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

[Jinjiang Tu <tujinjiang@huawei.com>]

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.

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