Re: [mm/slub] ae107fa919: BUG:unable_to_handle_page_fault_for_address

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sat, Mar 12, 2022 at 10:21:25AM +0100, Vlastimil Babka wrote:
> On 3/12/22 02:10, Hyeonggon Yoo wrote:
> > On Fri, Mar 11, 2022 at 04:46:00PM +0000, Hyeonggon Yoo wrote:
> >> On Fri, Mar 11, 2022 at 04:36:47PM +0100, Vlastimil Babka wrote:
> >>> On 3/11/22 15:54, Hyeonggon Yoo wrote:
> >>>> On Wed, Mar 09, 2022 at 10:15:31AM +0800, kernel test robot wrote:
> >>>>>
> >>>>>
> >>>>> Greeting,
> >>>>>
> >>>>> FYI, we noticed the following commit (built with gcc-9):
> >>>>>
> >>>>> commit: ae107fa91914f098cd54ab77e68f83dd6259e901 ("mm/slub: use stackdepot to save stack trace in objects")
> >>>>> https://git.kernel.org/cgit/linux/kernel/git/vbabka/linux.git slub-stackdepot-v3r0
> >>>>>
> >>>>> in testcase: boot
> >>>>>
> >>>>> on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
> >>>>>
> >>>>> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> >>>>>
> >>>>
> >>>> [+Cc Vlastimil and linux-mm]
> >>>
> >>> Thanks.
> >>> lkp folks: it would be nice if I was CC'd automatically on this, it's a
> >>> commit from my git tree and with by s-o-b :)
> >>>
> >>>> I _strongly_ suspect that this is because we don't initialize
> >>>> stack_table[i] = NULL when we allocate it from memblock_alloc().
> >>>
> >>> No, Mike (CC'd) suggested to drop the array init cycle, because
> >>> memblock_alloc would zero the area anyway.
> >>
> >> Ah, you are right. My mistake.
> >>
> >>> There has to be a different
> >>> reason. Wondering if dmesg contains the stack depot initialization message
> >>> at all...
> >>
> >> I think I found the reason.
> >> This is because of CONFIG_SLUB_DEBUG_ON.
> >> It can enable debugging without passing boot parameter.
> >>
> >> if CONFIG_SLUB_DEBUG_ON=y && slub_debug is not passed, we do not call
> >> stack_depot_want_early_init(), but the debugging flags are set.
> >>
> >> And we only call stack_depot_init() later in kmem_cache_create_usercopy().
> >>
> >> so it crashed while creating boot cache.
> > 
> > I tested this, and this was the reason.
> > It crashed on CONFIG_SLUB_DEBUG_ON=y because stackdepot always assume
> > that it was initialized in boot step, or failed
> > (stack_depot_disable=true).
> > 
> > But as it didn't even tried to initialize it, stack_table == NULL &&
> > stack_depot_disable == false. So accessing *(NULL + <hash value>)
> 
> Thanks for finding the cause!
> 

;)

> > Ideas? implementing something like kmem_cache_init_early() again?
> 
> I think we could simply make CONFIG_SLUB_DEBUG_ON select/depend on
> STACKDEPOT_ALWAYS_INIT?

Oh, sounds better.

If we make CONFIG_SLUB_DEBUG_ON select STACK_DEPOT_ALWAYS_INIT,
that is simple solution. but stackdepot will be initialized on
slub_debug=- too.

But I think no one will set CONFIG_SLUB_DEBUG_ON=y if not debugging...

I don't think making CONFIG_SLUB_DEBUG_ON depend on
CONFIG_STACKDEPOT_ALWAYS_INIT is good solution. only KASAN selects it.

-- 
Thank you, You are awesome!
Hyeonggon :-)