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//*********************************缺頁(yè)異常處理函數(shù)*******************************************
do_page_fault(struct pt_regs *regs, unsigned long error_code)
{
//獲取當(dāng)前cpu正在運(yùn)行的進(jìn)程的進(jìn)程描述符
//然后獲取該進(jìn)程的內(nèi)存描述符
tsk = current;
mm = tsk->mm;
/* Get the faulting address: */
//獲取出錯(cuò)的地址
address = read_cr2();
/*
* We fault-in kernel-space virtual memory on-demand. The
* 'reference' page table is init_mm.pgd.
*
* NOTE! We MUST NOT take any locks for this case. We may
* be in an interrupt or a critical region, and should
* only copy the information from the master page table,
* nothing more.
*
* This verifies that the fault happens in kernel space
* (error_code & 4) == 0, and that the fault was not a
* protection error (error_code & 9) == 0.
*/
//頁(yè)訪(fǎng)問(wèn)出錯(cuò)地址address在內(nèi)核空間
if (unlikely(fault_in_kernel_space(address))) {
//檢查標(biāo)志位確定訪(fǎng)問(wèn)發(fā)生在"內(nèi)核態(tài)"
if (!(error_code & (PF_RSVD | PF_USER | PF_PROT))) {
//如果是內(nèi)核空間"非連續(xù)內(nèi)存"的訪(fǎng)問(wèn),
//則直接拷貝"內(nèi)核頁(yè)表項(xiàng)"到"用戶(hù)頁(yè)表項(xiàng)"
//如果"內(nèi)核頁(yè)表項(xiàng)"為null,說(shuō)明內(nèi)核有BUG,返回-1
if (vmalloc_fault(address) >= 0)
return;
}
//如果在"用戶(hù)態(tài)"則直接進(jìn)入"非法訪(fǎng)問(wèn)"處理函數(shù)
//如果vmalloc_fault返回-1,則表示內(nèi)核BUG
bad_area_nosemaphore(regs, error_code, address);
//錯(cuò)誤處理函數(shù)
// 1 "用戶(hù)態(tài)"錯(cuò)誤-->直接終止進(jìn)程
// 2 "內(nèi)核態(tài)"錯(cuò)誤
// 系統(tǒng)調(diào)用參數(shù)錯(cuò)誤 ---->終止進(jìn)程/返回系統(tǒng)調(diào)用錯(cuò)誤碼
// 內(nèi)核BUG ---->內(nèi)核panic
return;
}
/*
* If we're in an interrupt, have no user context or are running
* in an atomic region then we must not take the fault:
*/
// 1 在中斷中,此時(shí)沒(méi)有進(jìn)程上下文
// 2 在原子操作流程中
// 都不允許處理缺頁(yè)異常
if (unlikely(in_atomic() || !mm)) {
bad_area_nosemaphore(regs, error_code, address);
return;
}
/*
* When running in the kernel we expect faults to occur only to
* addresses in user space. All other faults represent errors in
* the kernel and should generate an OOPS. Unfortunately, in the
* case of an erroneous fault occurring in a code path which already
* holds mmap_sem we will deadlock attempting to validate the fault
* against the address space. Luckily the kernel only validly
* references user space from well defined areas of code, which are
* listed in the exceptions table.
*
* As the vast majority of faults will be valid we will only perform
* the source reference check when there is a possibility of a
* deadlock. Attempt to lock the address space, if we cannot we then
* validate the source. If this is invalid we can skip the address
* space check, thus avoiding the deadlock:
*/
//此時(shí)可以確定缺頁(yè)地址address在"用戶(hù)空間"了
if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
//錯(cuò)誤發(fā)生在"內(nèi)核態(tài)",查看異常表
//如果在內(nèi)核態(tài)引起缺頁(yè),則引起缺頁(yè)的"指令地址"一定在"異常表"中
//如果"異常表"中返回指令地址,則說(shuō)明可能是"請(qǐng)求調(diào)頁(yè)",也可能是"非法訪(fǎng)問(wèn)"
//如果"異常表"中無(wú)地址,則肯定是內(nèi)核錯(cuò)誤
if ((error_code & PF_USER) == 0 &&
!search_exception_tables(regs->ip)) {
//內(nèi)核panic
bad_area_nosemaphore(regs, error_code, address);
return;
}
down_read(&mm->mmap_sem);
} else {
/*
* The above down_read_trylock() might have succeeded in
* which case we'll have missed the might_sleep() from
* down_read():
*/
might_sleep();
}
//尋找address所在的vma
vma = find_vma(mm, address);
//如果address之后無(wú)vma,則肯定是非法訪(fǎng)問(wèn)
if (unlikely(!vma)) {
bad_area(regs, error_code, address);
return;
}
// 1 如果vma->start_address<=address,則直接跳到 "合法訪(fǎng)問(wèn)"階段
// 2 如果vma->start_address>address,則也有可能是用戶(hù)的"入棧行為"導(dǎo)致缺頁(yè)
if (likely(vma->vm_start <= address))
goto good_area;
// "入棧"操作,則該vma的標(biāo)志為 "向下增長(zhǎng)"
if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) {
bad_area(regs, error_code, address);
return;
}
// 確定缺頁(yè)發(fā)生在"用戶(hù)態(tài)"
if (error_code & PF_USER) {
/*
* Accessing the stack below %sp is always a bug.
* The large cushion allows instructions like enter
* and pusha to work. ("enter $65535, $31" pushes
* 32 pointers and then decrements %sp by 65535.)
*/
//驗(yàn)證缺頁(yè)address和棧頂sp的關(guān)系
if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
bad_area(regs, error_code, address);
return;
}
}
//擴(kuò)展棧
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..
*/
good_area:
write = error_code & PF_WRITE;
// 再次驗(yàn)證"權(quán)限"
if (unlikely(access_error(error_code, write, vma))) {
bad_area_access_error(regs, error_code, address);
return;
}
/*
* If for any reason at all we couldn't handle the fault,
* make sure we exit gracefully rather than endlessly redo
* the fault:
*/
//分配新"頁(yè)框"
fault = handle_mm_fault(mm, vma, address, write ? FAULT_FLAG_WRITE : 0);
up_read(&mm->mmap_sem);
}
//*******************************訪(fǎng)問(wèn)權(quán)限驗(yàn)證函數(shù)********************************************
access_error(unsigned long error_code, int write, struct vm_area_struct *vma)
{
//如果是"寫(xiě)操作"引起的缺頁(yè),則該vma必須可寫(xiě)
if (write) {
/* write, present and write, not present: */
if (unlikely(!(vma->vm_flags & VM_WRITE)))
return 1;
return 0;
}
/* read, present: */
//檢查該頁(yè)是否已經(jīng)在RAM中,如果"特權(quán)位"置位表示頁(yè)框在RAM中
//表示進(jìn)程訪(fǎng)問(wèn)"有特權(quán)" 頁(yè)框
if (unlikely(error_code & PF_PROT))
return 1;
/* read, not present: */
//如果該頁(yè)不在內(nèi)存中,該線(xiàn)性區(qū)必須可"讀"
if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
return 1;
return 0;
}

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