[PATCH 6.18.y] mm/hugetlb: charge folios to the target mm's memcg

[Sasha Levin <sashal@kernel.org>]

From: Jinmeng Zhou <jinmengzhou22@gmail.com>

[ Upstream commit c2deb1356d82c9d54bd2fcf1241cc97eb66fd81f ]

HugeTLB folios are currently charged to the memcg of the allocating task.
This gives the wrong result when a userfaultfd handler populates a HugeTLB
VMA that belongs to another process.  The UFFDIO_COPY ioctl operates on
the userfaultfd context's mm, but get_mem_cgroup_from_current() charges
the folio to the handler's memcg instead.

This can be reproduced by placing the faulting process and its userfaultfd
handler in different memory cgroups.  Have the target process register a
HugeTLB mapping with userfaultfd, trigger a missing fault, and let the
handler resolve it with UFFDIO_COPY.  The hugepage usage is then reported
in the handler's memory.current instead of the target's.

The generic userfaultfd population path avoids this problem by charging
folios to dst_vma->vm_mm.

Pass the target mm through hugetlb_alloc_folio() and charge the folio by
using get_mem_cgroup_from_mm().  This preserves the existing charge timing
and error handling while making HugeTLB userfaultfd population consistent
with the generic path.

Link: https://lore.kernel.org/20260903075048.3316-1-zhoujinmeng@bytedance.com
Fixes: 8cba9576df60 ("hugetlb: memcg: account hugetlb-backed memory in memory controller")
Signed-off-by: Jinmeng Zhou <zhoujinmeng@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Hongfu Li <lihongfu@kylinos.cn>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org>
[ sashal: Passed vma->vm_mm directly to mem_cgroup_charge_hugetlb() in alloc_hugetlb_folio() because hugetlb_alloc_folio() is absent. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/memcontrol.h | 8 +++++---
 mm/hugetlb.c               | 2 +-
 mm/memcontrol.c            | 6 ++++--
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 706ddda4a60b1..d88f241e565bc 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -662,7 +662,8 @@ static inline int mem_cgroup_charge(struct folio *folio, struct mm_struct *mm,
 	return __mem_cgroup_charge(folio, mm, gfp);
 }
 
-int mem_cgroup_charge_hugetlb(struct folio* folio, gfp_t gfp);
+int mem_cgroup_charge_hugetlb(struct folio *folio, struct mm_struct *mm,
+			      gfp_t gfp);
 
 int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
 				  gfp_t gfp, swp_entry_t entry);
@@ -1172,9 +1173,10 @@ static inline int mem_cgroup_charge(struct folio *folio,
 	return 0;
 }
 
-static inline int mem_cgroup_charge_hugetlb(struct folio* folio, gfp_t gfp)
+static inline int mem_cgroup_charge_hugetlb(struct folio *folio,
+					    struct mm_struct *mm, gfp_t gfp)
 {
-        return 0;
+	return 0;
 }
 
 static inline int mem_cgroup_swapin_charge_folio(struct folio *folio,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bf1188e9b1c9b..8993bd37f6cd8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3124,7 +3124,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 		}
 	}
 
-	ret = mem_cgroup_charge_hugetlb(folio, gfp);
+	ret = mem_cgroup_charge_hugetlb(folio, vma->vm_mm, gfp);
 	/*
 	 * Unconditionally increment NR_HUGETLB here. If it turns out that
 	 * mem_cgroup_charge_hugetlb failed, then immediately free the page and
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 97ed5ad4053f0..2860de542a4af 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4768,6 +4768,7 @@ int __mem_cgroup_charge(struct folio *folio, struct mm_struct *mm, gfp_t gfp)
 /**
  * mem_cgroup_charge_hugetlb - charge the memcg for a hugetlb folio
  * @folio: folio being charged
+ * @mm: mm context of the allocation target
  * @gfp: reclaim mode
  *
  * This function is called when allocating a huge page folio, after the page has
@@ -4777,9 +4778,10 @@ int __mem_cgroup_charge(struct folio *folio, struct mm_struct *mm, gfp_t gfp)
  * Returns ENOMEM if the memcg is already full.
  * Returns 0 if either the charge was successful, or if we skip the charging.
  */
-int mem_cgroup_charge_hugetlb(struct folio *folio, gfp_t gfp)
+int mem_cgroup_charge_hugetlb(struct folio *folio, struct mm_struct *mm,
+			      gfp_t gfp)
 {
-	struct mem_cgroup *memcg = get_mem_cgroup_from_current();
+	struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
 	int ret = 0;
 
 	/*
-- 
2.53.0