Re: [PATCH v2 2/2] mm/vmpressure: split v1 userspace eventfd code into vmpressure-v1.c

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jun 29, 2026 at 05:59:37AM -0700, Usama Arif wrote:
> Clean up mm/vmpressure.c by separating the cgroup v1 userspace eventfd
> interface from the shared and v2 in-kernel code.
> 
> Currently, almost half of mm/vmpressure.c exists to serve tree=true:
> struct vmpressure_event, the events list and its mutex, the work_struct
> and vmpressure_work_fn that drains tree_scanned/tree_reclaimed, the
> parent walk, vmpressure_event(), vmpressure_register_event(),
> vmpressure_unregister_event(), and vmpressure_prio() (which always
> calls vmpressure() with tree=true).
> 
> Move it all into a new mm/vmpressure-v1.c built only when
> CONFIG_MEMCG_V1=y (following the existing memcontrol-v1.o pattern).
> 
> vmpressure.c keeps the shared bits (constants, vmpressure_calc_level,
> the runtime hierarchy check, the tree=false body, init/cleanup
> plumbing) and calls into three small v1 hooks for the tree=true
> accumulator and the v1 portions of init/cleanup. The hooks have
> static-inline no-op stubs in include/linux/vmpressure.h for the
> !MEMCG_V1 case, so callers don't need ifdefs. vmpressure_prio() gets
> the same treatment, which means vmscan.c's call site disappears at
> compile time on v2-only kernels.
> 
> The only #ifdef CONFIG_MEMCG_V1 in source remains around the v1-only
> fields inside struct vmpressure itself.
> 
> Memory savings on CONFIG_MEMCG_V1=n (measured with pahole):
> 
>   struct vmpressure :  112B ->   24B
>   struct mem_cgroup : 1664B -> 1536B
> 
> This split is the first step toward eventually making vmpressure
> CONFIG_MEMCG_V1 only. The v2 in-kernel socket pressure path
> (tree=false) cannot be removed today immediately: PSI is not an
> exact replacement for vmpressure, and switching networking socket-buffer
> back-off to PSI may regress networking performance or increase memory
> pressure in workloads that today rely on vmpressure's hysteresis. The
> medium-term plan is to introduce a PSI-based socket-pressure path, keep
> vmpressure available for v2 behind a defconfig as an opt-out for several
> releases, and only then drop the tree=false path entirely, at which point
> everything that remains in mm/vmpressure-v1.c is the whole subsystem.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>