{"id":54,"date":"2024-03-09T15:04:14","date_gmt":"2024-03-09T07:04:14","guid":{"rendered":"http:\/\/104.46.213.183\/?p=54"},"modified":"2024-03-09T15:04:14","modified_gmt":"2024-03-09T07:04:14","slug":"lab-copy-on-write","status":"publish","type":"post","link":"https:\/\/www.yewpo.top\/index.php\/54\/","title":{"rendered":"Lab Copy-on-Write"},"content":{"rendered":"

Lab Copy-on-Write<\/h1>\n
\n

\u5728\u5b9e\u9a8c\u521d\u671f\u7684\u64cd\u4f5c\u7cfb\u7edf\u4e2d\uff0c\u5f53\u4e00\u4e2a\u8fdb\u7a0b\u60f3\u8981\u521b\u5efa\u4e00\u4e2a\u5b50\u8fdb\u7a0b\u65f6\uff0c\u4f1a\u5c06\u7236\u8fdb\u7a0b\u7684\u8d44\u6e90\u5b8c\u5b8c\u5168\u5168\u7684\u590d\u5236\u4e00\u4efd\uff0c\u7136\u540e\u8fd9\u4e9b\u590d\u5236\u7684\u8d44\u6e90\u4e0d\u4e00\u5b9a\u90fd\u4f1a\u4f7f\u7528\uff0c\u5bfc\u81f4\u5185\u5b58\u7684\u7a7a\u95f4\u5229\u7528\u7387\u964d\u4f4e\u3002<\/p>\n

\u672c\u5b9e\u9a8c\u7684\u76ee\u7684\u662f\uff0c\u8ba9\u6211\u4eec\u4fee\u6539\u4e00\u4e9b\u5185\u6838\u4ee3\u7801\uff0c\u4f7f\u8fdb\u7a0b\u521b\u5efa\u5b50\u8fdb\u7a0b\u7684\u65f6\u5019\u91c7\u7528\u5199\u65f6\u590d\u5236\uff08copy on write\uff09\u7b56\u7565\uff0c\u8fd9\u4e2a\u7b56\u7565\u5141\u8bb8\u7236\u8fdb\u7a0b\u548c\u5b50\u8fdb\u7a0b\u6700\u521d\u5171\u4eab\u76f8\u540c\u7684\u9875\u9762\u6765\u5de5\u4f5c\uff0c\u8fd9\u4e9b\u5171\u4eab\u9875\u9762\u88ab\u6807\u8bb0\u4e3a\u5199\u65f6\u590d\u5236\uff0c\u8fd9\u610f\u5473\u7740\u5982\u679c\u4efb\u4f55\u4e00\u4e2a\u8fdb\u7a0b\u5199\u5165\u5171\u4eab\u9875\u9762\uff08\u5982\u679c\u8fd9\u4e2a\u9875\u9762\u4e4b\u524d\u662f\u5141\u8bb8\u88ab\u5199\u7684\u60c5\u51b5\u4e0b\uff09\uff0c\u90a3\u4e48\u5c31\u521b\u5efa\u5171\u4eab\u9875\u9762\u7684\u526f\u672c\u3002<\/p>\n

\u4e0b\u56fe\u4e3a\u521b\u5efa\u8fdb\u7a0b\u65f6\uff0c\u4e24\u4e2a\u8fdb\u7a0b\u5171\u4eab\u9875\u9762\u3002<\/p>\n

\"1609221978-71449\"<\/div><\/p>\n

\u5982\u679c\u8fdb\u7a0bA\u60f3\u8981\u8bfb\u5199\u9875\u9762Z\uff0c\u8fdb\u7a0bA\u5c31\u4f1a\u5f97\u5230\u4e00\u4e2aZ\u9875\u9762\u7684\u526f\u672c\uff0c\u7136\u540e\u5728\u8fd9\u4e2a\u526f\u672c\u4e0a\u5199\u5165\u3002\u5982\u679cB\u4e5f\u8981\u5199\u5165Z\u9875\u9762\u7684\u8bdd\uff0c\u4e5f\u4f1a\u5f97\u5230\u4e00\u4e2aZ\u9875\u9762\u7684\u526f\u672c\uff08\u4e0d\u540c\u4e8eA\u8fdb\u7a0b\u7684\u526f\u672c\uff09\u3002<\/p>\n

\"1609222774-71449\"<\/div><\/p>\n<\/blockquote>\n

\u8fd9\u4e2a\u5b9e\u9a8c\u6bd4\u8f83\u590d\u6742\uff0c\u6211\u5c06\u8fd9\u4e2a\u5b9e\u9a8c\u5206\u62104\u4e2a\u90e8\u5206\u5b9e\u73b0\uff0c\u6309\u7167\u4e2a\u4eba\u8ba4\u4e3a\u7684\u96be\u6613\uff08\u4e5f\u4e0d\u4e00\u5b9a\u662f\u96be\u6613\uff0c\u4e5f\u7b97\u662f\u5b9e\u9a8c\u7684\u8026\u5408\uff09\u7a0b\u5ea6\uff0c\u6309\u5148\u540e\u987a\u5e8f\u5206\u6210\u4e0b\u97624\u6b65\u3002<\/p>\n

    \n
  • \u5b9e\u73b0\u5f15\u7528\u8ba1\u6570\uff08kernel\/kalloc.c<\/code>\uff09<\/li>\n
  • \u5b9e\u73b0copy-on-write<\/code>\uff08kernel\/vm.c: uvmcopy()<\/code>\uff09<\/li>\n
  • \u5b9e\u73b0\u5bf9scuase<\/code>\u4e3a15(store\/AMO page fault<\/code>)\u7684\u9677\u9631\u5904\u7406\uff08kernel\/trap.c: usertrap()<\/code>\uff09<\/li>\n
  • \u5bf9kernel\/vm.c: copyout()<\/code>\u7684\u5904\u7406\uff0c\u4ee5\u9002\u5e94cow\u673a\u5236\u3002<\/li>\n<\/ul>\n

    1 \u5f15\u7528\u8ba1\u6570<\/h2>\n

    \u5f15\u5165cow\u673a\u5236\u4e4b\u540e\uff0c\u5c31\u4f1a\u53d1\u73b0\u4e00\u4e2a\u7269\u7406\u9875\u9762\u4e0d\u518d\u662f\u53ea\u88ab\u4e00\u4e2a\u8fdb\u7a0b\u4f7f\u7528\u4e86\uff0c\u4e5f\u53ef\u80fd\u591a\u4e2a\u8fdb\u7a0b\u5171\u4eab\u8fd9\u4e2a\u9875\u9762\u3002\u4e3a\u4e86\u7edf\u8ba1\u6709\u591a\u5c11\u4e2a\u8fdb\u7a0b\u5728\u4f7f\u7528\u8fd9\u4e2a\u9875\u9762\uff0c\u4ee5\u53ca\u5982\u679c\u8fd9\u4e2a\u9875\u9762\u5f15\u7528\u8ba1\u6570\u4e3a0\u7684\u65f6\u5019\u91ca\u653e\u8fd9\u4e2a\u7a7a\u95f4\uff0c\u5c06\u5176\u6dfb\u52a0\u81f3\u7a7a\u95f2\u94fe\u8868\u3002\u5f15\u7528\u8ba1\u6570\u7684\u5f15\u5165\u5728\u8fd9\u91cc\u5c31\u5f88\u91cd\u8981\u4e86\uff0c\u5bf9\u4e8e\u6574\u4e2a\u5b9e\u9a8c\u6765\u8bb2\uff0c\u8fd9\u4e00\u90e8\u5206\u76f8\u5bf9\u72ec\u7acb\u3002<\/p>\n

    1.1 \u5982\u4f55\u8ba1\u6570<\/h3>\n

    \u5728\u5b8f\u5b9a\u4e49\u4e2d\u5b9a\u4e49\u4e86\u7269\u7406\u5730\u5740\u7684\u6700\u5927\u503cPHYSTOP<\/code>\uff0c\u7269\u7406\u5730\u5740\u9876\u7aef\uff0c\u8fd8\u662f\u7269\u7406\u5730\u5740\u505c\u6b62\u4e4b\u5904\uff1f\uff08\u8fd9\u5f97\u770b\u662fphy-stop \u8fd8\u662f phys-top\uff09\u3002\u6bcf\u4e2a\u7269\u7406\u9875\u9762\u7684\u5927\u5c0f\u662fPGSIZE = 4096\u3002\u6211\u4eec\u53ef\u4ee5\u5b9a\u4e49\u4e00\u4e2a\u6570\u7ec4\u6765\u50a8\u5b58\u8fd9\u4e2a\u9875\u9762\u88ab\u5f15\u7528\u7684\u6b21\u6570\uff0c\u6570\u7ec4\u7684\u5927\u5c0f\u4e3aPHYSTOP \/ PGSIZE<\/code>\u3002<\/p>\n

    \/\/--------kernel\/vm.c----------\nuint refcount[PHYPGNUM];\n\n\/\/-------kernel\/mmelayout.h------\n#define PHYPGNUM (PHYSTOP \/ PGSIZE)<\/code><\/pre>\n
    1.2 \u521d\u59cb\u5316<\/h3>\n
    \u521b\u5efa\u7269\u7406\u5185\u5b58\u7684\u65f6\u5019\u4f1a\u5bf9\u8fd9\u4e9b\u7269\u7406\u5185\u5b58\u8fdb\u884c\u521d\u59cb\u5316kinit<\/code>\uff0c\u5bf9\u6bcf\u4e2a\u9875\u9762\u8c03\u7528\u4e00\u6b21kfree<\/code>\uff0c\u5982\u679c\u6211\u4eec\u8981\u5728kfree<\/code>\u51fd\u6570\u4e2d\u5bf9\u5f15\u7528\u8ba1\u6570\u51cf\u4e00\uff0c\u5e76\u5728\u51cf\u4e00\u4e4b\u540e\u5224\u65ad\u662f\u5426\u8981\u8981\u91ca\u653e\u7684\u8bdd\uff0c\u8fd9\u65f6\u6211\u4eec\u8981\u5c06\u521d\u59cb\u8ba1\u6570\u7f6e\u4e3a1\uff0c\u4ee5\u4fbf\u5728kfree<\/code>\u4e2d\u51cf\u4e00\u540e\u91ca\u653e\u3002<\/p>\n
    void\nfreerange(void *pa_start, void *pa_end)\n{\n  char *p;\n  p = (char*)PGROUNDUP((uint64)pa_start);\n  for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE) {\n    refcount[(uint64)p \/ PGSIZE] = 1; \/\/ set 1 for kfree dec 1\n    kfree(p);\n  }\n}<\/code><\/pre>\n
    1.3 \u5206\u914d<\/h3>\n
    \u5f53\u5206\u914d\u7a7a\u95f4\u7684\u65f6\u5019kalloc<\/code>\uff0c\u6211\u4eec\u5c06\u8fd9\u4e2a\u9875\u9762\u7684\u8ba1\u6570\u7f6e\u4e3a1,\uff08\u8fd9\u4e2a\u7a7a\u95f4\u4e00\u5b9a\u4f1a\u7528\u8fdb\u7a0b\u8981\u7684\uff0c\u4e0d\u7136\u5206\u914d\u4ed6\u505a\u4ec0\u4e48\uff09\u3002<\/p>\n
    void *\nkalloc(void)\n{\n  struct run *r;\n\n  acquire(&kmem.lock);\n  r = kmem.freelist;\n  if(r) {\n    kmem.freelist = r->next;\n    refcount[(uint64)r \/ PGSIZE] = 1; \/\/ set to 1\n  }\n  release(&kmem.lock);\n\n  if(r)\n    memset((char*)r, 5, PGSIZE); \/\/ fill with junk\n  return (void*)r;\n}<\/code><\/pre>\n
    NEED A LOCK<\/strong>\uff0c\u4e0d\u4e00\u5b9a\u53ea\u6709\u4e00\u4e2a\u8fdb\u7a0b\u5728\u8bbf\u95ee\u8fd9\u4e2a\u51fd\u6570\uff0c\u907f\u514d\u7ade\u4e89\u3002\uff08\u521d\u59cb\u5316\u6ca1\u6709\u7528\u9501\u662f\u56e0\u4e3a\uff0c\u53ea\u4f1a\u6709init<\/code>\u8fdb\u7a0b\u624d\u80fd\u521d\u59cb\u5316\u5185\u5b58\uff09\u3002\u4e0b\u5217\u51fd\u6570\u540c\u6837\u3002<\/p>\n
    1.4 \u91ca\u653e\uff08\u8ba1\u6570\u51cf\u4e00\uff09<\/h3>\n
    \u5f53\u6709\u8fdb\u7a0b\u4e0d\u9700\u8981\u67d0\u4e2a\u9875\u9762\u7684\u65f6\u5019\uff0c\u4e00\u822c\u4f1a\u5148\u89e3\u9664\u9875\u8868\u4e2d\u7684\u9875\u9762\u6620\u5c04\uff0c\u7136\u540e\u8c03\u7528uvmfree<\/code>\uff0c\u7136\u540e\u5c31\u4f1a\u8c03\u7528kfree<\/code>\u8fa3\u3002\u7b14\u8005\u6700\u5f00\u59cb\u8ba4\u4e3auvmunmap<\/code>\u51fd\u6570\u4e5f\u662f\u5fc5\u987b\u8981\u7ecf\u8fc7\u7684\u51fd\u6570\uff0c\u6240\u4ee5\u5728\u8fd9\u4e2a\u51fd\u6570\u91cc\u5bf9\u8ba1\u6570\u51cf\u4e00\uff0c\u7cfb\u7edf\u80fd\u8dd1\u8bc1\u660e\u8fd9\u4e2a\u6709\u4e00\u5b9a\u9053\u7406\uff0c\u5e76\u901a\u8fc7\u4e86cowtest<\/code>\u6d4b\u8bd5\uff0c\u5185\u6838\u6d4b\u8bd5\u5931\u8d25\u4e86\uff0c\u4f46\u4e5f\u4e0d\u4e00\u5b9a\u662f\u8fd9\u4e2a\u7684\u539f\u56e0\uff0c\u6240\u4ee5\u5927\u5bb6\u53ef\u4ee5\u5c1d\u8bd5\uff0c\u4f46\u662fkfree<\/code>\u51fd\u6570\u662f\u4e00\u5b9a\u4f1a\u7ecf\u8fc7\u7684\uff0c\u7f51\u4e0a\u666e\u904d\u89c2\u70b9\uff0c\u6240\u4ee5\u6211\u7684\u7b2c\u4e09\u7248\u4ee3\u7801\u4e3b\u8981\u9488\u5bf9\u7684\u662f\u8fd9\u4e2a\u7684\u95ee\u9898\u3002<\/p>\n
    \u4f46\u662f\u540e\u6765\u60f3\u4e00\u60f3\uff0cuvmunmap<\/code>\u662f\u4e0d\u662f\u53ea\u9488\u5bf9\u7528\u6237\u9875\u8868\u7684\uff0c\u5185\u6838\u5c31\u4e0d\u7528\u8fd9\u4e2a\u4e86\uff1f<\/p>\n
    \u7efc\u4e0a\uff0c\u5f53\u8c03\u7528kfree<\/code>\u7684\u65f6\u5019\uff0c\u6211\u4eec\u5bf9\u8ba1\u6570\u51cf\u4e00\uff0c\u5982\u679c\u8ba1\u6570\u4e3a0\uff0c\u6211\u4eec\u624d\u8fdb\u884c\u540e\u7eed\u7684\u91ca\u653e\u7a7a\u95f4\u7684\u64cd\u4f5c\uff0c\u5426\u5219\u6211\u4eec\u5c31\u8fd4\u56de\u3002<\/p>\n
    void\nkfree(void *pa)\n{\n    .....\n    acquire(&kmem.lock);\n    refcount[(uint64)pa \/ PGSIZE]--;\n    if (refcount[(uint64)pa \/ PGSIZE] > 0) {\n        release(&kmem.lock);\n        return;\n    }\n    release(&kmem.lock);\n    .....\n}<\/code><\/pre>\n
    1.5 \u8ba1\u6570\u7d2f\u52a0<\/h3>\n
    \u5f53\u6211\u4eecfork\u4e00\u4e2a\u8fdb\u7a0b\u7684\u65f6\u5019\uff0c\u6211\u4eec\u4f1a\u5c06\u5b50\u8fdb\u7a0b\u7684\u9875\u8868\u9875\u9762\u6620\u5c04\u5230\u7236\u8fdb\u7a0b\u7684\u7269\u7406\u9875\u9762\uff0c\u8be5\u9875\u9762\u7684\u5f15\u7528\u6b21\u6570\u589e\u52a0\uff0c\u6211\u4eec\u8981\u8c03\u7528\u8ba1\u6570\u589e\u52a0\u51fd\u6570\uff0c\u6765\u589e\u52a0\u8fd9\u4e2a\u9875\u9762\u7684\u5f15\u7528\u3002<\/p>\n
    void\nrefinc(void *pa)\n{\n  if ((uint64)pa % PGSIZE != 0) {\n    panic("refinc");\n  }\n\n  acquire(&kmem.lock);\n  refcount[(uint64)pa \/ PGSIZE]++;\n  release(&kmem.lock);\n}<\/code><\/pre>\n
    2 Copy-on-Write<\/h2>\n
    2.1 \u5220\u9664\u5206\u914d\u5185\u5b58\u90e8\u5206<\/h3>\n
    cow\u673a\u5236\u5df2\u4e0d\u9700\u8981\u5206\u914d\u5185\u5b58\uff0c\u76f4\u5230\u9700\u8981\u5199\u7684\u65f6\u5019\u3002\u6211\u4eec\u8981\u628a\u539f\u6765\u5206\u914d\u5185\u5b58\u7684\u90e8\u5206\u7ed9\u5220\u9664\u6389\u3002<\/p>\n
    2.2 \u6807\u8bb0\u53ea\u8bfb<\/h3>\n
    \u6309\u7167cow\u673a\u5236\u7684\u63cf\u8ff0\uff0c\u6240\u6709\u9875\u9762\u8981\u6807\u8bb0\u4f4d\u53ea\u8bfb\u3002<\/p>\n
    2.3 \u6dfb\u52a0COW\u4f4d<\/h3>\n
    \u5982\u679c\u8be5\u9875\u9762\u4e4b\u524d<\/strong>\u88ab\u6807\u8bb0\u4f4d\u53ef\u5199\u7684\u65f6\u5019\uff0c\u64e6\u9664\u53ef\u5199\u4f4d\uff0c\u6807\u8bb0C\u4f4d\uff08\u8868\u793a\u8fd9\u4e2a\u9875\u9762\u4f7f\u7528COW\u673a\u5236\uff09\u3002\u6211\u4eec\u53ef\u4ee5\u5728RSW<\/code>\uff08\u7ba1\u7406\u8f6f\u4ef6\u4fdd\u7559\u4f4d\uff09\u4e2d\u8bbe\u7f6e\u4e00\u4e2aC\u4f4d\u3002<\/p>\n
    \"image-20221105120656664\"<\/div><\/p>\n
    \/\/------------kernel\/risv.h------------------\n#define PTE_C (1L << 8)<\/code><\/pre>\n
    \u4fee\u6539\u6807\u8bb0\u7684\u65b9\u6cd5\u4e0d\u5728\u6b64\u5904\u8d58\u8ff0\u3002\uff08\u4f4d\u8fd0\u7b97\uff09<\/p>\n
    2.4 \u4fee\u6539PTE<\/h3>\n
    \u5bf9\u4e8e\u7236\u8fdb\u7a0b\u7684PTE\uff0c\u6211\u4eec\u53ef\u4ee5\u7528\u4f4d\u8fd0\u7b97\u4fee\u6539flags\u4f4d\u5373\u53ef\uff0c\u6ca1\u6709\u5fc5\u8981\u8c03\u7528\u89e3\u9664\u6620\u5c04\u548c\u6620\u5c04\u51fd\u6570\u3002<\/p>\n
    \u5bf9\u4e8e\u5b50\u8fdb\u7a0b\u7684PTE\uff0c\u6211\u4eec\u5c31\u9700\u8981\u7528mappages<\/code>\u51fd\u6570\u8fdb\u884c\u6620\u5c04\u4e86\uff0c\u6620\u5c04\u5b8c\u4e86\u4e4b\u540e\u4e0d\u8981\u5fd8\u8bb0\u589e\u52a0\u8fd9\u4e2a\u7269\u7406\u9875\u9762\u7684\u5f15\u7528\u8ba1\u6570<\/strong>\u3002<\/p>\n
    2.5 \u4fee\u6539\u540e\u7684\u4ee3\u7801<\/h3>\n
    int\nuvmcopy(pagetable_t old, pagetable_t new, uint64 sz)\n{\n  pte_t *pte;\n  uint64 pa, i;\n  uint flags;\n\n  for(i = 0; i < sz; i += PGSIZE){\n    if((pte = walk(old, i, 0)) == 0)\n      panic("uvmcopy: pte should exist");\n    if((*pte & PTE_V) == 0)\n      panic("uvmcopy: page not present");\n    pa = PTE2PA(*pte);\n    flags = PTE_FLAGS(*pte);\n\n      \/\/--------------------------- change