linux-kernel - Re: [PATCH v3 11/21] slab: remove SLUB_CPU

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Message-ID: <aXAkwLsGP9rqamKL@hyeyoo>
Date: Wed, 21 Jan 2026 09:58:40 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Petr Tesarik <ptesarik@...e.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
        bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH v3 11/21] slab: remove SLUB_CPU_PARTIAL

On Tue, Jan 20, 2026 at 10:25:27PM +0000, Suren Baghdasaryan wrote:
> On Fri, Jan 16, 2026 at 2:40 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> > @@ -5744,10 +5553,9 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
> >
> >         /*
> >          * Objects left in the slab. If it was not on the partial list before
> > -        * then add it. This can only happen when cache has no per cpu partial
> > -        * list otherwise we would have put it there.
> > +        * then add it.
> >          */
> > -       if (!IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL) && unlikely(was_full)) {
> > +       if (unlikely(was_full)) {
> 
> This is not really related to your change but I wonder why we check
> for was_full to detect that the slab was not on partial list instead
> of checking !on_node_partial... They might be equivalent at this point
> but it's still a bit confusing.

If we only know that a slab is not on the partial list, we cannot
manipulate its list because it may be on a linked list that cannot
handle list manipulation outside function
(e.g., pc.slabs in __refill_objects()).

If it's not on the partial list, we can safely manipulate the list
only when we know it was full. It's safe because full slabs are not
supposed to be on any list (except for debug caches, where frees are
done via free_to_partial_list()).

-- 
Cheers,
Harry / Hyeonggon