Re: [PATCH bpf-next v5 1/2] mm/bpf: Add bpf_proactive_reclaim kfunc

[bot+bpf-ci@kernel.org]

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> mm/bpf: Add bpf_proactive_reclaim kfunc
>
> Add bpf_proactive_reclaim(), a sleepable kfunc which performs one
> proactive reclaim pass on a given memory cgroup, similar to a write
> to memory.reclaim but without retrying until the target is reached.
>
> The kfunc refuses to reclaim if the calling task is already in a
> reclaim context, as a nested reclaim would corrupt the outer reclaim
> state.
>
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>

This isn't a bug, but could the changelog say why this kfunc is needed -
the intended consumer, and why a single non-retrying pass is preferred
over what a write to memory.reclaim does - rather than restating what the
function does?

> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> index 716df49d76477..297ff7f050427 100644
> --- a/mm/bpf_memcontrol.c
> +++ b/mm/bpf_memcontrol.c

[ ... ]

> @@ -159,6 +160,49 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg)
>  	mem_cgroup_flush_stats(memcg);
>  }
>
> +/*
> + * Reclaim must not recurse: try_to_free_mem_cgroup_pages() overwrites
> + * current->reclaim_state, so a nested call would corrupt the outer
> + * reclaim state. Reclaim windows are marked with PF_MEMALLOC;
> + * reclaim_state is also checked because it is installed slightly
> + * before PF_MEMALLOC.
> + */
> +static bool bpf_in_reclaim_context(void)
> +{
> +	return (current->flags & PF_MEMALLOC) || current->reclaim_state;
> +}
> +
> +/**
> + * 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
> + *
> + * Trigger one proactive reclaim pass on @memcg, similar to a write to
> + * memory.reclaim, but without retrying until @size is reached.
> + * Must not be called with a filesystem lock held: the reclaim path
> + * may deadlock on it via filesystem shrinkers.
> + *
> + * Return: The amount of memory reclaimed, in bytes, or 0 if @size is
> + * smaller than a page or the task is already in a reclaim context.
> + */
> +__bpf_kfunc unsigned long bpf_proactive_reclaim(struct mem_cgroup *memcg,
> +						unsigned long size)
> +{
> +	unsigned long nr_reclaimed;
> +
> +	if (size < PAGE_SIZE || unlikely(bpf_in_reclaim_context()))
> +		return 0;
> +
> +	nr_reclaimed = try_to_free_mem_cgroup_pages(memcg, size / PAGE_SIZE,
> +						    GFP_KERNEL,
> +						    MEMCG_RECLAIM_MAY_SWAP |
> +						    MEMCG_RECLAIM_PROACTIVE,
> +						    NULL);
> +
> +	return nr_reclaimed * PAGE_SIZE;
> +}
> +
>  __bpf_kfunc_end_defs();
>
>  BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
> @@ -172,6 +216,8 @@ BTF_ID_FLAGS(func, bpf_mem_cgroup_usage)
>  BTF_ID_FLAGS(func, bpf_mem_cgroup_page_state)
>  BTF_ID_FLAGS(func, bpf_mem_cgroup_flush_stats, KF_SLEEPABLE)
>
> +BTF_ID_FLAGS(func, bpf_proactive_reclaim, KF_SLEEPABLE)
> +

This isn't a bug, but would it be worth extending
tools/testing/selftests/bpf/progs/cgroup_iter_memcg.c (or adding a small
test) to exercise bpf_proactive_reclaim?  The sibling kfuncs registered in
the same set (bpf_get_mem_cgroup, bpf_mem_cgroup_flush_stats,
bpf_mem_cgroup_page_state, bpf_mem_cgroup_vm_events) are all exercised by
that test, and a test would pin down the byte units and the 'returns 0'
cases.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/33064477499