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Date:   Mon, 15 Mar 2021 10:16:13 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Vlastimil Babka <vbabka@suse.cz>
cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Paul E . McKenney" <paulmck@kernel.org>,
        linux-btrfs@vger.kernel.org, David Sterba <dsterba@suse.com>,
        Oliver Glitta <glittao@gmail.com>
Subject: Re: [PATCH] [PATCH] mm, slub: enable slub_debug static key when
 creating cache with explicit debug flags
In-Reply-To: <20210315153415.24404-1-vbabka@suse.cz>
Message-ID: <2d80f81a-ed85-a36f-6527-b75da3ae209e@google.com>
References: <20210315153415.24404-1-vbabka@suse.cz>
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On Mon, 15 Mar 2021, Vlastimil Babka wrote:

> Commit ca0cab65ea2b ("mm, slub: introduce static key for slub_debug()")
> introduced a static key to optimize the case where no debugging is enabled for
> any cache. The static key is enabled when slub_debug boot parameter is passed,
> or CONFIG_SLUB_DEBUG_ON enabled.
> 
> However, some caches might be created with one or more debugging flags
> explicitly passed to kmem_cache_create(), and the commit missed this. Thus the
> debugging functionality would not be actually performed for these caches unless
> the static key gets enabled by boot param or config.
> 
> This patch fixes it by checking for debugging flags passed to
> kmem_cache_create() and enabling the static key accordingly.
> 
> Note such explicit debugging flags should not be used outside of debugging and
> testing as they will now enable the static key globally. btrfs_init_cachep()
> creates a cache with SLAB_RED_ZONE but that's a mistake that's being corrected
> [1]. rcu_torture_stats() creates a cache with SLAB_STORE_USER, but that is a
> testing module so it's OK and will start working as intended after this patch.
> 
> Also note that in case of backports to kernels before v5.12 that don't have
> 59450bbc12be ("mm, slab, slub: stop taking cpu hotplug lock"),
> static_branch_enable_cpuslocked() should be used.
> 

Since this affects 5.9+, is the plan to propose backports to stable with 
static_branch_enable_cpuslocked() once this is merged?  (I notice the 
absence of the stable tag here, which I believe is intended.)

> [1] https://lore.kernel.org/linux-btrfs/20210315141824.26099-1-dsterba@suse.com/
> 
> Reported-by: Oliver Glitta <glittao@gmail.com>
> Fixes: ca0cab65ea2b ("mm, slub: introduce static key for slub_debug()")
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: David Rientjes <rientjes@google.com>

> ---
>  mm/slub.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 350a37f30e60..cd6694ad1a0a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3827,6 +3827,15 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  
>  static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
>  {
> +#ifdef CONFIG_SLUB_DEBUG
> +	/*
> +	 * If no slub_debug was enabled globally, the static key is not yet
> +	 * enabled by setup_slub_debug(). Enable it if the cache is being
> +	 * created with any of the debugging flags passed explicitly.
> +	 */
> +	if (flags & SLAB_DEBUG_FLAGS)
> +		static_branch_enable(&slub_debug_enabled);
> +#endif
>  	s->flags = kmem_cache_flags(s->size, flags, s->name);
>  #ifdef CONFIG_SLAB_FREELIST_HARDENED
>  	s->random = get_random_long();