Re: [PATCH bpf-next v4 3/7] bpf: Implement cgroup storage available to non-cgroup-attached bpf progs

Yosry Ahmed
2022-10-25T01:25:28Z

On Mon, Oct 24, 2022 at 5:48 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Mon, Oct 24, 2022 at 5:32 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
> >
> > On 10/24/22 5:21 PM, Yosry Ahmed wrote:
> > > On Mon, Oct 24, 2022 at 2:15 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
> > >>
> > >> On 10/23/22 11:05 AM, Yonghong Song wrote:
> > >>> +void bpf_cgrp_storage_free(struct cgroup *cgroup)
> > >>> +{
> > >>> + struct bpf_local_storage *local_storage;
> > >>> + struct bpf_local_storage_elem *selem;
> > >>> + bool free_cgroup_storage = false;
> > >>> + struct hlist_node *n;
> > >>> + unsigned long flags;
> > >>> +
> > >>> + rcu_read_lock();
> > >>> + local_storage = rcu_dereference(cgroup->bpf_cgrp_storage);
> > >>> + if (!local_storage) {
> > >>> + rcu_read_unlock();
> > >>> + return;
> > >>> + }
> > >>> +
> > >>> + /* Neither the bpf_prog nor the bpf_map's syscall
> > >>> + * could be modifying the local_storage->list now.
> > >>> + * Thus, no elem can be added to or deleted from the
> > >>> + * local_storage->list by the bpf_prog or by the bpf_map's syscall.
> > >>> + *
> > >>> + * It is racing with __bpf_local_storage_map_free() alone
> > >>> + * when unlinking elem from the local_storage->list and
> > >>> + * the map's bucket->list.
> > >>> + */
> > >>> + bpf_cgrp_storage_lock();
> > >>> + raw_spin_lock_irqsave(&local_storage->lock, flags);
> > >>> + hlist_for_each_entry_safe(selem, n, &local_storage->list, snode) {
> > >>> + bpf_selem_unlink_map(selem);
> > >>> + /* If local_storage list has only one element, the
> > >>> + * bpf_selem_unlink_storage_nolock() will return true.
> > >>> + * Otherwise, it will return false. The current loop iteration
> > >>> + * intends to remove all local storage. So the last iteration
> > >>> + * of the loop will set the free_cgroup_storage to true.
> > >>> + */
> > >>> + free_cgroup_storage =
> > >>> + bpf_selem_unlink_storage_nolock(local_storage, selem, false, false);
> > >>> + }
> > >>> + raw_spin_unlock_irqrestore(&local_storage->lock, flags);
> > >>> + bpf_cgrp_storage_unlock();
> > >>> + rcu_read_unlock();
> > >>> +
> > >>> + if (free_cgroup_storage)
> > >>> + kfree_rcu(local_storage, rcu);
> > >>> +}
> > >>
> > >> [ ... ]
> > >>
> > >>> +/* *gfp_flags* is a hidden argument provided by the verifier */
> > >>> +BPF_CALL_5(bpf_cgrp_storage_get, struct bpf_map *, map, struct cgroup *, cgroup,
> > >>> + void *, value, u64, flags, gfp_t, gfp_flags)
> > >>> +{
> > >>> + struct bpf_local_storage_data *sdata;
> > >>> +
> > >>> + WARN_ON_ONCE(!bpf_rcu_lock_held());
> > >>> + if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE))
> > >>> + return (unsigned long)NULL;
> > >>> +
> > >>> + if (!cgroup)
> > >>> + return (unsigned long)NULL;
> > >>> +
> > >>> + if (!bpf_cgrp_storage_trylock())
> > >>> + return (unsigned long)NULL;
> > >>> +
> > >>> + sdata = cgroup_storage_lookup(cgroup, map, true);
> > >>> + if (sdata)
> > >>> + goto unlock;
> > >>> +
> > >>> + /* only allocate new storage, when the cgroup is refcounted */
> > >>> + if (!percpu_ref_is_dying(&cgroup->self.refcnt) &&
> > >>> + (flags & BPF_LOCAL_STORAGE_GET_F_CREATE))
> > >>> + sdata = bpf_local_storage_update(cgroup, (struct bpf_local_storage_map *)map,
> > >>> + value, BPF_NOEXIST, gfp_flags);
> > >>> +
> > >>> +unlock:
> > >>> + bpf_cgrp_storage_unlock();
> > >>> + return IS_ERR_OR_NULL(sdata) ? (unsigned long)NULL : (unsigned long)sdata->data;
> > >>> +}
> > >>
> > >> [ ... ]
> > >>
> > >>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > >>> index 764bdd5fd8d1..32145d066a09 100644
> > >>> --- a/kernel/cgroup/cgroup.c
> > >>> +++ b/kernel/cgroup/cgroup.c
> > >>> @@ -5227,6 +5227,10 @@ static void css_free_rwork_fn(struct work_struct *work)
> > >>> struct cgroup_subsys *ss = css->ss;
> > >>> struct cgroup *cgrp = css->cgroup;
> > >>>
> > >>> +#ifdef CONFIG_BPF_SYSCALL
> > >>> + bpf_cgrp_storage_free(cgrp);
> > >>> +#endif
> > >>
> > >>
> > >> After revisiting comment 4bfc0bb2c60e, some of the commit message came to my mind:
> > >>
> > >> " ...... it blocks a possibility to implement
> > >> the memcg-based memory accounting for bpf objects, because a circular
> > >> reference dependency will occur. Charged memory pages are pinning the
> > >> corresponding memory cgroup, and if the memory cgroup is pinning
> > >> the attached bpf program, nothing will be ever released."
> > >>
> > >> Considering the bpf_map_kzalloc() is used in bpf_local_storage_map.c and it can
> > >> charge the memcg, I wonder if the cgrp_local_storage will have similar refcnt
> > >> loop issue here.
> > >>
> > >> If here is the right place to free the cgrp_local_storage() and enough to break
> > >> this refcnt loop, it will be useful to add some explanation and its
> > >> consideration in the commit message.
> > >>
> > >
> > > I think a similar refcount loop issue can happen here as well. IIUC,
> > > this function will only be run when the css is released after all
> > > references are dropped. So if memcg charging is enabled the cgroup
> > > will never be removed. This one might be trickier to handle though..
> >
> > How about removing all storage from cgrp->bpf_cgrp_storage in
> > cgroup_destroy_locked()?
>
> The problem here is that you lose information for cgroups that went
> offline but still exist in the kernel (i.e offline cgroups). The
> commit log 4bfc0bb2c60e mentions that such cgroups can have live
> sockets attached, so this might be a problem? From a memory
> perspective, offline memcgs can still undergo memory operations like
> reclaim. If we are using BPF to collect cgroup statistics for memory
> reclaim, we can't do so for offline memcgs, which is not the end of
> the world, but the cgroup storages become slightly less powerful. We
> might also lose some data that we have already stored for such offline
> memcgs. Also BPF programs now need to handle the case where they have
> a valid cgroup pointer but they cannot retrieve a cgroup storage for
> it because it went offline.
>
> We ideally want to be able to charge the memory to the cgroup without
> holding a ref to it, which is against the cgroup memory charging
> model.

+Roman Gushchin

Wait a second. I think Roman implemented reparenting of BPF map memcg
charges when a memcg is offlined. In this case, when a cgroup goes
offline (aka rmdir()'d), the charges will actually move to the parent,
and the local storage will no longer be holding any refs to the
cgroup. In that case, the current freeing location should be fine and
the cgroup local storage can continue to work with offlined cgroups.

Roman, could you please confirm or deny my thesis?