Re: [RFC PATCH 3/4] memcg_ext: allow BPF to defer memory.high enforcement

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

On Thu, Sep 24, 2026 at 01:14:59PM -0700, JP Kobryn wrote:
[...]
> > +static void bpf_memcg_ctx_init(struct bpf_memcg_ctx *ctx,
> > +			       struct mem_cgroup *memcg,
> > +			       struct mem_cgroup *over_limit, gfp_t gfp_mask)
> > +{
> > +	ctx->memcg = memcg;
> > +	ctx->memcg_over_limit = over_limit;
> > +	ctx->task = current;
> > +	ctx->cgroup_id = cgroup_id(memcg->css.cgroup);
> > +	ctx->over_limit_cgroup_id = over_limit ?
> > +		cgroup_id(over_limit->css.cgroup) : 0;
> > +	ctx->nr_pages_over_high = current->memcg_nr_pages_over_high;
> > +	ctx->gfp_flags = (__force u32)gfp_mask;
> > +}
> > +
> > +u32 bpf_memcg_high_policy(struct mem_cgroup *memcg,
> > +			  struct mem_cgroup *over_limit, gfp_t gfp_mask)
> > +{
> > +	const struct bpf_prog_array_item *item;
> > +	const struct bpf_memcg_ops *ops;
> > +	struct bpf_memcg_ctx ctx;
> > +	u32 acc = BPF_MEMCG_HIGH_NO_OPINION;
> > +	struct cgroup *cgrp;
> > +
> > +	if (!cgroup_bpf_enabled(CGROUP_MEMCG_OPS))
> > +		return acc;
> > +
> > +	/*
> > +	 * Only the default hierarchy has a cgroup_bpf, and the static key is
> > +	 * global, so one policy anywhere turns this on for v1 memcgs too.  A
> > +	 * v1 memcg still cannot get here, because memory.high and swap.high
> > +	 * are both v2-only and so it never builds the debt that leads to this
> > +	 * call.  A hook on a path v1 can reach needs its own cgroup_on_dfl()
> > +	 * test: a v1 cgroup has no effective array and an uninitialised
> > +	 * cgrp->bpf.refcnt.
> > +	 */
> > +	cgrp = memcg->css.cgroup;
> > +
> > +	/*
> > +	 * A program can allocate and re-enter the charge path.  Skip the
> > +	 * nested call.  This guards the callbacks only.
> > +	 */
> > +	if (current->in_bpf_memcg)
> > +		return acc;
> > +	current->in_bpf_memcg = 1;
> > +
> > +	rcu_read_lock_dont_migrate();
> > +
> > +	/*
> > +	 * A memcg outlives its cgroup while it has charges, and
> > +	 * cgroup_bpf_release() frees the arrays when the cgroup goes.
> > +	 */
> > +	if (!cgroup_bpf_tryget_live(cgrp))
> > +		goto out;
> > +
> > +	bpf_memcg_ctx_init(&ctx, memcg, over_limit, gfp_mask);
> > +
> > +	bpf_cgroup_struct_ops_foreach(ops, item, cgrp, CGROUP_MEMCG_OPS) {
> > +		if (ops->high_policy)
> > +			acc |= ops->high_policy(&ctx) &
> > +			       BPF_MEMCG_HIGH_VALID_MASK;
> > +	}
> 
> If I'm reading correctly, the gfp_mask at this point doesn't account for
> task restrictions, so the BPF program may see __GFP_FS, __GFP_IO, etc
> which may later be cleared when setting up the scan_control instance.

I am passing the same gfp_mask which has been passed to try_charge_memcg(), so
it should be same as what reclaim_high (reclaim internal) sees.