Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE for weighted interleaving

[Huang, Ying]

Gregory Price <gregory.price@xxxxxxxxxxxx> writes:

> On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
>> Gregory Price <gregory.price@xxxxxxxxxxxx> writes:
>> 
>> >
>> > I think this is handled already? It is definitely an explicit race
>> > condition that is documented elsewhere:
>> >
>> > /*
>> >  * mpol_rebind_policy - Migrate a policy to a different set of nodes
>> >  *
>> >  * Per-vma policies are protected by mmap_lock. Allocations using per-task
>> >  * policies are protected by task->mems_allowed_seq to prevent a premature
>> >  * OOM/allocation failure due to parallel nodemask modification.
>> >  */
>> 
>> Thanks for pointing this out!
>> 
>> If we use task->mems_allowed_seq reader side in
>> weighted_interleave_nodes() we can guarantee the consistency of
>> policy->nodes.  That may be not deserved, because it's not a big deal to
>> allocate 1 page in a wrong node.
>> 
>> It makes more sense to do that in
>> alloc_pages_bulk_array_weighted_interleave(), because a lot of pages may
>> be allocated there.
>> 
>
> To save the versioning if there are issues, here are the 3 diffs that
> I have left. If you are good with these changes, I'll squash the first
> 2 into the third commit, keep the last one as a separate commit (it
> changes the interleave_nodes() logic too), and submit v5 w/ your
> reviewed tag on all of them.
>
>
> Fix one (pedantic?) warning from syzbot:
> ----------------------------------------
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index b1437396c357..dfd097009606 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2391,7 +2391,7 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>         unsigned long nr_allocated = 0;
>         unsigned long rounds;
>         unsigned long node_pages, delta;
> -       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

More details can be found in

https://www.kernel.org/doc/html/latest/dev-tools/sparse.html

Per my understanding, we shouldn't use "__rcu" here.  Please search
"__rcu" in the following document.

https://www.kernel.org/doc/html/latest/RCU/checklist.html

>         unsigned int weight_total = 0;
>         unsigned long rem_pages = nr_pages;
>         nodemask_t nodes;
>
>
>
> Simplifying resume_node/weight logic:
> -------------------------------------
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 2c1aef8eab70..b0ca9bcdd64c 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2405,15 +2405,9 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>                 page_array += nr_allocated;
>                 total_allocated += nr_allocated;
>                 /* if that's all the pages, no need to interleave */
> -               if (rem_pages < weight) {
> -                       /* stay on current node, adjust il_weight */
> +               if (rem_pages <= weight) {
>                         me->il_weight -= rem_pages;
>                         return total_allocated;
> -               } else if (rem_pages == weight) {
> -                       /* move to next node / weight */
> -                       me->il_prev = next_node_in(node, nodes);
> -                       me->il_weight = get_il_weight(me->il_prev);
> -                       return total_allocated;
>                 }
>                 /* Otherwise we adjust remaining pages, continue from there */
>                 rem_pages -= weight;
> @@ -2460,17 +2454,10 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>                         node_pages += weight;
>                         delta -= weight;
>                 } else if (delta) {
> +                       /* when delta is deleted, resume from that node */
                                           ~~~~~~~
                                           depleted?


>                         node_pages += delta;
> -                       /* delta may deplete on a boundary or w/ a remainder */
> -                       if (delta == weight) {
> -                               /* boundary: resume from next node/weight */
> -                               resume_node = next_node_in(node, nodes);
> -                               resume_weight = weights[resume_node];
> -                       } else {
> -                               /* remainder: resume this node w/ remainder */
> -                               resume_node = node;
> -                               resume_weight = weight - delta;
> -                       }
> +                       resume_node = node;
> +                       resume_weight = weight - delta;
>                         delta = 0;
>                 }
>                 /* node_pages can be 0 if an allocation fails and rounds == 0 */
>
>
>
>
>
> task->mems_allowed_seq protection (added as 4th patch)
> ------------------------------------------------------
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index b0ca9bcdd64c..b1437396c357 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1879,10 +1879,15 @@ 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;
> +       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(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.

> -               /* 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;
> @@ -1896,10 +1901,17 @@ 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;
> @@ -2388,10 +2401,17 @@ static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
>         int prev_node;
>         int i;
>
> +

Change by accident?

>         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;

--
Best Regards,
Huang, Ying