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

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Date:   Mon, 15 Mar 2021 10:32:07 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        David Sterba <dsterba@...e.com>,
        Oliver Glitta <glittao@...il.com>
Subject: Re: [PATCH] [PATCH] mm, slub: enable slub_debug static key when
 creating cache with explicit debug flags

On Mon, Mar 15, 2021 at 06:28:42PM +0100, Vlastimil Babka wrote:
> On 3/15/21 6:16 PM, David Rientjes wrote:
> > 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.)
> 
> I was thinking about it, and since the rcutorture user is only in -next (AFAICS)
> and btrfs user was unintended, it didn't seem to meet stable criteria to me. But
> I won't mind if it's backported.

I had better ask...  Should rcutorture be doing something different?

							Thanx, Paul

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