Re: [PATCH v2 1/2] mm/huge_memory: use folio's memcg inside __folio_split()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Jul 30, 2026 at 10:18:00PM -0400, Zi Yan wrote:
> During a pagecache folio split, an xarray node allocation can happen and
> needs to charge at folio's memcg instead of folio split invoker's memcg,
> because for example folio split can happen during reclaim and reclaim's
> active memcg might not be folio's memcg. Switch to folio's memcg at the
> beginning and switch back afterwards.

I assume this is the only allocation? I guess in general it makes sense to have
the folio's memcg be active here regardless.

>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Fixes: 6b24ca4a1a8d4 ("mm: Use multi-index entries in the page cache")

Cc: stable?

> Signed-off-by: Zi Yan <ziy@nvidia.com>

Change seems reasonable overall.

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/huge_memory.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 04e8a6b553435..b9c2d8908e564 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4063,34 +4063,42 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>  	struct folio *end_folio = folio_next(folio);
>  	bool is_anon = folio_test_anon(folio);
> +	struct mem_cgroup *memcg, *old_memcg;
>  	struct address_space *mapping = NULL;
>  	struct anon_vma *anon_vma = NULL;
>  	int old_order = folio_order(folio);
>  	struct folio *new_folio, *next;
>  	int nr_shmem_dropped = 0;
>  	enum ttu_flags ttu_flags = 0;
> -	int ret;
>  	pgoff_t end = 0;
> +	int ret;
>
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
>
>  	if (folio != page_folio(split_at) || folio != page_folio(lock_at)) {
>  		ret = -EINVAL;
> -		goto out;
> +		goto out_no_memcg;
>  	}
>
>  	if (new_order >= old_order) {
>  		ret = -EINVAL;
> -		goto out;
> +		goto out_no_memcg;
>  	}
>
>  	ret = folio_check_splittable(folio, new_order, split_type);
>  	if (ret) {
>  		VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
> -		goto out;
> +		goto out_no_memcg;

This function really badly needs splitting up and probably some cleanup.h work :)

>  	}
>
> +	/*
> +	 * switch to folio's memcg as xarray node allocation can happen and
> +	 * needs to charge to it.
> +	 */
> +	memcg = get_mem_cgroup_from_folio(folio);
> +	old_memcg = set_active_memcg(memcg);
> +
>  	if (is_anon) {
>  		/*
>  		 * The caller does not necessarily hold an mmap_lock that would
> @@ -4231,6 +4239,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	if (mapping)
>  		i_mmap_unlock_read(mapping);
>  out:
> +	/* restore to caller's old_memcg */
> +	set_active_memcg(old_memcg);
> +	mem_cgroup_put(memcg);
> +out_no_memcg:
>  	xas_destroy(&xas);
>  	if (is_pmd_order(old_order))
>  		count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
>
> --
> 2.53.0
>

--
Cheers, Lorenzo