linux-kernel - Re: [PATCH RFC] mm: slub: fix flush_cpu_slab()/__free

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Date:   Tue, 30 Aug 2022 12:24:29 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Maurizio Lombardi <mlombard@...hat.com>
Cc:     vbabka@...e.cz, linux-mm@...ck.org, rientjes@...gle.com,
        penberg@...nel.org, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de
Subject: Re: [PATCH RFC] mm: slub: fix flush_cpu_slab()/__free_slab()
 invocations in task context.

On 2022-08-29 17:48:05 [+0200], Maurizio Lombardi wrote:
> diff --git a/mm/slub.c b/mm/slub.c
> index 862dbd9af4f5..d46ee90651d2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2681,30 +2681,34 @@ struct slub_flush_work {
>  	bool skip;
>  };
>  
> +static void flush_cpu_slab(void *d)
> +{
> +	struct kmem_cache *s = d;
> +	struct kmem_cache_cpu *c = this_cpu_ptr(s->cpu_slab);
> +
> +	if (c->slab)
> +		flush_slab(s, c);
> +
> +	unfreeze_partials(s);
> +}
…
> @@ -2721,13 +2725,18 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
>  	lockdep_assert_cpus_held();
>  	mutex_lock(&flush_lock);
>  
> +	if (in_task()) {
> +		on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);

This blocks with disabled preemption until it completes flush_cpu_slab()
on all CPUs. That function acquires a local_lock_t which can not be
acquired from in-IRQ which is where this function will be invoked due to
on_each_cpu_cond().

Couldn't we instead use a workqueue with that WQ_MEM_RECLAIM bit? It may
reclaim memory after all ;)

Sebastian