Re: [PATCH bpf-next 02/36] bpf: summarize may write stack slots in insn_aux_data

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2026-09-26 at 15:51 +0000, Alexei Starovoitov wrote:
> On Sat, Sep 26, 2026 at 07:20 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > @@ -662,6 +662,11 @@ struct bpf_insn_aux_data {
> >  	};
> >  	struct btf_struct_meta *kptr_struct_meta;
> >  	u64 map_key_state; /* constant (32 bit) key tracking for maps */
> > +	/*
> > +	 * Per-instruction summary of stack slots in the current frame
> > +	 * that this instruction may write to.
> > +	 */
> > +	DECLARE_BITMAP(may_write_mask, MAX_BPF_STACK_SLOTS);
> 
> That's 32 bytes per insn and the next patch adds 32 more,
> for every prog whether it has loops or not.
> FM_MAY_WRITE in patch 1 makes every frame_masks a third bigger too.
> commit 481ceda77aeb sized the liveness masks by the stack the frame
> uses to avoid exactly that.
> 
> bpf_may_write_mask() is the only accessor.
> Can the summary stay in liveness.c, as wide as the subprog's stack,
> and only for insns with scc != 0 ?

I did some profiling and may_write_mask does not show as a big hog in
total memory consumption increase. (+50% is the biggest observed
increase). The biggest increase is from a loop_stack_entry array in
bpf_func_state, and from a fact that I pushed bpf_func_state to a 2KB
allocation bucket. I'll see what can be done there.