Re: [RFC PATCH 06/11] mm/mempolicy: modify do_mbind to operate on task argument instead of current

Michal Hocko
2023-11-28T14:12:31Z

On Wed 22-11-23 16:11:55, Gregory Price wrote:
[...]
> + * Like get_vma_policy and get_task_policy, must hold alloc/task_lock
> + * while calling this.
> + */
> +static struct mempolicy *get_task_vma_policy(struct task_struct *task,
> + struct vm_area_struct *vma,
> + unsigned long addr, int order,
> + pgoff_t *ilx)
[...]

You should add lockdep annotation for alloc_lock/task_lock here for clarity and
also...
> @@ -1844,16 +1899,7 @@ struct mempolicy *__get_vma_policy(struct vm_area_struct *vma,
> struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
> unsigned long addr, int order, pgoff_t *ilx)
> {
> - struct mempolicy *pol;
> -
> - pol = __get_vma_policy(vma, addr, ilx);
> - if (!pol)
> - pol = get_task_policy(current);
> - if (pol->mode == MPOL_INTERLEAVE) {
> - *ilx += vma->vm_pgoff >> order;
> - *ilx += (addr - vma->vm_start) >> (PAGE_SHIFT + order);
> - }
> - return pol;
> + return get_task_vma_policy(current, vma, addr, order, ilx);

I do not think that all get_vma_policy take task_lock (just random check
dequeue_hugetlb_folio_vma->huge_node->get_vma_policy AFAICS)

Also I do not see policy_nodemask to be handled anywhere. That one is
used along with get_vma_policy (sometimes hidden like in
alloc_pages_mpol). It has a dependency on
cpuset_nodemask_valid_mems_allowed. That means that e.g. mbind on a
remote task would be constrained by current task cpuset when allocating
migration targets for the target task. I am wondering how many other
dependencies like that are lurking there.
--
Michal Hocko
SUSE Labs