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From: "Huang, Ying" <ying.huang@intel.com>
To: Gregory Price <gourry.memverge@gmail.com>
Cc: linux-mm@kvack.org,  linux-kernel@vger.kernel.org,
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  hyeongtak.ji@sk.com,  mhocko@kernel.org,  vtavarespetr@micron.com,
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  hannes@cmpxchg.org,  dan.j.williams@intel.com
Subject: Re: [PATCH v5 4/4] mm/mempolicy: protect task interleave functions
 with tsk->mems_allowed_seq
In-Reply-To: <20240202170238.90004-5-gregory.price@memverge.com> (Gregory
	Price's message of "Fri, 2 Feb 2024 12:02:38 -0500")
References: <20240202170238.90004-1-gregory.price@memverge.com>
	<20240202170238.90004-5-gregory.price@memverge.com>
Date: Mon, 05 Feb 2024 13:48:44 +0800
Message-ID: <87r0hr31hf.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Gregory Price <gourry.memverge@gmail.com> writes:

> In the event of rebind, pol->nodemask can change at the same time as an
> allocation occurs.  We can detect this with tsk->mems_allowed_seq and
> prevent a miscount or an allocation failure from occurring.
>
> The same thing happens in the allocators to detect failure, but this
> can prevent spurious failures in a much smaller critical section.
>
> Suggested-by: "Huang, Ying" <ying.huang@intel.com>
> Signed-off-by: Gregory Price <gregory.price@memverge.com>
> ---
>  mm/mempolicy.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index d8cc3a577986..ed0d5d2d456a 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1878,11 +1878,17 @@ bool apply_policy_zone(struct mempolicy *policy, enum zone_type zone)
>  
>  static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
>  {
> -	unsigned int node = current->il_prev;
> -
> -	if (!current->il_weight || !node_isset(node, policy->nodes)) {
> +	unsigned int node;
> +	unsigned int cpuset_mems_cookie;
> +
> +retry:
> +	/* to prevent miscount use tsk->mems_allowed_seq to detect rebind */
> +	cpuset_mems_cookie = read_mems_allowed_begin();
> +	node = current->il_prev;
> +	if (!node || !node_isset(node, policy->nodes)) {
            ~~~~~
            !current->il_weight ?

--
Best Regards,
Huang, Ying

>  		node = next_node_in(node, policy->nodes);
> -		/* can only happen if nodemask is being rebound */
> +		if (read_mems_allowed_retry(cpuset_mems_cookie))
> +			goto retry;
>  		if (node == MAX_NUMNODES)
>  			return node;
>  		current->il_prev = node;
> @@ -1896,8 +1902,14 @@ static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
>  static unsigned int interleave_nodes(struct mempolicy *policy)
>  {
>  	unsigned int nid;
> +	unsigned int cpuset_mems_cookie;
> +
> +	/* to prevent miscount, use tsk->mems_allowed_seq to detect rebind */
> +	do {
> +		cpuset_mems_cookie = read_mems_allowed_begin();
> +		nid = next_node_in(current->il_prev, policy->nodes);
> +	} while (read_mems_allowed_retry(cpuset_mems_cookie));
>  
> -	nid = next_node_in(current->il_prev, policy->nodes);
>  	if (nid < MAX_NUMNODES)
>  		current->il_prev = nid;
>  	return nid;
> @@ -2374,6 +2386,7 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>  		struct page **page_array)
>  {
>  	struct task_struct *me = current;
> +	unsigned int cpuset_mems_cookie;
>  	unsigned long total_allocated = 0;
>  	unsigned long nr_allocated = 0;
>  	unsigned long rounds;
> @@ -2391,7 +2404,13 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>  	if (!nr_pages)
>  		return 0;
>  
> -	nnodes = read_once_policy_nodemask(pol, &nodes);
> +	/* read the nodes onto the stack, retry if done during rebind */
> +	do {
> +		cpuset_mems_cookie = read_mems_allowed_begin();
> +		nnodes = read_once_policy_nodemask(pol, &nodes);
> +	} while (read_mems_allowed_retry(cpuset_mems_cookie));
> +
> +	/* if the nodemask has become invalid, we cannot do anything */
>  	if (!nnodes)
>  		return 0;