kmalloc是通过cache来实现的, 只不过每次kmalloc的大小不同, 因此是从不同的cache中分配:
/* include/linux/slab.h */
// 注意kmalloc是在头文件中定义的
static inline void *kmalloc(size_t size, gfp_t flags)
{
if (__builtin_constant_p(size)) {
/*__builtin_constant_p 用于判断一个值是否为编译时常数,如果参数size的值是常数,函数返
* 回 1,否则返回 0。__builtin_constant_p是GCC的内建函数*/
// 以下是找一个对象大小刚好大于等于size的cache
int i = 0;
#define CACHE(x)
if (size <= x)
goto found;
else
i++;
#include "kmalloc_sizes.h"
#undef CACHE
{
extern void __you_cannot_kmalloc_that_much(void);
__you_cannot_kmalloc_that_much();
}
found:
// 实际还是通过kmem_cache_alloc来分配内存空间, 因此也是cache
return kmem_cache_alloc((flags & GFP_DMA) ?
malloc_sizes[i].cs_dmacachep :
malloc_sizes[i].cs_cachep, flags);
}
// 通过该函数最后也是由__cache_alloc()函数来分配空间
return __kmalloc(size, flags);
}
// 这是kmalloc_sizes.h文件内容, 实际就是定义CACHE中可用的对象大小
// 普通情况下最大是128K, 也就是kmalloc能分配的最大内存量
#if (PAGE_SIZE == 4096)
CACHE(32)
#endif
CACHE(64)
#if L1_CACHE_BYTES < 64
CACHE(96)
#endif
CACHE(128)
#if L1_CACHE_BYTES < 128
CACHE(192)
#endif
CACHE(256)
CACHE(512)
CACHE(1024)
CACHE(2048)
CACHE(4096)
CACHE(8192)
CACHE(16384)
CACHE(32768)
CACHE(65536)
CACHE(131072)
#if (NR_CPUS > 512) || (MAX_NUMNODES > 256) || !defined(CONFIG_MMU)
CACHE(262144)
#endif
#ifndef CONFIG_MMU
CACHE(524288)
CACHE(1048576)
#ifdef CONFIG_LARGE_ALLOCS
CACHE(2097152)
CACHE(4194304)
CACHE(8388608)
CACHE(16777216)
CACHE(33554432)
#endif /* CONFIG_LARGE_ALLOCS */
#endif /* CONFIG_MMU
继续调用
void *__kmalloc(size_t size, gfp_t flags)
{
return __do_kmalloc(size, flags, NULL);
}
static __always_inline void *__do_kmalloc(size_t size, gfp_t flags,
void *caller)
{
struct kmem_cache *cachep;
/* If you want to save a few bytes .text space: replace
* __ with kmem_.
* Then kmalloc uses the uninlined functions instead of the inline
* functions.
*/
cachep = __find_general_cachep(size, flags);
if (unlikely(cachep == NULL))
return NULL;
return __cache_alloc(cachep, flags, caller);
}
static inline struct kmem_cache *__find_general_cachep(size_t size, gfp_t gfpflags)
{
struct cache_sizes *csizep = malloc_sizes;
#if DEBUG
/* This happens if someone tries to call
* kmem_cache_create(), or __kmalloc(), before
* the generic caches are initialized.
*/
BUG_ON(malloc_sizes[INDEX_AC].cs_cachep == NULL);
#endif
while (size > csizep->cs_size)
csizep++;
/*
* Really subtle: The last entry with cs->cs_size==ULONG_MAX
* has cs_{dma,}cachep==NULL. Thus no special case
* for large kmalloc calls required.
*/
if (unlikely(gfpflags & GFP_DMA))
return csizep->cs_dmacachep;
return csizep->cs_cachep;
}
__find_general_cachep确定可以分配的空间大小由malloc_sizes结构体数组指定,在阅读mm/slab.c中它的定义如下