kernel 3.10内核源码分析--缺页异常(page fault)处理流程
基本原理
1、page fault由硬件产生,是一种“异常”。产生条件为:当CPU访问某线性地址,而该线性地址还没有对应的页表项,即还没有分配相应的物理内存并进行映射时,自动产生异常。
2、page fault基本流程:
从cr2中获取发生异常的地址
缺页地址位于内核态
位于vmalloc区?->从主内核页表同步数据到进程页表
非vmalloc区 ->不应该产生page fault->oops
缺页地址位于用户态
缺页上下文发生在内核态
exception table中有相应的处理项? ->进行修正
没有 ->oops
查找vma
找到?-> 是否expand stack?->堆栈扩展
不是->正常的缺页处理:handle_mm_fault
没找到->bad_area
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/*
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*缺页异常主处理函数。
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*regs:异常时的寄存器信息;
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*error_code-当异常发生时,硬件压入栈中的错误代码。
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* 当第0位被清0时,则异常是由一个不存在的页所引起的。否则是由无效的访问权限引起的。
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* 如果第1位被清0,则异常由读访问或者执行访问所引起,如果被设置,则异常由写访问引起。
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* 如果第2位被清0,则异常发生在内核态,否则异常发生在用户态。
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*/
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static void __kprobes
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__do_page_fault(struct pt_regs *regs, unsigned long error_code)
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{
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struct vm_area_struct *vma;
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struct task_struct *tsk;
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unsigned long address;
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struct mm_struct *mm;
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int fault;
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int write = error_code & PF_WRITE;
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unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE |
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(write ? FAULT_FLAG_WRITE : 0);
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tsk = current;
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mm = tsk->mm;
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/* Get the faulting address: */
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//缺页异常的地址默认存放于CR2寄存器中,x86硬件决定
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address = read_cr2();
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/*
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* Detect and handle instructions that would cause a page fault for
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* both a tracked kernel page and a userspace page.
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*/
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if (kmemcheck_active(regs))
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kmemcheck_hide(regs);
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prefetchw(&mm->mmap_sem);
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// mmio不应该发生缺页,通常都会ioremap到vmalloc区,然后进行访问
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if (unlikely(kmmio_fault(regs, address)))
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return;
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/*
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* We fault-in kernel-space virtual memory on-demand. The
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* 'reference' page table is init_mm.pgd.
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*
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* We MUST NOT take any locks for this case. We may
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* be in an interrupt or a critical region, and should
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* only copy the information from the master page table,
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* nothing more.
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*
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* This verifies that the fault happens in kernel space
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* (error_code & 4) == 0, and that the fault was not a
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* protection error (error_code & 9) == 0.
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*/
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/*
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* 缺页地址位于内核空间。并不代表异常发生于内核空间,有可能是用户
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* 态访问了内核空间的地址。
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*/
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if (unlikely(fault_in_kernel_space(address))) {
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if (!(error_code & (PF_RSVD | PF_USER | PF_PROT))) {
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/*
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* 检查发生缺页的地址是否在vmalloc区,是则进行相应的处理
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* 主要是从内核主页表向进程页表同步数据
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*/
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if (vmalloc_fault(address) >= 0)
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return;
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if (kmemcheck_fault(regs, address, error_code))
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return;
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}
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/* Can handle a stale RO->RW TLB: */
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/*
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* 检查是否是由于陈旧的TLB导致的假的pagefault(由于TLB的延迟flush导致,
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* 因为提前flush会有比较大的性能代价)。
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*/
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if (spurious_fault(error_code, address))
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return;
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/* kprobes don't want to hook the spurious faults: */
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if (notify_page_fault(regs))
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return;
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/*
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* Don't take the mm semaphore here. If we fixup a prefetch
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* fault we could otherwise deadlock:
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*/
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/*
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* 有问题了: 由于异常地址位于内核态,触发内核异常,因为vmalloc
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* 区的缺页异常前面已经处理过了,内核态的缺页异常只能发生在
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* vmalloc区,如果不是,那就是内核异常了。
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*/
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bad_area_nosemaphore(regs, error_code, address);
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return;
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}
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// 进入到这里,说明异常地址位于用户态
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/* kprobes don't want to hook the spurious faults: */
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if (unlikely(notify_page_fault(regs)))
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return;
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/*
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* It's safe to allow irq's after cr2 has been saved and the
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* vmalloc fault has been handled.
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*
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* User-mode registers count as a user access even for any
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* potential system fault or CPU buglet:
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*/
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/*
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* 开中断,这种情况下,是安全的,可以缩短因缺页异常导致的关中断时长。
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* 老内核版本中(2.6.11)没有这样的操作
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*/
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if (user_mode_vm(regs)) {
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local_irq_enable();
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error_code |= PF_USER;
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} else {
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if (regs->flags & X86_EFLAGS_IF)
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local_irq_enable();
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}
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-
if (unlikely(error_code & PF_RSVD))
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pgtable_bad(regs, error_code, address);
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-
if (static_cpu_has(X86_FEATURE_SMAP)) {
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if (unlikely(smap_violation(error_code, regs))) {
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bad_area_nosemaphore(regs, error_code, address);
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return;
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}
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}
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perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
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/*
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* If we're in an interrupt, have no user context or are running
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* in an atomic region then we must not take the fault:
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*/
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/*
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* 当缺页异常发生于中断或其它atomic上下文中时,则产生异常。
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* 这种情况下,不应该再产生page fault
-
*/
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if (unlikely(in_atomic() || !mm)) {
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bad_area_nosemaphore(regs, error_code, address);
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return;
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}
-
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/*
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* When running in the kernel we expect faults to occur only to
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* addresses in user space. All other faults represent errors in
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* the kernel and should generate an OOPS. Unfortunately, in the
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* case of an erroneous fault occurring in a code path which already
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* holds mmap_sem we will deadlock attempting to validate the fault
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* against the address space. Luckily the kernel only validly
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* references user space from well defined areas of code, which are
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* listed in the exceptions table.
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*
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* As the vast majority of faults will be valid we will only perform
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* the source reference check when there is a possibility of a
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* deadlock. Attempt to lock the address space, if we cannot we then
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* validate the source. If this is invalid we can skip the address
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* space check, thus avoiding the deadlock:
-
*/
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if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
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/*
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* 缺页发生在内核上下文,这种情况发生缺页的地址只能位于用户态地址空间
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* 这种情况下,也只能为exceptions table中预先定义好的异常,如果exceptions
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* table中没有预先定义的处理,或者缺页的地址位于内核态地址空间,则表示
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* 错误,进入oops流程。
-
*/
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if ((error_code & PF_USER) == 0 &&
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!search_exception_tables(regs->ip)) {
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bad_area_nosemaphore(regs, error_code, address);
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return;
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}
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retry:
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// 如果发生在用户态或者有exception table,说明不是内核异常
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down_read(&mm->mmap_sem);
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} else {
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/*
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* The above down_read_trylock() might have succeeded in
-
* which case we'll have missed the might_sleep() from
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* down_read():
-
*/
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might_sleep();
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}
-
-
// 在当前进程的地址空间中寻找发生异常的地址对应的VMA。
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vma = find_vma(mm, address);
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// 如果没找到VMA,则释放mem_sem信号量后,进入__bad_area_nosemaphore处理。
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if (unlikely(!vma)) {
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bad_area(regs, error_code, address);
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return;
-
}
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/* 找到VMA,且发生异常的虚拟地址位于vma的有效范围内,则为正常的缺页
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* 异常,请求调页,分配物理内存 */
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if (likely(vma->vm_start <= address))
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goto good_area;
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/* 如果异常地址不是位于紧挨着堆栈区的那个区域,同时又没有相应VMA,则
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* 进程访问了非法地址,进入bad_area处理
-
*/
-
if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) {
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bad_area(regs, error_code, address);
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return;
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}
-
if (error_code & PF_USER) {
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/*
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* Accessing the stack below %sp is always a bug.
-
* The large cushion allows instructions like enter
-
* and pusha to work. ("enter $65535, $31" pushes
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* 32 pointers and then decrements %sp by 65535.)
-
*/
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/*
-
* 压栈操作时,操作的地址最大的偏移为65536+32*sizeof(unsigned long),
-
* 该操作由pusha命令触发(老版本中,pusha命令最大只能操作32字节,即
-
* 同时压栈8个寄存器)。如果访问的地址距栈顶的距离超过了,则肯定是非法
-
* 地址访问了。
-
*/
-
if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
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bad_area(regs, error_code, address);
-
return;
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}
-
}
-
-
/*
-
* 运行到这里,说明设置了VM_GROWSDOWN标记,表示缺页异常地址位于堆栈区
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* 需要扩展堆栈。说明: 堆栈区的虚拟地址空间也是动态分配和扩展的,不是
-
* 一开始就分配好的。
-
*/
-
if (unlikely(expand_stack(vma, address))) {
-
bad_area(regs, error_code, address);
-
return;
-
}
-
-
/*
-
* Ok, we have a good vm_area for this memory access, so
-
* we can handle it..
-
*/
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/*
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* 运行到这里,说明是正常的缺页异常,则进行请求调页,分配物理内存
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*/
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good_area:
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if (unlikely(access_error(error_code, vma))) {
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bad_area_access_error(regs, error_code, address);
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return;
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}
-
-
/*
-
* If for any reason at all we couldn't handle the fault,
-
* make sure we exit gracefully rather than endlessly redo
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* the fault:
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*/
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/*
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* 分配物理内存,缺页异常的正常处理主函数
-
* 可能的情况有:1、请求调页/按需分配;2、COW;3、缺的页位于交换分区,
-
* 需要换入。
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*/
-
fault = handle_mm_fault(mm, vma, address, flags);
-
-
if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
-
if (mm_fault_error(regs, error_code, address, fault))
-
return;
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}
-
-
/*
-
* Major/minor page fault accounting is only done on the
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* initial attempt. If we go through a retry, it is extremely
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* likely that the page will be found in page cache at that point.
-
*/
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if (flags & FAULT_FLAG_ALLOW_RETRY) {
-
if (fault & VM_FAULT_MAJOR) {
-
tsk->maj_flt++;
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perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
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regs, address);
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} else {
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tsk->min_flt++;
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perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
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regs, address);
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}
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if (fault & VM_FAULT_RETRY) {
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/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
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* of starvation. */
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flags &= ~FAULT_FLAG_ALLOW_RETRY;
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flags |= FAULT_FLAG_TRIED;
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goto retry;
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}
-
}
-
-
// VM86模式(兼容老环境)相关检查
-
check_v8086_mode(regs, address, tsk);
-
-
up_read(&mm->mmap_sem);
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}
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