Re: [PATCH v2 2/2] mm/memcg: migrate per-node hugetlb lruvec stat together with hugetlb folio

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

On 9/23/26 3:56 PM, Muchun Song wrote:
>
>> On Sep 23, 2026, at 11:40, Hongfu Li <hongfu.li@linux.dev> wrote:
>>
>>
>> On 9/23/26 10:40 AM, Muchun Song wrote:
>>>
>>> On 2026/9/23 10:05, Hongfu Li wrote:
>>>> From: Hongfu Li <lihongfu@kylinos.cn>
>>>>
>>>> memory.numa_stat exposes per-node hugetlb counters from per-node lruvec
>>>> stats. These stats are accounted against folio_nid(): incremented on
>>>> the folio's node when handed to a user, decremented when the folio is
>>>> returned to the pool.
>>>>
>>>> During hugetlb folio migration, mem_cgroup_migrate() moves the charge
>>>> to the new folio and drops the memcg data of the old one, so the free
>>>> of the old folio right after the migration skips the memcg per-node
>>>> lruvec decrement. The hugetlb count stays attributed to the old node
>>>> for the rest of the life of the charge, while the target folio gets no
>>>> increment on the new node; its later free decrements a counter that
>>>> was never incremented.
>>>>
>>>> Migrate the per-node lruvec accounting alongside migration. Global
>>>> memcg totals remain balanced because they track resource consumption,
>>>> not node placement.
>>>>
>>>> Fixes: 05d4532b60e3 ("memcg/hugetlb: add hugeTLB counters to memcg")
>>>> Cc: stable@vger.kernel.org
>>>> Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
>>>> Tested-by: Joshua Hahn <joshua.hahnjy@gmail.com>
>>>> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
>>>> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>>>> ---
>>>>    include/linux/memcontrol.h |  8 ++++++++
>>>>    mm/hugetlb.c               | 25 +++++++++++++++++++++++++
>>>>    mm/memcontrol.c            |  5 ++---
>>>>    3 files changed, 35 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>>> index a8358f297b65..74110a324f9e 100644
>>>> --- a/include/linux/memcontrol.h
>>>> +++ b/include/linux/memcontrol.h
>>>> @@ -984,6 +984,9 @@ unsigned long lruvec_page_state_monotonic(const struct lruvec *lruvec,
>>>>    unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
>>>>                          enum node_stat_item idx);
>>>>    +void mod_memcg_lruvec_state(struct lruvec *lruvec,
>>>> +                enum node_stat_item idx, int val);
>>>> +
>>>>    void mem_cgroup_flush_stats(struct mem_cgroup *memcg);
>>>>    void mem_cgroup_flush_stats_ratelimited(struct mem_cgroup *memcg);
>>>>    @@ -1452,6 +1455,11 @@ static inline unsigned long lruvec_page_state_local(const struct lruvec *lruvec,
>>>>        return node_page_state(lruvec_pgdat(lruvec), idx);
>>>>    }
>>>>    +static inline void mod_memcg_lruvec_state(struct lruvec *lruvec,
>>>> +                      enum node_stat_item idx, int val)
>>>> +{
>>>> +}
>>>> +
>>>>    static inline void mem_cgroup_flush_stats(struct mem_cgroup *memcg)
>>>>    {
>>>>    }
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 519c30b338a8..76d019594b39 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -23,6 +23,7 @@
>>>>    #include <linux/mmdebug.h>
>>>>    #include <linux/sched/signal.h>
>>>>    #include <linux/rmap.h>
>>>> +#include <linux/rcupdate.h>
>>>>    #include <linux/string_choices.h>
>>>>    #include <linux/string_helpers.h>
>>>>    #include <linux/swap.h>
>>>> @@ -7378,12 +7379,36 @@ void folio_putback_hugetlb(struct folio *folio)
>>>>        folio_put(folio);
>>>>    }
>>>>    +static void move_hugetlb_lruvec_stat(struct folio *old_folio,
>>>> +                     struct folio *new_folio)
>>>> +{
>>>> +    struct mem_cgroup *memcg;
>>>> +    long nr_pages = folio_nr_pages(old_folio);
>>>> +    int old_nid = folio_nid(old_folio);
>>>> +    int new_nid = folio_nid(new_folio);
>>>> +
>>>> +    if (old_nid == new_nid)
>>>> +        return;
>>>> +
>>>> +    guard(rcu)();
>>>> +
>>>> +    memcg = folio_memcg(new_folio);
>>>> +    if (!memcg)
>>>> +        return;
>>>> +
>>>> +    mod_memcg_lruvec_state(mem_cgroup_lruvec(memcg, NODE_DATA(old_nid)),
>>>> +                   NR_HUGETLB, -nr_pages);
>>> Why not use mod_lruvec_state? mod_memcg_lruvec_state is an internal
>>> API for memcg, I don't want it to be exported.
>> Thank you for the review.
>>
>> mod_lruvec_state() would update the node counter a second time.  It calls
>> mod_node_page_state() as well, and the target's node counter is already
>> updated in alloc_hugetlb_folio_nodemask() (patch 1/2):
>>
>>      lruvec_stat_mod_folio(folio, NR_HUGETLB, folio_nr_pages(folio));
>>
>> For an uncharged folio lruvec_stat_mod_folio() only updates the node
>> counter.  The target folio is not charged to any memcg at that point; its
>> charge only appears later in mem_cgroup_migrate().  So the node side is
>> already covered and only the per-memcg attribution has to follow the
>> charge here.
> Looking at the first patch again, I do not think the
> overall accounting is incorrect. My concern is where NR_HUGETLB
> accounting is performed.
>
> The low-level allocation helpers only take a folio from the
> hugetlb pool. They do not have enough context to decide how
> the folio should be accounted. That decision should be left
> to their callers, once the folio enters the relevant usage
> lifecycle.
>
> hugetlb_alloc_folio() is a good example. It first obtains a
> folio, then calls mem_cgroup_charge_hugetlb(), and only
> afterwards calls lruvec_stat_mod_folio(). At that point,
> the folio has the correct memcg and lruvec, so all relevant
> counters can be updated together.
>
> After this series, alloc_hugetlb_folio_reserve() and
> alloc_hugetlb_folio_nodemask() account NR_HUGETLB before
> returning the folio. This makes them unsuitable for callers
> that need to charge the folio afterwards. Before the charge,
> lruvec_stat_mod_folio() can update only the node counter.
> The later charge does not associate that earlier update with
> the new memcg.
>
> If a caller accounts again after charging, the node counter is
> updated twice. If it does not, the memcg and per-node lruvec
> counters remain missing. The caller then needs a memcg-only
> correction, creating a special accounting protocol between
> the helper and its callers.
>
> Migration exposes the same issue. The target gets node
> accounting before it inherits the source memcg. The second patch
> must update the lruvec state and then cancel its node changes,
> because the target allocation already updated the node counter.
> This works, but makes the accounting lifecycle harder to follow.
>
> Would it be cleaner to keep these helpers focused on allocation
> and let each caller account at the correct lifecycle point?
> Normal allocation can account after a successful memcg charge.
> Migration can move the complete lruvec state after it succeeds.

