mm/hugetlb_cgroup: move per-node usage on cross node migration

Patch series "mm/hugetlb_cgroup: move the per-node usage along with the
folio", v2.

The per-node usage reported by hugetlb.<size>.numa_stat is accounted
against folio_nid() in __hugetlb_cgroup_commit_charge() and
__hugetlb_cgroup_uncharge_folio(), so it is only correct while a folio
stays charged on the same node and in the same hugetlb_cgroup.

Two paths move a folio which stays charged, and neither moves the usage
with it.  hugetlb_cgroup_migrate() only moves the hugetlb_cgroup pointers
of a folio migrated to another node, and hugetlb_cgroup_move_parent() only
moves the page_counter charges and the hugetlb_cgroup pointer of the
folios of a dying cgroup.  In both cases the node (or cgroup) which was
charged keeps a usage which never goes away, while the node (or cgroup)
which ends up uncharging the folio underflows as soon as the folio is
freed.

Patch 1/2 moves the usage along with the folio on cross node migration,
patch 2/2 does the same for the folios a dying cgroup reparents.


This patch (of 2):

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().

Link: https://lore.kernel.org/20260918-for-hugetlb-charge-v2-0-2b6d8c2bdc36@kylinos.cn
Link: https://lore.kernel.org/20260918-for-hugetlb-charge-v2-1-2b6d8c2bdc36@kylinos.cn
Fixes: f47761999052 ("hugetlb: add hugetlb.*.numa_stat file")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions