前面分析了不连续页面管理的初始化以及申请的实现,最后以释放不连续页面空间vfree()收尾。
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【file:/mm/vmalloc.c】
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/**
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* vfree - release memory allocated by vmalloc()
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* @addr: memory base address
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*
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* Free the virtually continuous memory area starting at @addr, as
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* obtained from vmalloc(), vmalloc_32() or __vmalloc(). If @addr is
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* NULL, no operation is performed.
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*
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* Must not be called in NMI context (strictly speaking, only if we don't
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* have CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG, but making the calling
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* conventions for vfree() arch-depenedent would be a really bad idea)
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*
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* NOTE: assumes that the object at *addr has a size >= sizeof(llist_node)
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*/
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void vfree(const void *addr)
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{
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BUG_ON(in_nmi());
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kmemleak_free(addr);
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if (!addr)
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return;
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if (unlikely(in_interrupt())) {
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struct vfree_deferred *p = &__get_cpu_var(vfree_deferred);
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if (llist_add((struct llist_node *)addr, &p->list))
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schedule_work(&p->wq);
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} else
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__vunmap(addr, 1);
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}
该函数首先对kmemleak内存泄漏跟踪进行解除,继而保证释放的地址空间不为空。如若当前释放操作在中断中,那么将释放的内存空间信息加入到当前CPU的vfree_deferred管理链表中,继而通过schedule_work()唤醒工作队列,对内存进行异步释放操作;但如果当前操作不在中断中,将直接通过__vunmap()进行内存释放。
对于工作队列,是在vmalloc_init()中创建的free_work(),其具体实现。
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【file:/mm/vmalloc.c】
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static void free_work(struct work_struct *w)
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{
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struct vfree_deferred *p = container_of(w, struct vfree_deferred, wq);
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struct llist_node *llnode = llist_del_all(&p->list);
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while (llnode) {
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void *p = llnode;
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llnode = llist_next(llnode);
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__vunmap(p, 1);
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}
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}
可以看到该函数实际上也是调用__vunmap()进行不连续内存页面进行释放的。
具体分析一下__vunmap()的实现,其实现也是非常简单的,实则是vmalloc()的倒序操作。
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【file:/mm/vmalloc.c】
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static void __vunmap(const void *addr, int deallocate_pages)
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{
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struct vm_struct *area;
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if (!addr)
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return;
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if (WARN(!PAGE_ALIGNED(addr), "Trying to vfree() bad address (%p)\n",
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addr))
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return;
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area = remove_vm_area(addr);
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if (unlikely(!area)) {
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WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
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addr);
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return;
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}
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debug_check_no_locks_freed(addr, area->size);
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debug_check_no_obj_freed(addr, area->size);
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if (deallocate_pages) {
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int i;
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for (i = 0; i < area->nr_pages; i++) {
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struct page *page = area->pages[i];
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BUG_ON(!page);
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__free_page(page);
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}
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if (area->flags & VM_VPAGES)
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vfree(area->pages);
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else
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kfree(area->pages);
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}
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kfree(area);
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return;
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}
可以看到该函数先是通过remove_vm_area()将虚拟地址空间从管理结构中移除,继而是相关调测。最后判断入参deallocate_pages,如果该值为0,则表示它是vumap()调用释放,否则是vfree()释放内存,其需要将申请的不连续物理内存页面释放掉。释放不连续物理内存页面的操作也很简单,通过循环遍历物理页面数组,将各页面逐一返回。页面释放完之后,最后将数组空间和vm_struct信息接口空间释放。
深入看一下remove_vm_area()的实现。
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【file:/mm/vmalloc.c】
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/**
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* remove_vm_area - find and remove a continuous kernel virtual area
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* @addr: base address
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*
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* Search for the kernel VM area starting at @addr, and remove it.
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* This function returns the found VM area, but using it is NOT safe
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* on SMP machines, except for its size or flags.
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*/
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struct vm_struct *remove_vm_area(const void *addr)
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{
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struct vmap_area *va;
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va = find_vmap_area((unsigned long)addr);
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if (va && va->flags & VM_VM_AREA) {
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struct vm_struct *vm = va->vm;
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spin_lock(&vmap_area_lock);
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va->vm = NULL;
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va->flags &= ~VM_VM_AREA;
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spin_unlock(&vmap_area_lock);
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vmap_debug_free_range(va->va_start, va->va_end);
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free_unmap_vmap_area(va);
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vm->size -= PAGE_SIZE;
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return vm;
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}
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return NULL;
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}
该函数先是通过find_vmap_area()查找前面注册到红黑树的不连续内存的vmap_area管理信息,继而清除VM_VM_AREA标记,最后调用free_unmap_vmap_area()释放相关的内存页面。
具体看一下find_vmap_area()实现。
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【file:/mm/vmalloc.c】
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static struct vmap_area *find_vmap_area(unsigned long addr)
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{
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struct vmap_area *va;
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spin_lock(&vmap_area_lock);
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va = __find_vmap_area(addr);
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spin_unlock(&vmap_area_lock);
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return va;
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}
该函数主要是加保护锁后调用__find_vmap_area()进行查找。
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【file:/mm/vmalloc.c】
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static struct vmap_area *__find_vmap_area(unsigned long addr)
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{
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struct rb_node *n = vmap_area_root.rb_node;
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while (n) {
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struct vmap_area *va;
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va = rb_entry(n, struct vmap_area, rb_node);
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if (addr < va->va_start)
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n = n->rb_left;
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else if (addr >= va->va_end)
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n = n->rb_right;
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else
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return va;
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}
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return NULL;
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}
可以看到很熟悉的代码,类似前面分配虚拟内存空间一样,遍历vmap_area_root红黑树进行查找。
现在回到free_unmap_vmap_area()进行收尾。
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【file:/mm/vmalloc.c】
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/*
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* Free and unmap a vmap area
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*/
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static void free_unmap_vmap_area(struct vmap_area *va)
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{
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flush_cache_vunmap(va->va_start, va->va_end);
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free_unmap_vmap_area_noflush(va);
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}
该函数主要用于清除告诉缓冲的指定的虚拟地址空间,当然这取决于硬件体系结构。其中flush_cache_vunmap()在x86环境中是空函数。而free_unmap_vmap_area_noflush()函数。
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【file:/mm/vmalloc.c】
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/*
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* Free and unmap a vmap area, caller ensuring flush_cache_vunmap had been
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* called for the correct range previously.
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*/
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static void free_unmap_vmap_area_noflush(struct vmap_area *va)
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{
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unmap_vmap_area(va);
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free_vmap_area_noflush(va);
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}
unmap_vmap_area()主要是通过vunmap_page_range()指定范围,最终通过vunmap_page_range()将该虚拟地址空间释放到内核页表。其中unmap_vmap_area()实现。
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【file:/mm/vmalloc.c】
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/*
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* Clear the pagetable entries of a given vmap_area
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*/
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static void unmap_vmap_area(struct vmap_area *va)
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{
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vunmap_page_range(va->va_start, va->va_end);
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}
而vunmap_page_range()实现了内存页面的映射。
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【file:/mm/vmalloc.c】
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static void vunmap_page_range(unsigned long addr, unsigned long end)
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{
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pgd_t *pgd;
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unsigned long next;
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BUG_ON(addr >= end);
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pgd = pgd_offset_k(addr);
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do {
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next = pgd_addr_end(addr, end);
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if (pgd_none_or_clear_bad(pgd))
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continue;
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vunmap_pud_range(pgd, addr, next);
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} while (pgd++, addr = next, addr != end);
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}
至于free_vmap_area_noflush()的实现。
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【file:/mm/vmalloc.c】
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/*
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* Free a vmap area, caller ensuring that the area has been unmapped
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* and flush_cache_vunmap had been called for the correct range
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* previously.
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*/
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static void free_vmap_area_noflush(struct vmap_area *va)
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{
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va->flags |= VM_LAZY_FREE;
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atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
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if (unlikely(atomic_read(&vmap_lazy_nr) > lazy_max_pages()))
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try_purge_vmap_area_lazy();
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}
则是将vmp_area加入到vmap_purge_list链表,当加入该链表的数量超过一定值之后,才会通过try_purge_vmap_area_lazy()将vmap_area真正释放掉。这是一个延迟的释放过程,最终会把vmap_area及相关的页面全部释放掉。
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