current->mm with new mm, and the task_struct isn't changed,
>> thus task mempolicy is same, so it is reasonable to use get_user_pages_remote() in exec().
>>
>>> Do you have some practical issue with pin/get_user_pages_remote()?
>> Yes, I have a driver to pin_user_pages_remote() for other task.
> Please give more details of your issue to help us to understand it.  For
> example, why cannot you use pin_user_pages()?

+ CC Ruilin, he understands the usage scenario better.

RuiLin, could you please explain why we couldn' use pin_user_pages()?

> ---
> Best Regards,
> Huang, Ying