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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

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().

Do you have some practical issue with pin/get_user_pages_remote()?

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Best Regards,
Huang, Ying