Re: [PATCH 2/3] mm: memcontrol: correct the parameter type of __mod_memcg{_lruvec}_state()

["Harry Yoo (Oracle)" <harry@kernel.org>]

On Tue, Mar 24, 2026 at 07:31:28PM +0800, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> The __mod_memcg_state() and __mod_memcg_lruvec_state() were used to
> reparent non-hierarchical stats, the values passed to them might exceed
> the upper limit of the type int, so correct the val parameter type of them
> to long.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  include/trace/events/memcg.h | 10 +++++-----
>  mm/memcontrol.c              |  8 ++++----
>  2 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7fb9cbc10dfbb..4a78550f6174e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -527,7 +527,7 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec,
>  
>  #ifdef CONFIG_MEMCG_V1
>  static void __mod_memcg_lruvec_state(struct mem_cgroup_per_node *pn,
> -				     enum node_stat_item idx, int val);
> +				     enum node_stat_item idx, long val);
>  
>  void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg,
>  				       struct mem_cgroup *parent, int idx)
> @@ -784,7 +784,7 @@ static int memcg_page_state_unit(int item);
>   * Normalize the value passed into memcg_rstat_updated() to be in pages. Round
>   * up non-zero sub-page updates to 1 page as zero page updates are ignored.
>   */
> -static int memcg_state_val_in_pages(int idx, int val)
> +static long memcg_state_val_in_pages(int idx, long val)
>  {
>  	int unit = memcg_page_state_unit(idx);

Sashiko AI made an interesting argument [1] that this could lead to
incorrectly returning a very large positive number. Let me verify that.

[1] https://sashiko.dev/#/patchset/cover.1774342371.git.zhengqi.arch%40bytedance.com

Sashiko wrote:
> Does this change inadvertently break the handling of negative byte-sized
> updates?
> Looking at the rest of the function:
> 	if (!val || unit == PAGE_SIZE)
> 		return val;
> 	else
> 		return max(val * unit / PAGE_SIZE, 1UL);

> PAGE_SIZE is defined as an unsigned long.

Right, it's defined as 1UL << PAGE_SHIFT.

> When val is negative, such as during uncharging of byte-sized stats like
> MEMCG_ZSWAP_B, the expression val * unit is a negative long.

Right.

> Dividing a signed long by an unsigned long causes the signed long to be
> promoted to unsigned before division,

Right.

> resulting in a massive positive
> number instead of a small negative one.

Let's look at an example (assuming unit is 1).

val = val * unit = -16384 (-16 KiB)
val * unit / PAGE_SIZE = 0xFFFFFFFFFFFFC000 / PAGE_SIZE = 0x3FFFFFFFFFFFFF
max(0x3FFFFFFFFFFFFF, 1UL) = 0x3FFFFFFFFFF

Yeah, that's a massive positive number.

Hmm but how did it work when it was int?

val = val * unit = -16384 (-16KiB)
val * unit / PAGE_SIZE = 0xFFFFFFFFFFFFC000 / PAGE_SIZE = 0x3FFFFFFFFFFFFF
max(val * unit / PAGE_SIZE, 1UL) = 0x3FFFFFFFFFFFFF
(int)0x3FFFFFFFFFFFFF = 0xFFFFFFFF = (-1)

That's incorrect. It should have been -4?

> Before this change, the function returned an int, which implicitly truncated
> the massive unsigned 64-bit result to a 32-bit int, accidentally yielding the
> correct negative arithmetic value.

So... "accidentally yielding the correct negative arithemetic value"
is wrong.

Sounds like it's been subtly broken even before this patch and nobody
noticed.

> By changing the return type to long, this implicit truncation is removed,
> and the huge positive value is returned unaltered.

That's true.

> Could this corrupt tracepoint logs when passed to trace_mod_memcg_state?

I'm not sure if that's critical but yeah that's true.

> Also, would passing this huge positive value to memcg_rstat_updated instantly
> exceed the charge batch threshold and trigger endless, expensive global
> cgroup_rstat flushing, severely degrading system performance?

It would lead to more frequent flushes, at least.

-- 
Cheers,
Harry / Hyeonggon