linux-kernel - Re: [PATCH 0/5] SLUB debugfs improvements based on stackdepot

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Date:   Wed, 2 Mar 2022 10:09:37 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Rapoport <rppt@...ux.ibm.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Oliver Glitta <glittao@...il.com>,
        Faiyaz Mohammed <faiyazm@...eaurora.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Karolina Drobnik <karolinadrobnik@...il.com>
Subject: Re: [PATCH 0/5] SLUB debugfs improvements based on stackdepot

On 3/2/22 09:37, Mike Rapoport wrote:
> On Mon, Feb 28, 2022 at 09:27:02PM +0000, Hyeonggon Yoo wrote:
>> On Mon, Feb 28, 2022 at 08:10:18PM +0100, Vlastimil Babka wrote:
>> > On 2/26/22 08:19, Hyeonggon Yoo wrote:
>> > > On Fri, Feb 25, 2022 at 07:03:13PM +0100, Vlastimil Babka wrote:
>> > >> Hi,
>> > >> 
>> > >> this series combines and revives patches from Oliver's last year
>> > >> bachelor thesis (where I was the advisor) that make SLUB's debugfs
>> > >> files alloc_traces and free_traces more useful.
>> > >> The resubmission was blocked on stackdepot changes that are now merged,
>> > >> as explained in patch 2.
>> > >> 
>> > > 
>> > > Hello. I just started review/testing this series.
>> > > 
>> > > it crashed on my system (arm64)
>> > 
>> > Hmm, interesting. On x86_64 this works for me and stackdepot is allocated
>> > from memblock. arm64 must have memblock freeing happen earlier or something.
>> > (CCing memblock experts)
>> > 
>> > > I ran with boot parameter slub_debug=U, and without KASAN.
>> > > So CONFIG_STACKDEPOT_ALWAYS_INIT=n.
>> > > 
>> > > void * __init memblock_alloc_try_nid(
>> > >                         phys_addr_t size, phys_addr_t align,
>> > >                         phys_addr_t min_addr, phys_addr_t max_addr,
>> > >                         int nid)
>> > > {
>> > >         void *ptr;
>> > > 
>> > >         memblock_dbg("%s: %llu bytes align=0x%llx nid=%d from=%pa max_addr=%pa %pS\n",
>> > >                      __func__, (u64)size, (u64)align, nid, &min_addr,
>> > >                      &max_addr, (void *)_RET_IP_);
>> > >         ptr = memblock_alloc_internal(size, align,
>> > >                                            min_addr, max_addr, nid, false);
>> > >         if (ptr)
>> > >                 memset(ptr, 0, size); <--- Crash Here
>> > > 
>> > >         return ptr;
>> > > }
>> > > 
>> > > It crashed during create_boot_cache() -> stack_depot_init() ->
>> > > memblock_alloc().
>> > > 
>> > > I think That's because, in kmem_cache_init(), both slab and memblock is not
>> > > available. (AFAIU memblock is not available after mem_init() because of
>> > > memblock_free_all(), right?)
>> > 
>> > Hm yes I see, even in x86_64 version mem_init() calls memblock_free_all().
>> > But then, I would expect stack_depot_init() to detect that memblock_alloc()
>> > returns NULL, we print ""Stack Depot hash table allocation failed,
>> > disabling" and disable it. Instead it seems memblock_alloc() returns
>> > something that's already potentially used by somebody else? Sounds like a bug?
>> 
>> 
>> By the way, I fixed this by allowing stack_depot_init() to be called in
>> kmem_cache_init() too [1] and Marco suggested that calling
>> stack_depot_init() depending on slub_debug parameter for simplicity. [2]
>> 
>> I would prefer [2], Would you take a look?
>> 
>> [1] https://lkml.org/lkml/2022/2/27/31
>> 
>> [2] https://lkml.org/lkml/2022/2/28/717
> 
> I have the third version :)

While simple, it changes the timing of stack_depot_early_init() that was
supposed to be at a single callsite - now it's less predictable and depends
on e.g. kernel parameter ordering. Some arch/config combo could break,
dunno. Setting a variable that stack_depot_early_init() checks should be
more robust.

> 
> diff --git a/mm/slub.c b/mm/slub.c
> index a74afe59a403..0c3ab2335b46 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1548,6 +1548,10 @@ static int __init setup_slub_debug(char *str)
>  	}
>  out:
>  	slub_debug = global_flags;
> +
> +	if (slub_flags & SLAB_STORE_USER && IS_ENABLED(CONFIG_STACKDEPOT))
> +		stack_depot_early_init();
> +
>  	if (slub_debug != 0 || slub_debug_string)
>  		static_branch_enable(&slub_debug_enabled);
>  	else
> @@ -4221,9 +4225,6 @@ 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();
> -
>  	/* Initialize the pre-computed randomized freelist if slab is up */
>  	if (slab_state >= UP) {
>  		if (init_cache_random_seq(s))
>  
>> -- 
>> Thank you, You are awesome!
>> Hyeonggon :-)
>