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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Dec 04, 2023 at 04:30:44PM -0800, Chris Li wrote:
> On Thu, Nov 30, 2023 at 12:35 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > 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@infradead.org> 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:
> 
> That is my first thought as well. If we are having two different
> flavors of LRU add, one has memcg and one without. The list_lru_add()
> vs list_lru_add_memcg() is the common way to do it.
> > > >
> > > > +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.
> 
> We can add it to the document if that is desirable.

It would help, but it still violates the "easy to use, hard to misuse"
principle. And I think it does the API layering backwards.

list_lru_add() is the "default" API function. It makes sense to keep
that simple and robust, then add add convenience wrappers for
additional, specialized functionality like memcg lookups for charged
slab objects - even if that's a common usecase.

It's better for a new user to be paused by the require memcg argument
in the default function and then go and find list_lru_add_obj(), than
it is for somebody to quietly pass an invalid object to list_lru_add()
and have subtle runtime problems and crashes (which has happened twice
now already).