Hi Muchun,

Thanks a lot for the detailed explanation.

One thing keeps the target accounting in alloc_hugetlb_folio_nodemask(): the
node counter is paired with the folio's free, and a failed migration 
frees its
target right away (put_new_folio()/folio_put() -> free_huge_folio(), which
always decrements the node counter). If the target were accounted only 
after a
successful migration, every failed migration would decrement a node counter
that was never incremented.

Unless there is a way to let free_huge_folio() tell that the folio comes
from a failed migration, and skip the decrement for it.

> An uncharged path can explicitly update only the node counter
> if global accounting is still required.
>
> This would keep policy out of the low-level allocator and make
> the accounting lifecycle easier to follow.
>
> Thanks.
>
>>> Thanks.
>>>
>>>> + mod_memcg_lruvec_state(mem_cgroup_lruvec(memcg, NODE_DATA(new_nid)),
>>>> +                   NR_HUGETLB, nr_pages);
>>>> +}
>>>> +
>>>>    void move_hugetlb_state(struct folio *old_folio, struct folio *new_folio,
>>>>                enum migrate_reason reason)
>>>>    {
>>>>        struct hstate *h = folio_hstate(old_folio);
>>>>          hugetlb_cgroup_migrate(old_folio, new_folio);
>>>> +    move_hugetlb_lruvec_stat(old_folio, new_folio);
>>>>        folio_set_owner_migrate_reason(new_folio, reason);
>>>>          /*
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 88824f783571..a5335da5d425 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -1015,9 +1015,8 @@ static void __mod_memcg_lruvec_state(struct mem_cgroup_per_node *pn,
>>>>        put_cpu();
>>>>    }
>>>>    -static void mod_memcg_lruvec_state(struct lruvec *lruvec,
>>>> -                     enum node_stat_item idx,
>>>> -                     int val)
>>>> +void mod_memcg_lruvec_state(struct lruvec *lruvec,
>>>> +                enum node_stat_item idx, int val)
>>>>    {
>>>>        struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>>>>        struct mem_cgroup_per_node *pn;
>>>>
>> -- 
>> Best regards,
>> Hongfu

-- 
Best regards,
Hongfu