linux-kernel - Re: [RFC Patch v2 1/4] mm/mempolicy: skip nodemask intersect check for 'interleave' when oom

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Date:   Sun, 23 May 2021 22:15:00 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Feng Tang <feng.tang@...el.com>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com
Subject: Re: [RFC Patch v2 1/4] mm/mempolicy: skip nodemask intersect check
 for 'interleave' when oom

On Thu, 20 May 2021, Feng Tang wrote:

> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index d79fa29..1964cca 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2098,7 +2098,7 @@ bool init_nodemask_of_mempolicy(nodemask_t *mask)
>   *
>   * If tsk's mempolicy is "default" [NULL], return 'true' to indicate default
>   * policy.  Otherwise, check for intersection between mask and the policy
> - * nodemask for 'bind' or 'interleave' policy.  For 'preferred' or 'local'
> + * nodemask for 'bind' policy.  For 'interleave', 'preferred' or 'local'
>   * policy, always return true since it may allocate elsewhere on fallback.
>   *
>   * Takes task_lock(tsk) to prevent freeing of its mempolicy.
> @@ -2111,29 +2111,13 @@ bool mempolicy_nodemask_intersects(struct task_struct *tsk,
>  
>  	if (!mask)
>  		return ret;
> +
>  	task_lock(tsk);
>  	mempolicy = tsk->mempolicy;
> -	if (!mempolicy)
> -		goto out;
> -
> -	switch (mempolicy->mode) {
> -	case MPOL_PREFERRED:
> -		/*
> -		 * MPOL_PREFERRED and MPOL_F_LOCAL are only preferred nodes to
> -		 * allocate from, they may fallback to other nodes when oom.
> -		 * Thus, it's possible for tsk to have allocated memory from
> -		 * nodes in mask.
> -		 */
> -		break;
> -	case MPOL_BIND:
> -	case MPOL_INTERLEAVE:
> +	if (mempolicy && mempolicy->mode == MPOL_BIND)
>  		ret = nodes_intersects(mempolicy->v.nodes, *mask);

If MPOL_INTERLEAVE is deemed only a suggestion, the same could be 
considered true of MPOL_BIND intersection as well, no?

> -		break;
> -	default:
> -		BUG();
> -	}
> -out:
>  	task_unlock(tsk);
> +
>  	return ret;
>  }
>  
> -- 
> 2.7.4
> 
>