linux-kernel - Re: [RFC 2/6] mm, mempolicy: stop adjusting current->il_next in mpol_rebind

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Date:   Wed, 12 Apr 2017 16:16:26 -0500 (CDT)
From:   Christoph Lameter <cl@...ux.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC 2/6] mm, mempolicy: stop adjusting current->il_next in
 mpol_rebind_nodemask()

On Wed, 12 Apr 2017, Vlastimil Babka wrote:

> >> Well, interleave_nodes() will then potentially return a node outside of
> >> the allowed memory policy when its called for the first time after
> >> mpol_rebind_.. . But thenn it will find the next node within the
> >> nodemask and work correctly for the next invocations.
> >
> > Hmm, you're right. But that could be easily fixed if il_next became il_prev, so
> > we would return the result of next_node_in(il_prev) and also store it as the new
> > il_prev, right? I somehow assumed it already worked that way.

Yup that makes sense and I thought about that when I saw the problem too.

> @@ -863,6 +856,18 @@ static int lookup_node(unsigned long addr)
>  	return err;
>  }
>
> +/* Do dynamic interleaving for a process */
> +static unsigned interleave_nodes(struct mempolicy *policy, bool update_prev)

Why do you need an additional flag? Would it not be better to always
update and switch the update_prev=false case to simply use
next_node_in()?

> +{
> +	unsigned next;
> +	struct task_struct *me = current;
> +
> +	next = next_node_in(me->il_prev, policy->v.nodes);
> +	if (next < MAX_NUMNODES && update_prev)
> +		me->il_prev = next;
> +	return next;
> +}
> +
>  /* Retrieve NUMA policy */
>  static long do_get_mempolicy(int *policy, nodemask_t *nmask,
>  			     unsigned long addr, unsigned long flags)
> @@ -916,7 +921,7 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
>  			*policy = err;
>  		} else if (pol == current->mempolicy &&
>  				pol->mode == MPOL_INTERLEAVE) {
> -			*policy = current->il_next;
> +			*policy = interleave_nodes(current->mempolicy, false);

Here