Re: [PATCH v8 1/6] list_lru: allows explicit memcg and NUMA node selection

[Johannes Weiner]

On Thu, Nov 30, 2023 at 12:07:41PM -0800, Nhat Pham wrote:
> On Thu, Nov 30, 2023 at 11:57 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 30, 2023 at 11:40:18AM -0800, Nhat Pham wrote:
> > > This patch changes list_lru interface so that the caller must explicitly
> > > specify numa node and memcg when adding and removing objects. The old
> > > list_lru_add() and list_lru_del() are renamed to list_lru_add_obj() and
> > > list_lru_del_obj(), respectively.
> >
> > Wouldn't it be better to add list_lru_add_memcg() and
> > list_lru_del_memcg() and have:
> >
> > +bool list_lru_del(struct list_lru *lru, struct list_head *item)
> > +{
> > +       int nid = page_to_nid(virt_to_page(item));
> > +       struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
> > +               mem_cgroup_from_slab_obj(item) : NULL;
> > +
> > +       return list_lru_del_memcg(lru, item, nid, memcg);
> > +}
> >
> > Seems like _most_ callers will want the original versions and only
> > a few will want the explicit memcg/nid versions.  No?
> >
> 
> I actually did something along that line in earlier iterations of this
> patch series (albeit with poorer naming - __list_lru_add() instead of
> list_lru_add_memcg()). The consensus after some back and forth was
> that the original list_lru_add() was not a very good design (the
> better one was this new version that allows for explicit numa/memcg
> selection). So I agreed to fix it everywhere as a prep patch.
> 
> I don't have strong opinions here to be completely honest, but I do
> think this new API makes more sense (at the cost of quite a bit of
> elbow grease to fix every callsites and extra reviewing).

Maybe I can shed some light since I was pushing for doing it this way.

The quiet assumption that 'struct list_head *item' is (embedded in) a
slab object that is also charged to a cgroup is a bit much, given that
nothing in the name or documentation of the function points to that.

It bit us in the THP shrinker where that list head is embedded in a
tailpage (virt_to_page(page) is fun to debug). And it caused some
confusion in this case as well, where the zswap entry is a slab object
but not charged (the entry descriptor is not attractive for cgroup
accounting, only the backing memory it points to.)

Yes, for most users - at least right now - the current assumption is
accurate. The thinking was just that if we do have to differentiate
callers now anyway, we might as well make the interface a bit more
self-documenting and harder to misuse going forward, even if it's a
bit more churn now.