kmalloc和vmalloc的区别

kmalloc和vmalloc完全不同。

kmalloc

kmalloc申请一段物理地址和逻辑地址连续的内存空间。

 /* 申请一段物理地址连续的内存空间 */ static __always_inline void *kmalloc(size_t size, gfp_t flags) { if (__builtin_constant_p(size)) { if (size > KMALLOC_MAX_CACHE_SIZE) //申请空间大于一个page_size return kmalloc_large(size, flags); #ifndef CONFIG_SLOB /* 在未配置slob情况下：申请空间小于一个page_size：从slab中kmalloc_caches分配 */ if (!(flags & GFP_DMA)) { int index = kmalloc_index(size); if (!index) return ZERO_SIZE_PTR; return kmem_cache_alloc_trace(kmalloc_caches[index], flags, size); } #endif } /* 在配置slob情况下：申请空间小于一个page_size：从slob中kmalloc_caches分配 */ return __kmalloc(size, flags); } 

kmalloc_caches的具体设置如下：

struct kmem_cache *kmalloc_caches[KMALLOC_SHIFT_HIGH + 1]; EXPORT_SYMBOL(kmalloc_caches); 

vmalloc

vmalloc申请一段逻辑地址连续、物理地址不连续的内存空间。

/ 申请虚拟地址连续的一段内存（物理地址并不连续，和malloc不同） 其实申请若干个page，使得N*page的大小大于等于size * vmalloc - allocate virtually contiguous memory * @size: allocation size * Allocate enough pages to cover @size from the page level * allocator and map them into contiguous kernel virtual space. * * For tight control over page level allocator and protection flags * use __vmalloc() instead. */ void *vmalloc(unsigned long size) { return __vmalloc_node_flags(size, NUMA_NO_NODE, GFP_KERNEL | __GFP_HIGHMEM); } static inline void *__vmalloc_node_flags(unsigned long size, int node, gfp_t flags) { return __vmalloc_node(size, 1, flags, PAGE_KERNEL, node, __builtin_return_address(0)); } static void *__vmalloc_node(unsigned long size, unsigned long align, gfp_t gfp_mask, pgprot_t prot, int node, const void *caller) { return __vmalloc_node_range(size, align, VMALLOC_START, VMALLOC_END, gfp_mask, prot, 0, node, caller); } void *__vmalloc_node_range(unsigned long size, unsigned long align, unsigned long start, unsigned long end, gfp_t gfp_mask, pgprot_t prot, unsigned long vm_flags, int node, const void *caller) { struct vm_struct *area; void *addr; unsigned long real_size = size; /* 和page大小对齐，分配整数个page */ size = PAGE_ALIGN(size); if (!size || (size >> PAGE_SHIFT) > totalram_pages) goto fail; area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNINITIALIZED | vm_flags, start, end, node, gfp_mask, caller); if (!area) goto fail; addr = __vmalloc_area_node(area, gfp_mask, prot, node); if (!addr) return NULL; /* * In this function, newly allocated vm_struct has VM_UNINITIALIZED * flag. It means that vm_struct is not fully initialized. * Now, it is fully initialized, so remove this flag here. */ clear_vm_uninitialized_flag(area); /* * A ref_count = 2 is needed because vm_struct allocated in * __get_vm_area_node() contains a reference to the virtual address of * the vmalloc'ed block. */ kmemleak_alloc(addr, real_size, 2, gfp_mask); return addr; fail: warn_alloc_failed(gfp_mask, 0, "vmalloc: allocation failure: %lu bytes\n", real_size); return NULL; } //进入__vmalloc_area_node函数内部 static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask, pgprot_t prot, int node) { const int order = 0; struct page pages; unsigned int nr_pages, array_size, i; const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO; const gfp_t alloc_mask = gfp_mask | __GFP_NOWARN; nr_pages = get_vm_area_size(area) >> PAGE_SHIFT; array_size = (nr_pages * sizeof(struct page *)); area->nr_pages = nr_pages; /* Please note that the recursion is strictly bounded. */ //为pages数组申请连续的空间 if (array_size > PAGE_SIZE) { pages = __vmalloc_node(array_size, 1, nested_gfp|__GFP_HIGHMEM, PAGE_KERNEL, node, area->caller); } else { pages = kmalloc_node(array_size, nested_gfp, node); } area->pages = pages; if (!area->pages) { remove_vm_area(area->addr); kfree(area); return NULL; } for (i = 0; i < area->nr_pages; i++) { struct page *page; if (node == NUMA_NO_NODE) //alloc_page申请单个page page = alloc_kmem_pages(alloc_mask, order); else page = alloc_kmem_pages_node(node, alloc_mask, order); if (unlikely(!page)) { /* Successfully allocated i pages, free them in __vunmap() */ area->nr_pages = i; goto fail; } area->pages[i] = page; if (gfpflags_allow_blocking(gfp_mask)) cond_resched(); } //修改内核使用的页表项，建立vmalloc区域的逻辑地址到page物理地址的映射 if (map_vm_area(area, prot, pages)) goto fail; return area->addr; fail: warn_alloc_failed(gfp_mask, order, "vmalloc: allocation failure, allocated %ld of %ld bytes\n", (area->nr_pages*PAGE_SIZE), area->size); vfree(area->addr); return NULL; } 

需要使用map_vm_area修改内核使用的页表项，建立vmalloc区域的逻辑地址到page物理地址的映射。