start ------------------------------\n    if (flags & PTE_W) { \/\/ mask C tag , unmask W tag\n      flags &= ~PTE_W;\n      flags |= PTE_C;\n    }\n\n    flags |= PTE_R;  \/\/ make it read only\n\n    *pte = PA2PTE(pa) | flags; \/\/ chage father process's flags\n\n    mappages(new, i, PGSIZE, pa, flags);\/\/ map child's pages to the father process's physical pages\n    refinc((void *)pa); \/\/ add count to physics page, important here!!!\n\n      \/\/---------------------- chage end -----------------------------------\n  }\n  return 0;\n\n    \/\/ delete the err part for that we do not need to alloc a physical page here.\n}<\/code><\/pre>\n
    3 scause(kernel\/trap.c)<\/h2>\n
    \u5f53\u6211\u4eec\u5199\u597d\u4e0a\u8ff0\u4e24\u90e8\u5206\u4ee3\u7801\u540e\uff0c\u6211\u4eec\u5c1d\u8bd5\u8fd0\u884c\u8fd9\u4e2a\u7cfb\u7edf\uff0c\u4f1a\u53d1\u73b0\u4ea7\u751f\u4e86scause = oxf<\/code>\u7684fault\uff0c\u67e5\u8be2scause\u8868\uff0c0xf \u7684\u60c5\u51b5\u8868\u793astore\/AMO page fault<\/code>\u3002<\/p>\n
    \"image-20221105122032103\"<\/div><\/p>\n
    \u4e3a\u4ec0\u4e48\u4f1a\u4ea7\u751f\u8fd9\u79cd\u60c5\u51b5\u5462\uff1f\u539f\u56e0\u662f\u6211\u4eec\u5c06\u8fdb\u7a0b\u7684\u9875\u9762\u90fd\u6807\u8bb0\u4e3a\u4e86\u53ea\u8bfb\uff0c\u800c\u8fdb\u7a0b\u6709\u5199\u7684\u64cd\u4f5c\uff0c\u5f88\u660e\u663e\uff0c\u8fd9\u4e2a\u64cd\u4f5c\u662f\u4e0d\u88ab\u5141\u8bb8\u7684\uff0c\u6240\u4ee5\u4ea7\u751f\u4e86\u8fd9\u4e2a\u9519\u8bef\uff0c\u540c\u65f6\u4e5f\u53ef\u80fd\u4f1a\u4ea7\u751fscause = 0x2<\/code>\u7684\u9519\u8bef\uff0c\u51fa\u73b0\u4e86\u975e\u6cd5\u6307\u4ee4\u3002<\/p>\n
    \u51fa\u73b0\u4e860xf<\/code>\u7684\u9519\u8bef\u4e0d\u4e00\u5b9a\u662f\u4e0d\u80fd\u5199\uff0c\u4e5f\u6709\u53ef\u80fd\u8fd9\u4e2a\u9875\u9762\u4f1a\u5728\u8981\u5199\u5165\u65f6\u8fdb\u884c\u590d\u5236\u64cd\u4f5c\uff0c\u6240\u4ee5\u6211\u4eec\u8981\u5728trap.c<\/code>\u91cc\u5904\u7406\u8fd9\u4e2a\u9519\u8bef\u3002<\/p>\n
    \u6211\u4eec\u4e0d\u59a8\u5c01\u88c5\u4e00\u4e2acowpagefault(pagetable_t pagetable, void * va)<\/code>\u51fd\u6570\uff0c\u4f20\u9012\u4e24\u4e2a\u53c2\u6570\uff0c\u4e00\u4e2a\u9875\u8868\uff0c\u4e00\u4e2a\u865a\u62df\u5730\u5740\uff0c\u7528\u6765\u5224\u65ad\u8fd9\u4e2a\u865a\u62df\u5730\u5740\u4f1a\u4e0d\u4f1a\u89e6\u53d1cow\u673a\u5236\u3002<\/p>\n
