Re: [PATCH 1/2] mm/hugetlb_cgroup: move per-node usage on cross node migration

[Hongfu Li <hongfu.li@linux.dev>]

On 9/17/26 1:58 PM, Muchun Song wrote:
>
>> On Sep 16, 2026, at 16:30, Hongfu Li <hongfu.li@linux.dev> wrote:
>>
>> From: Hongfu Li <lihongfu@kylinos.cn>
>>
>> hugetlb.<size>.numa_stat uses folio_nid() to account usage in
>> __hugetlb_cgroup_commit_charge() and __hugetlb_cgroup_uncharge_folio().
>> hugetlb_cgroup_migrate() only moves hugetlb_cgroup pointers, leaving
>> per-node usage behind on the source node during cross-node migration.
>>
>> When the migrated folio gets uncharged, we subtract usage from the
>> destination node counter. This creates stale usage on the source node
>> and unsigned long counter underflow on the destination node.
>> The hugetlb.<size>.numa_stat interface exposes these incorrect per-node
>> usage values to userspace.
>>
>> Add a hugetlb_cgroup_move_usage() helper which moves the usage from the
>> old node to the new node, and call it from hugetlb_cgroup_migrate().
>>
>> Fixes: f47761999052 ("hugetlb: add hugetlb.*.numa_stat file")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
>> ---
>> mm/hugetlb_cgroup.c | 31 +++++++++++++++++++++++++++++++
>> 1 file changed, 31 insertions(+)
>>
>> diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
>> index ecb6e0b7819a..1040406e7e1d 100644
>> --- a/mm/hugetlb_cgroup.c
>> +++ b/mm/hugetlb_cgroup.c
>> @@ -179,6 +179,34 @@ static void hugetlb_cgroup_css_free(struct cgroup_subsys_state *css)
>> hugetlb_cgroup_free(hugetlb_cgroup_from_css(css));
>> }
>>
>> +static void hugetlb_cgroup_move_usage(struct hugetlb_cgroup *from,
>> + 				     struct hugetlb_cgroup *to,
>> + 				     struct folio *from_folio,
>> +  			             struct folio *to_folio)
>> +{
>> + 	int idx = hstate_index(folio_hstate(from_folio));
>> + 	unsigned long nr_pages = folio_nr_pages(from_folio);
>> + 	int from_nid = folio_nid(from_folio);
>> + 	int to_nid = folio_nid(to_folio);
>> + 	unsigned long usage;
>> +
>> + 	lockdep_assert_held(&hugetlb_lock);
>> +
>> + 	if (!from || !to)
>> + 		return;
>> +
>> + 	if (from == to && from_nid == to_nid)
>> + 		return;
>> +
>> + 	usage = READ_ONCE(from->nodeinfo[from_nid]->usage[idx]);
> READ_ONCE is unnecessary because there is no concurrent writer.

Thanks for the review.

You're right, the READ_ONCE is indeed unnecessary here. I'll drop it in v2.

>> + 	if (WARN_ON_ONCE(usage < nr_pages))
>> + 		return;
>> + 	WRITE_ONCE(from->nodeinfo[from_nid]->usage[idx], usage - nr_pages);
>> +
>> + 	usage = READ_ONCE(to->nodeinfo[to_nid]->usage[idx]);
> Same here.
>
>> + 	WRITE_ONCE(to->nodeinfo[to_nid]->usage[idx], usage + nr_pages);
>> +}
>> +
>> /*
>>   * Should be called with hugetlb_lock held.
>>   * Since we are holding hugetlb_lock, pages cannot get moved from
>> @@ -906,6 +934,9 @@ void hugetlb_cgroup_migrate(struct folio *old_folio, struct folio *new_folio)
>> 	/* move the h_cg details to new cgroup */
>> 	set_hugetlb_cgroup(new_folio, h_cg);
>> 	set_hugetlb_cgroup_rsvd(new_folio, h_cg_rsvd);
>> +
>> + 	hugetlb_cgroup_move_usage(h_cg, h_cg, old_folio, new_folio);
>> +
>> 	list_move(&new_folio->lru, &h->hugepage_activelist);
>> 	spin_unlock_irq(&hugetlb_lock);
>> }
>>
>> -- 
>> 2.54.0
>>
-- 
Best regards,
Hongfu