Re: [PATCH v2 2/4] memcg: uint16_t for nr_bytes in obj_stock_pcp

[Muchun Song]

> On May 22, 2026, at 09:19, Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> 
> Currently struct obj_stock_pcp stores nr_bytes in an 'unsigned int'
> which is 4 bytes on 64-bit machines. Switch the field to uint16_t to
> shrink the per-CPU cache.
> 
> The kernel supports PAGE_SIZE_4KB, _8KB, _16KB, _32KB, _64KB and
> _256KB (see HAVE_PAGE_SIZE_* in arch/Kconfig). After the
> PAGE_SIZE-aligned flush in __refill_obj_stock(), the sub-page
> remainder fits in uint16_t up through 64KiB pages where PAGE_SIZE - 1
> == U16_MAX, but on 256KiB pages PAGE_SIZE - 1 == 0x3FFFF exceeds
> U16_MAX. The accumulator also needs to stay within uint16_t between
> page-aligned flushes on 64KiB pages where PAGE_SIZE itself is
> U16_MAX + 1.
> 
> Accumulate the new total in an 'unsigned int' local, then:
> 
>  1. Flush whenever the accumulator would hit U16_MAX. Together with
>     the existing allow_uncharge flush at PAGE_SIZE, this keeps the
>     uint16_t safe on PAGE_SIZE <= 64KiB.
> 
>  2. On configs with PAGE_SHIFT > 16 (PAGE_SIZE_256KB on hexagon and
>     powerpc 44x), push any sub-page remainder above U16_MAX into
>     objcg->nr_charged_bytes via atomic_add before storing back, so
>     the store cannot silently truncate. The PAGE_SHIFT > 16 guard
>     folds the branch out at compile time on smaller page sizes.
> 
> Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
> Tested-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Reviewed-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.