On Wed, Aug 27, 2025 at 04:34:48PM +0100, Lorenzo Stoakes wrote:
> > +__bpf_kfunc_start_defs();
> > +
> > +/**
> > + * bpf_mm_get_mem_cgroup - Get the memory cgroup associated with a mm_struct.
> > + * @mm: The mm_struct to query
> > + *
> > + * The obtained mem_cgroup must be released by calling bpf_put_mem_cgroup().
> > + *
> > + * Return: The associated mem_cgroup on success, or NULL on failure. Note that
> > + * this function depends on CONFIG_MEMCG being enabled - it will always return
> > + * NULL if CONFIG_MEMCG is not configured.
>
> What kind of locking is assumed here?
>
> Are we protected against mmdrop() clearing out the mm?
No locking is needed. Just the valid mm object or NULL. Usually the
underlying function (get_mem_cgroup_from_mm) is called in page fault
context where the current is holding mm. Here the only requirement is
that mm is valid either through explicit reference or the context.
>
> > + */
> > +__bpf_kfunc struct mem_cgroup *bpf_mm_get_mem_cgroup(struct mm_struct *mm)
> > +{
> > + return get_mem_cgroup_from_mm(mm);
> > +}
> > +
> > +/**
> > + * bpf_put_mem_cgroup - Release a memory cgroup obtained from bpf_mm_get_mem_cgroup()
> > + * @memcg: The memory cgroup to release
> > + */
> > +__bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
> > +{
> > +#ifdef CONFIG_MEMCG
> > + if (!memcg)
> > + return;
> > + css_put(&memcg->css);
>
> Feels weird to have an ifdef here but not elsewhere, maybe the whole thing
> should be ifdef...?
>
> Is there not a put equivalent for get_mem_cgroup_from_mm()? That is a bit weird.
>
> Also do we now refrence the memcg global? That's pretty gross, could we not
> actually implement such a helper?
>
> Is it valid to do this also? Maybe cgroup people can chime in.
There is mem_cgroup_put() which should handle !CONFIG_MEMCG configs.
