linux-kernel - Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED

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Date: Thu, 01 Feb 2024 11:02:47 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gregory.price@...verge.com>
Cc: Gregory Price <gourry.memverge@...il.com>,  <linux-mm@...ck.org>,
  <linux-kernel@...r.kernel.org>,  <linux-doc@...r.kernel.org>,
  <linux-fsdevel@...r.kernel.org>,  <linux-api@...r.kernel.org>,
  <corbet@....net>,  <akpm@...ux-foundation.org>,  <honggyu.kim@...com>,
  <rakie.kim@...com>,  <hyeongtak.ji@...com>,  <mhocko@...nel.org>,
  <vtavarespetr@...ron.com>,  <jgroves@...ron.com>,
  <ravis.opensrc@...ron.com>,  <sthanneeru@...ron.com>,
  <emirakhur@...ron.com>,  <Hasan.Maruf@....com>,
  <seungjun.ha@...sung.com>,  <hannes@...xchg.org>,
  <dan.j.williams@...el.com>,  Srinivasulu Thanneeru
 <sthanneeru.opensrc@...ron.com>
Subject: Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE
 for weighted interleaving

Gregory Price <gregory.price@...verge.com> writes:

> On Thu, Feb 01, 2024 at 09:55:07AM +0800, Huang, Ying wrote:
>> Gregory Price <gregory.price@...verge.com> writes:
>> > -       u8 __rcu *table, *weights, weight;
>> > +       u8 __rcu *table, __rcu *weights, weight;
>> 
>> The __rcu usage can be checked with `sparse` directly.  For example,
>> 
>> make C=1 mm/mempolicy.o
>> 
>
> fixed and squashed, all the __rcu usage i had except the global pointer
> have been used.  Thanks for the reference material, was struggling to
> understand that.
>
>> > task->mems_allowed_seq protection (added as 4th patch)
>> > ------------------------------------------------------
>> >
>> > +       cpuset_mems_cookie = read_mems_allowed_begin();
>> >         if (!current->il_weight || !node_isset(node, policy->nodes)) {
>> >                 node = next_node_in(node, policy->nodes);
>> 
>> node will be changed in the loop.  So we need to change the logic here.
>> 
>
> new patch, if it all looks good i'll ship it in v5
>
> ~Gregory
>
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index d8cc3a577986..4e5a640d10b8 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)) {
> -               node = next_node_in(node, policy->nodes);
> -               /* can only happen if nodemask is being rebound */
> +       unsigned int node;

IIUC, "node" may be used without initialization.

--
Best Regards,
Huang, Ying

> +       unsigned int cpuset_mems_cookie;
> +
> +retry:
> +       /* to prevent miscount use tsk->mems_allowed_seq to detect rebind */
> +       cpuset_mems_cookie = read_mems_allowed_begin();
> +       if (!current->il_weight ||
> +           !node_isset(current->il_prev, policy->nodes)) {
> +               node = next_node_in(current->il_prev, policy->nodes);
> +               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;