Re: [PATCH bpf-next v12 1/2] mm/bpf: Add bpf_proactive_reclaim kfunc

[JP Kobryn <inwardvessel@gmail.com>]

On 9/17/26 11:58 PM, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@kylinos.cn>
> 
> BPF programs can observe memory pressure on a cgroup, e.g. refault
> stats via bpf_mem_cgroup_page_state(), but cannot act on it:
> triggering reclaim requires writing to memory.reclaim, which BPF
> cannot do.
> 
> Add bpf_proactive_reclaim(), a sleepable kfunc performing one
> proactive reclaim pass on a memcg, like a write to memory.reclaim
> but without retrying until the target is reached, so that when and
> how hard to reclaim is BPF policy rather than hard-coded thresholds.
> The reclaim target of a single call is capped at MEMCG_CHARGE_BATCH,
> as high_work_func() does for memory.high; reclaiming more is left to
> the program, which can call the kfunc once per bpf_wq callback and
> stop at any point. It is limited to BPF_PROG_TYPE_SYSCALL, because
> other sleepable programs may run with filesystem locks held, on
> which the reclaim path could deadlock via filesystem shrinkers.
> 
> Convert MIN_SWAPPINESS, MAX_SWAPPINESS and SWAPPINESS_ANON_ONLY from
> macros to an enum so that they are emitted into BTF and usable from
> BPF programs via vmlinux.h.
> 
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---
>   mm/bpf_memcontrol.c | 62 ++++++++++++++++++++++++++++++++++++++++++++-
>   mm/internal.h       | 10 +++++---
>   2 files changed, 67 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> index 716df49d7647..c5d7f29ade85 100644
> --- a/mm/bpf_memcontrol.c
> +++ b/mm/bpf_memcontrol.c
> @@ -8,6 +8,8 @@
>   #include <linux/memcontrol.h>
>   #include <linux/bpf.h>
>   
> +#include "internal.h"
> +
>   __bpf_kfunc_start_defs();
>   
>   /**
> @@ -159,6 +161,48 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg)
>   	mem_cgroup_flush_stats(memcg);
>   }
>   
> +/**
> + * bpf_proactive_reclaim - proactively reclaim memory from a memory cgroup
> + * @memcg: the target memory cgroup to reclaim from.
> + * @size: the amount of memory to reclaim, in bytes, clamped to
> + *        MEMCG_CHARGE_BATCH.
> + * @swappiness: the reclaim swappiness, in the range [MIN_SWAPPINESS,
> + *              SWAPPINESS_ANON_ONLY], or -1 to use the memcg's own
> + *              swappiness.

The ANON_ONLY enumerator shouldn't be included as part of the range. I
would change this to [MIN_SWAPPINESS, MAX_SWAPPINESS] and then specify
that ANON_ONLY is a special mode like -1 is.

> + *
> + * Performs one proactive reclaim pass on @memcg, like a write to
> + * memory.reclaim but without retrying until @size is reached. Call it
> + * repeatedly to reclaim more than one batch.
> + *
> + * Only available to BPF_PROG_TYPE_SYSCALL, because other sleepable programs
> + * may run with filesystem locks held, which the reclaim path can deadlock
> + * on via filesystem shrinkers.
> + *
> + * Return: The amount of memory reclaimed, in bytes, or a negative error.
> + */
> +__bpf_kfunc long bpf_proactive_reclaim(struct mem_cgroup *memcg,
> +				       unsigned long size,
> +				       int swappiness)
> +{
> +	unsigned long nr_reclaimed;
> +	unsigned long nr_pages;
> +
> +	if (swappiness < -1 || swappiness > SWAPPINESS_ANON_ONLY)
> +		return -EINVAL;

Related to the previous comment, you treat the special values as part of
the range. It works currently, but creates a layout dependency on the
enum. I think it would be more future-proof if you did:

if (swappiness != -1 && swappiness != SWAPPINESS_ANON_ONLY) {
	if (swappiness < MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
		return -EINVAL;
}

The previous comments I brought up are now resolved, so assuming you'll
make the changes above you can include:

Reviewed-by: JP Kobryn <jp.kobryn@linux.dev>