Re: [PATCH] lib/stackdepot: Use page allocator if both slab and memblock is unavailable

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

On Mon, Feb 28, 2022 at 11:50:49AM +0100, Marco Elver wrote:
> On Mon, 28 Feb 2022 at 11:05, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> [...]
> > > This is odd - who is calling stack_depot_init() while neither slab nor
> > > memblock are available?
> >
> > It's not merged yet - but Oliver's patch (2/5) in his series [1] does:
> > If user is debugging cache, it calls stack_depot_init() when creating
> > cache.
> >
> > > @@ -4221,6 +4220,9 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
> > >       s->remote_node_defrag_ratio = 1000;
> > >  #endif
> > >
> > > +     if (s->flags & SLAB_STORE_USER && IS_ENABLED(CONFIG_STACKDEPOT))
> > > +             stack_depot_init();
> > > +
> >
> > Oliver's patch series enables stack depot when arch supports stacktrace,
> > to store slab objects' stack traces. (as slub debugging feature.)
> >
> > Because slub debugging is turned on by default, the commit 2dba5eb1c73b
> > ("lib/stackdepot: allow optional init and stack_table allocation by
> > kvmalloc()") made stack_depot_init() can be called later.
> >
> > With Oliver's patch applied, stack_depot_init() can be called in
> > contexts below:
> >
> >   1) only memblock available (for kasan)
> >   2) only buddy available, vmalloc/memblock unavailable (for boot caches)
> >   3) buddy/slab available, vmalloc/memblock unavailable (vmap_area cache)
> >   4) buddy/slab/vmalloc available, memblock unavailable (other caches)
> >
> > SLUB supports enabling debugging for specific cache by passing
> > slub_debug boot parameter. As slab caches can be created in
> > various context, stack_depot_init() should consider all contexts above.
> >
> > Writing this, I realized my patch does not handle case 3).. I'll send v3.
> >
> > [1] https://lore.kernel.org/linux-mm/YhoakP7Kih%2FYUgiN@ip-172-31-19-208.ap-northeast-1.compute.internal/T/#t
> > [2] https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-stackdepot-v1
> >
> > > Do you have a stacktrace?
> >
> > Yeah, here:
> >
> > You can reproduce this on vbabka's slab-stackdepot-v1 branch [2] with
> > slub_debug=U, and CONFIG_STACKDEPOT_ALWAYS_INIT=n
> >
> [...]
> > [    0.000000] Call trace:
> > [    0.000000]  __memset+0x16c/0x188
> > [    0.000000]  stack_depot_init+0xc8/0x100
> > [    0.000000]  __kmem_cache_create+0x454/0x570
> > [    0.000000]  create_boot_cache+0xa0/0xe0
> 
> I think even before this point you have all the information required
> to determine if stackdepot will be required. It's available after
> setup_slub_debug().
> 
> So why can't you just call stack_depot_init() somewhere else and avoid
> all this complexity?
>

You are right. That is much simpler and sound good as SLUB does not
support enabling SLAB_STORE_USER flag when system is up.

I'll try this approach.
Thank you!

> > [    0.000000]  kmem_cache_init+0xf8/0x204
> > [    0.000000]  start_kernel+0x3ec/0x668
> > [    0.000000]  __primary_switched+0xc0/0xc8
> > [    0.000000] Code: 91010108 54ffff4a 8b040108 cb050042 (d50b7428)
> > [    0.000000] ---[ end trace 0000000000000000 ]---
> > [    0.000000] Kernel panic - not syncing: Attempted to kill the idle task!
> > [    0.000000] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

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