linux-kernel - Re: [PATCH 5.10 518/530] mm, slub: enable slub_debug static key when creating cache with explicit debug flags

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Message-ID: <cb4f0543-de13-73fd-6b59-6fe2eb078f64@suse.cz>
Date:   Wed, 12 May 2021 17:37:57 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, Oliver Glitta <glittao@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.10 518/530] mm, slub: enable slub_debug static key when
 creating cache with explicit debug flags

On 5/12/21 5:35 PM, Vlastimil Babka wrote:
> On 5/12/21 4:50 PM, Greg Kroah-Hartman wrote:
>> From: Vlastimil Babka <vbabka@...e.cz>
>> 
>> [ Upstream commit 1f0723a4c0df36cbdffc6fac82cd3c5d57e06d66 ]
>> 
>> 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.

Geez, and this part was also missed. Also why I chose not to Cc stable in the
first place.

>> [1] https://lore.kernel.org/linux-btrfs/20210315141824.26099-1-dsterba@suse.com/
>> 
>> Link: https://lkml.kernel.org/r/20210315153415.24404-1-vbabka@suse.cz
>> Fixes: ca0cab65ea2b ("mm, slub: introduce static key for slub_debug()")
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> Reported-by: Oliver Glitta <glittao@...il.com>
>> Acked-by: David Rientjes <rientjes@...gle.com>
>> Cc: Christoph Lameter <cl@...ux.com>
>> Cc: Pekka Enberg <penberg@...nel.org>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
> 
> Uh, rather not release this to stable without the followup fix:
> https://lore.kernel.org/linux-mm/20210504120019.26791-1-vbabka@suse.cz/
> 
>> ---
>>  mm/slub.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 05a501b67cd5..e4f7978d43c2 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3779,6 +3779,15 @@ 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();
>> 
>