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Date:   Mon, 15 Mar 2021 10:59:32 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     David Rientjes <rientjes@google.com>,
        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, 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
Message-ID: <20210315175932.GO2696@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
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On Mon, Mar 15, 2021 at 06:36:34PM +0100, Vlastimil Babka wrote:
> On 3/15/21 6:32 PM, Paul E. McKenney wrote:
> > 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
> 
> No, I think it's fine if a testing module such as rcutorture flips the static
> key for the rest of the kernel's uptime. I only CC'd you as FYI in case you were
> wondering why you can't see any alloc/free stacks in its output :)

Ah, all of my recent tests have been for sufficient duration that all
was well by the time that that code was invoked.  But thank you for the
heads up -- someone will hit this sooner or later, and I freely confess
that I would have been clueless.

							Thanx, Paul