memcg: uint16_t for nr_bytes in obj_stock_pcp

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 on PAGE_SHIFT <=
16 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 configs with PAGE_SHIFT > 16 (PAGE_SIZE_256KB on hexagon and powerpc
44x, both 32-bit), uint16_t cannot represent the sub-page remainder. 
Define obj_stock_bytes_t as 'unsigned int' on those archs so nr_bytes can
hold the full remainder and the normal page-boundary flush in
__refill_obj_stock() and the page extraction in drain_obj_stock() both
work correctly.

The single-cache-line layout target only applies to PAGE_SHIFT <= 16;
those archs are 32-bit embedded and not the optimization target.

Link: https://lore.kernel.org/20260526033931.1760588-3-shakeel.butt@linux.dev
Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: kernel test robot <oliver.sang@intel.com>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions