Re: [PATCH 1/1] memcg/hugetlb: Adding hugeTLB counters to memory controller

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Oct 17, 2024 at 09:04:38AM -0700, Joshua Hahn wrote:
> HugeTLB is added as a metric in memcg_stat_item, and is updated in the
> alloc and free methods for hugeTLB, after (un)charging has already been
> committed. Changes are batched and updated / flushed like the rest of
> the memcg stats, which makes additional overhead by the infrequent
> hugetlb allocs / frees minimal.
> 
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> ---
>  include/linux/memcontrol.h | 3 +++
>  mm/hugetlb.c               | 5 +++++
>  mm/memcontrol.c            | 6 ++++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 34d2da05f2f1..66e925ae499a 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -39,6 +39,9 @@ enum memcg_stat_item {
>  	MEMCG_KMEM,
>  	MEMCG_ZSWAP_B,
>  	MEMCG_ZSWAPPED,
> +#ifdef CONFIG_HUGETLB_PAGE
> +	MEMCG_HUGETLB,
> +#endif
>  	MEMCG_NR_STAT,
>  };

It would be better to add a native vmstat counter for this, as there
is no strong reason to make this memcg specific. This would also make
it NUMA-node-aware.

IOW, add a new item to enum node_stat_item (plus the string in
vmstat.c and memcontrol.c).

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 190fa05635f4..ca7151096712 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1887,6 +1887,7 @@ void free_huge_folio(struct folio *folio)
>  	struct hstate *h = folio_hstate(folio);
>  	int nid = folio_nid(folio);
>  	struct hugepage_subpool *spool = hugetlb_folio_subpool(folio);
> +	struct mem_cgroup *memcg = get_mem_cgroup_from_current();
>  	bool restore_reserve;
>  	unsigned long flags;
>  
> @@ -1926,6 +1927,8 @@ void free_huge_folio(struct folio *folio)
>  	hugetlb_cgroup_uncharge_folio_rsvd(hstate_index(h),
>  					  pages_per_huge_page(h), folio);
>  	mem_cgroup_uncharge(folio);
> +	mod_memcg_state(memcg, MEMCG_HUGETLB, -pages_per_huge_page(h));
> +	mem_cgroup_put(memcg);
>  	if (restore_reserve)
>  		h->resv_huge_pages++;

This goes wrong if the folio is freed by somebody other than the
owning cgroup. For example if the task moved between cgroups after the
memory was charged.

It's better to use the folio->memcg linkage that was established by
the allocation path.

Use lruvec_stat_mod_folio(), it will handle all of this.

> @@ -3093,6 +3096,8 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>  
>  	if (!memcg_charge_ret)
>  		mem_cgroup_commit_charge(folio, memcg);
> +
> +	mod_memcg_state(memcg, MEMCG_HUGETLB, nr_pages);

And here as well.