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
