Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time

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

On Thu, May 13, 2021 at 02:29:13PM +0200, Marco Elver wrote:
> This doesn't solve the problem. We want the compiler to complain
> whenever kmalloc_index() is used with non-constant in normal code. But
> it should be possible to use it in allocator tests regardless of size.
> Either that or export kmalloc_slab(), but I think that's worse. I'll
> send my patch with an updated comment.


to explain in more detail,

in include/linux/slab.h:

  static __always_inline void *kmalloc(size_t size, gfp_t flags)                                                                                                                                           
  {                                                                               
        if (__builtin_constant_p(size)) {                                         
  #ifndef CONFIG_SLOB                                                             
              unsigned int index;                                                 
  #endif                                                                          
              if (size > KMALLOC_MAX_CACHE_SIZE)                                  
                    return kmalloc_large(size, flags);                            
  #ifndef CONFIG_SLOB                                                             
              index = kmalloc_index(size);  


it checks if size is bigger than KMALLOC_MAX_CACHE_SIZE.
so kmalloc_index works safely because the size was already checked.

and definition of KMALLOC_MAX_CACHE_SIZE is

in include/linux/slab.h:
  #ifdef CONFIG_SLAB                                                              
  #define KMALLOC_SHIFT_HIGH    ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \           
                          (MAX_ORDER + PAGE_SHIFT - 1) : 25)                      
  #define KMALLOC_SHIFT_MAX     KMALLOC_SHIFT_HIGH                                
  #ifndef KMALLOC_SHIFT_LOW                                                       
  #define KMALLOC_SHIFT_LOW     5                                                 
  #endif                                                                          
  #endif                                                                          
                                                                                  
  #ifdef CONFIG_SLUB                                                              
  #define KMALLOC_SHIFT_HIGH    (PAGE_SHIFT + 1)                                  
  #define KMALLOC_SHIFT_MAX     (MAX_ORDER + PAGE_SHIFT - 1)                      
  #ifndef KMALLOC_SHIFT_LOW                                                       
  #define KMALLOC_SHIFT_LOW     3                                                 
  #endif                                                                          
  #endif                                                                          
                                                                                  
  #ifdef CONFIG_SLOB                                                              
  #define KMALLOC_SHIFT_HIGH    PAGE_SHIFT                                        
  #define KMALLOC_SHIFT_MAX     (MAX_ORDER + PAGE_SHIFT - 1)                      
  #ifndef KMALLOC_SHIFT_LOW                                                       
  #define KMALLOC_SHIFT_LOW     3                                                 
  #endif                                                                          
  #endif

so if kmalloc_index is called from another place other than kmalloc,
it's not safe to assume that the supported size is 32MB.

Thanks, Hyeonggon