KMALLOC size allocation

Question

Does KMALLOC allocates only in page size memory or it can allocate less ? What are the sizes that the kmalloc can allocate ? Where can I find description of it, as everywher

Answer 1

It all depends on the allocator used in your kernel. Slab, Slub or Slob. See Following kernel configuration variables:

CONFIG_SLAB
CONFIG_SLUB
CONFIG_SLOB

All above allocators use MAX_ORDER and PAGE_SHIFT to decide the maximum limit of kmalloc()

The largest kmalloc size supported by the SLAB allocators is 32 megabyte (2^25) or the maximum allocatable page order if that is less than 32 MB.

#define KMALLOC_SHIFT_HIGH      ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
  (MAX_ORDER + PAGE_SHIFT - 1) : 25)
#define KMALLOC_SHIFT_MAX       KMALLOC_SHIFT_HIGH

SLUB directly allocates requests fitting in to an order-1 page (PAGE_SIZE*2). Larger requests are passed to the page allocator.

#define KMALLOC_SHIFT_HIGH      (PAGE_SHIFT + 1)
#define KMALLOC_SHIFT_MAX       (MAX_ORDER + PAGE_SHIFT)

SLOB passes all requests larger than one page to the page allocator. No kmalloc array is necessary since objects of different sizes can be allocated from the same page.

#define KMALLOC_SHIFT_HIGH      PAGE_SHIFT
#define KMALLOC_SHIFT_MAX       30

Maximum allocatable size by kmalloc() is

#define KMALLOC_MAX_SIZE        (1UL << KMALLOC_SHIFT_MAX)

Minimum allocatable size from kmalloc() is

#define KMALLOC_MIN_SIZE (1 << KMALLOC_SHIFT_LOW)

Default KMALLOC_SHIFT_LOW for slab is 5, and for slub and slob it is 3.