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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20231130203522.GC543908@cmpxchg.org>
Date:   Thu, 30 Nov 2023 15:35:22 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
        cerasuolodomenico@...il.com, yosryahmed@...gle.com,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        mhocko@...nel.org, roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, chrisl@...nel.org, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org
Subject: Re: [PATCH v8 1/6] list_lru: allows explicit memcg and NUMA node
 selection

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@...radead.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:
> >
> > +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.