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

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Message-ID: <ZWX0-hEjqkmnR1Nq@tiehlicka>
Date:   Tue, 28 Nov 2023 15:11:06 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Gregory Price <gourry.memverge@...il.com>
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, arnd@...db.de, tglx@...utronix.de,
        luto@...nel.org, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        tj@...nel.org, ying.huang@...el.com,
        Gregory Price <gregory.price@...verge.com>
Subject: Re: [RFC PATCH 06/11] mm/mempolicy: modify do_mbind to operate on
 task argument instead of current

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