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

[Feng Tang]

On Thu, Nov 03, 2022 at 09:33:28AM +0100, Vlastimil Babka wrote:
[...]
> >> 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. 
> 
> No I mean here:
> 
> #else /* CONFIG_TRACING */
> /* Save a function call when CONFIG_TRACING=n */
> static __always_inline __alloc_size(3)                                   
> void *kmalloc_trace(struct kmem_cache *s, gfp_t flags, size_t size)
> {       
>         void *ret = kmem_cache_alloc(s, flags);
>                     
>         ret = kasan_kmalloc(s, ret, size, flags);
>         return ret;
> }
> 
> we call kmem_cache_alloc() and discard the size parameter, so it will assume
> s->object_size (and as the side-effect, crash if s is NULL). We shouldn't
> add "s is NULL?" checks, but fix passing the size - probably switch to
> __kmem_cache_alloc_node()? and in the following kmalloc_node_trace() analogically.
 
Got it, thanks! I might have missed it during some rebasing for the
kmalloc wastage debug patch.

How about the following fix?

Thanks,
Feng

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