On Thu, Aug 27, 2026 at 06:36:29PM +0800, Hui Zhu wrote:
> From: Hui Zhu <[email protected]>
>
> 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 <[email protected]>
> ---
> mm/bpf_memcontrol.c | 46 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 46 insertions(+)
>
> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> index 716df49d7647..297ff7f05042 100644
> --- a/mm/bpf_memcontrol.c
> +++ b/mm/bpf_memcontrol.c
> @@ -6,6 +6,7 @@
> */
>
> #include <linux/memcontrol.h>
> +#include <linux/swap.h>
> #include <linux/bpf.h>
>
> __bpf_kfunc_start_defs();
> @@ -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;
I have been thinking about this more and more and after looking at the reasoning
behind your check current->reclaim_state and also Sashiko's comment on NOIO/NOFS
contexts, I am more convinced that this kfunc can not be a simple sleepable
function. We need more than that. We need clean process context as well similar
to the userspace poking memory.reclaim. Something like kthread or workqueue.
Kumar & Andrii, is there a way to restrict a kfunc to only be called from
special BPF threads/workqueues? Is there some similar concept in BPF world?
> +
> + 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)
> +
> BTF_KFUNCS_END(bpf_memcontrol_kfuncs)
>
> static const struct btf_kfunc_id_set bpf_memcontrol_kfunc_set = {
> --
> 2.53.0
>
