kmalloc question

[Borislav Petkov <petkov@xxxxxxxxxxxxxxx>]

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Hi there people,
i only need a confirmation on some unclear issues concerning the following 
function:

<include/linux/slab.h>
77 static inline void *kmalloc(size_t size, int flags)
 78 {
 79         if (__builtin_constant_p(size)) {
 80                 int i = 0;
 81 #define CACHE(x) \
 82                 if (size <= x) \
 83                         goto found; \
 84                 else \
 85                         i++;
 86 #include "kmalloc_sizes.h"
 87 #undef CACHE
 88                 {
 89                         extern void __you_cannot_kmalloc_that_much(void);
 90                         __you_cannot_kmalloc_that_much();
 91                 }
 92 found:
 93                 return kmem_cache_alloc((flags & GFP_DMA) ?
 94                         malloc_sizes[i].cs_dmacachep :
 95                         malloc_sizes[i].cs_cachep, flags);
 96         }
 97         return __kmalloc(size, flags);
 98 }

I understand that the CACHE macro is used to search the malloc_sizes array to 
find a slab object that is (size <= x) and if it is found, we jump to the 
found label where we call kmem_cache_alloc(). After we #undef CACHE, we have 
a block with that external function that is declared nowhere. After googling 
a while, I think I know what this does but i'm not sure - when, at compile 
time, something requests a slab object that is bigger than the alloc'd ones, 
we break compilation and force the caller to change its call to kmalloc in 
order to request a more suitable size object. I don't understand the thoughts 
behind that, though, thus my question.

Thanks in advance for your help,
Boris.
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