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From: "Huang, Ying" <ying.huang@intel.com>
To: Gregory Price <gourry.memverge@gmail.com>
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Subject: Re: [PATCH 2/3] mm/mempolicy: refactor a read-once mechanism into a
 function for re-use
In-Reply-To: <20240112210834.8035-3-gregory.price@memverge.com> (Gregory
	Price's message of "Fri, 12 Jan 2024 16:08:33 -0500")
References: <20240112210834.8035-1-gregory.price@memverge.com>
	<20240112210834.8035-3-gregory.price@memverge.com>
Date: Mon, 15 Jan 2024 12:13:06 +0800
Message-ID: <87h6jf1bfx.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Gregory Price <gourry.memverge@gmail.com> writes:

> move the use of barrier() to force policy->nodemask onto the stack into
> a function `read_once_policy_nodemask` so that it may be re-used.
>
> Suggested-by: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Gregory Price <gregory.price@memverge.com>
> ---
>  mm/mempolicy.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 5da4fd79fd18..0abd3a3394ef 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1907,6 +1907,20 @@ unsigned int mempolicy_slab_node(void)
>  	}
>  }
>  
> +static unsigned int read_once_policy_nodemask(struct mempolicy *pol,
> +					      nodemask_t *mask)

It may be more useful if we define this as memcpy_once().  That can be
used not only for nodemask, but also other data structure.

> +{
> +	/*
> +	 * barrier stabilizes the nodemask locally so that it can be iterated
> +	 * over safely without concern for changes. Allocators validate node
> +	 * selection does not violate mems_allowed, so this is safe.
> +	 */
> +	barrier();
> +	__builtin_memcpy(mask, &pol->nodes, sizeof(nodemask_t));

We don't use __builtin_memcpy() in kernel itself directly.  Although it
is used in kernel tools.  So, I think it's better to use memcpy() here.

> +	barrier();
> +	return nodes_weight(*mask);
> +}
> +
>  /*
>   * Do static interleaving for interleave index @ilx.  Returns the ilx'th
>   * node in pol->nodes (starting from ilx=0), wrapping around if ilx
> @@ -1914,20 +1928,12 @@ unsigned int mempolicy_slab_node(void)
>   */
>  static unsigned int interleave_nid(struct mempolicy *pol, pgoff_t ilx)
>  {
> -	nodemask_t nodemask = pol->nodes;
> +	nodemask_t nodemask;
>  	unsigned int target, nnodes;
>  	int i;
>  	int nid;
> -	/*
> -	 * The barrier will stabilize the nodemask in a register or on
> -	 * the stack so that it will stop changing under the code.
> -	 *
> -	 * Between first_node() and next_node(), pol->nodes could be changed
> -	 * by other threads. So we put pol->nodes in a local stack.
> -	 */
> -	barrier();
>  
> -	nnodes = nodes_weight(nodemask);
> +	nnodes = read_once_policy_nodemask(pol, &nodemask);
>  	if (!nnodes)
>  		return numa_node_id();
>  	target = ilx % nnodes;

--
Best Regards,
Huang, Ying