Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc

[Feng Tang]

On Wed, Nov 02, 2022 at 04:22:37PM +0800, Vlastimil Babka wrote:
> On 11/1/22 11:33, John Thomson wrote:
[...]
> > 
> > [    0.000000] Linux version 6.1.0-rc3+ (john@john) (mipsel-buildroot-linux-gnu-gcc.br_real (Buildroot 2021.11-4428-g6b6741b) 12.2.0, GNU ld (GNU Binutils) 2.39) #62 SMP Tue Nov  1 19:49:52 AEST 2022
> > [    0.000000] slub: __kmem_cache_alloc_lru called with kmem_cache ptr: 0x0
> > [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc3+ #62
> > [    0.000000] Stack : 810fff78 80084d98 80889d00 00000004 00000000 00000000 80889d5c 80c90000
> > [    0.000000]         80920000 807bd380 8089d368 80923bd3 00000000 00000001 80889d08 00000000
> > [    0.000000]         00000000 00000000 807bd380 8084bd51 00000002 00000002 00000001 6d6f4320
> > [    0.000000]         00000000 80c97ce9 80c97d14 fffffffc 807bd380 00000000 00000003 00000dc0
> > [    0.000000]         00000000 a0000000 80910000 8110a0b4 00000000 00000020 80010000 80010000
> > [    0.000000]         ...
> > [    0.000000] Call Trace:
> > [    0.000000] [<80008260>] show_stack+0x28/0xf0
> > [    0.000000] [<8070cdc0>] dump_stack_lvl+0x60/0x80
> > [    0.000000] [<801c1428>] kmem_cache_alloc+0x5c0/0x740
> > [    0.000000] [<8092856c>] prom_soc_init+0x1fc/0x2b4
> > [    0.000000] [<80928060>] prom_init+0x44/0xf0
> > [    0.000000] [<80929214>] setup_arch+0x4c/0x6a8
> > [    0.000000] [<809257e0>] start_kernel+0x88/0x7c0
> > [    0.000000] 
> > [    0.000000] SoC Type: MediaTek MT7621 ver:1 eco:3
> 
> The stack means CONFIG_TRACING=n, is that right?
 
Yes, from the kconfig, CONFIG_TRACING is not set.

> That would mean
> prom_soc_init()
>   soc_dev_init()
>     kzalloc() -> kmalloc()
>       kmalloc_trace()  // after #else /* CONFIG_TRACING */
>         kmem_cache_alloc(s, flags);
> 
> Looks like this path is a small bug in the wasting detection patch, as we
> throw away size there.

Yes, from the code reading and log from John, it is.

One strange thing is, I reset the code to v6.0, and found that 
__kmem_cache_alloc_lru() also access the 's->object_size'

void *__kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
			     gfp_t gfpflags)
{
	void *ret = slab_alloc(s, lru, gfpflags, _RET_IP_, s->object_size);
	...
}

And from John's dump_stack() info, this call is also where the NULL pointer
happens, which I still can't figue out.

> AFAICS before this patch, we "survive" "kmem_cache *s" being NULL as
> slab_pre_alloc_hook() will happen to return NULL and we bail out from
> slab_alloc_node(). But this is a side-effect, not an intended protection.
> Also the CONFIG_TRACING variant of kmalloc_trace() would have called
> trace_kmalloc dereferencing s->size anyway even before this patch.
> 
> I don't think we should add WARNS in the slab hot paths just to prevent this
> rare error of using slab too early. At most VM_WARN... would be acceptable
> but still not necessary as crashing immediately from a NULL pointer is
> sufficient.
> 
> So IMHO mips should fix their soc init, 

Yes, for the mips fix, John has proposed to defer the calling of prom_soc_init(),
which looks reasonable.

> and we should look into the
> CONFIG_TRACING=n variant of kmalloc_trace(), to pass orig_size properly.

You mean check if the pointer is NULL and bail out early. 

Thanks,
Feng