    3.1 \u89e6\u53d1COW\u673a\u5236\u68c0\u67e5<\/h3>\n
    \u5982\u679cscause == 0xf<\/code>\uff0c\u89e6\u53d1cow\u673a\u5236\u68c0\u67e5\uff1a<\/p>\n
    if (r_scause() == 0xf) {\n    if (cowpagefault(p->pagetable, r_stval()) != 0) {\n        setkilled(p);\n    }\n}<\/code><\/pre>\n
    \u5982\u679c\u8fd9\u4e2a\u865a\u62df\u5730\u5740\u4e0d\u6ee1\u8db3cow\u673a\u5236\uff0c\u5219\u6211\u4eec\u6740\u6b7b\u8fd9\u4e2a\u8fdb\u7a0b\u3002\uff08\u6bd4\u5982\uff0c\u8fd9\u4e2a\u9875\u9762\u53ea\u8bfb\uff0c\u65e0\u6548\uff0c\u6216\u8005\u7528\u6237\u6001\u4e0d\u80fd\u8bbf\u95ee\uff09<\/p>\n
    3.2 \u68c0\u67e5PTE<\/h3>\n
    \u901a\u8fc7walk<\/code>\u51fd\u6570\u5728\u8fd9\u4e2a\u9875\u8868\u4e2d\u627e\u5230\u8fd9\u4e2a\u865a\u62df\u5730\u5740va\u7684PTE\uff0c\u5e76\u68c0\u67e5\u8fd9\u4e2aPTE\u662f\u5426\u5b58\u5728\uff0c\u662f\u5426\u6709\u6548\uff0c\u662f\u5426\u7528\u6237\u53ef\u4ee5\u8bbf\u95ee\uff0c\u5982\u679c\u4ee5\u4e0a\u90fd\u4e0d\u6ee1\u8db3\uff0c\u5219\u4e0d\u6ee1\u8db3cow\u673a\u5236\u3002\u5982\u679c\u4ee5\u4e0a\u5747\u6ee1\u8db3\uff0c\u5219\u68c0\u67e5\u662f\u5426\u6709PTE_C<\/code>\u4f4d\uff0c\u5982\u679c\u8be5\u6807\u8bb0\u4f4d\u8bbe\u7acb\uff0c\u5219\u6ee1\u8db3cow\u673a\u5236\u3002<\/p>\n
    3.3 \u5206\u914d\u5e76\u6620\u5c04<\/h3>\n
    \u9996\u5148\uff0c\u6211\u4eec\u5206\u914d\u4e00\u4e2a\u9875\u9762\uff0c\u5982\u679c\u5206\u914d\u5931\u8d25\uff0c\u8868\u793a\u7269\u7406\u5185\u5b58\u5df2\u7ecf\u6ee1\u4e86\uff0c\u5219\u8fd4\u56de\u9519\u8bef\uff0c\u540c\u6837\u5bfc\u81f4\u6740\u6b7b\u8fdb\u7a0b\u3002\u5982\u679c\u5206\u914d\u6210\u529f\uff0c\u5219\u5c06\u539f\u6765\u7684\u7269\u7406\u9875\u9762\u7684\u5185\u5bb9\u7528memmove<\/code>\u51fd\u6570\u590d\u5236\u5230\u65b0\u7684\u7269\u7406\u9875\u9762\u4e0a\uff0c\u540c\u65f6\uff0c\u4f7f\u539f\u6765\u7684\u7269\u7406\u9875\u9762\u7684\u5f15\u7528\u6b21\u6570\u51cf\u4e00(kfree)<\/strong>\u3002\u4fee\u6539PTE\uff0c\u4f7f\u4e4b\u6620\u5c04\u5230\u65b0\u7684\u7269\u7406\u9875\u9762\u4e0a\u3002\uff08\u6b64\u6b65\u9aa4\u4e5f\u53ea\u9700\u8981\u4fee\u6539PTE\u5c31\u597d\uff0c\u6ca1\u6709\u5fc5\u8981\u8c03\u7528\u76f8\u5173PTE\u51fd\u6570\uff09\u3002<\/p>\n
    3.4 \u4ee3\u7801\u5b9e\u73b0<\/h3>\n
    int\ncowpagefault(pagetable_t pagetable, uint64 va)\n{\n  pte_t *pte = walk(pagetable, va, 0);\n\n  if (pte == 0) {\n    return -1;\n  }\n\n  if ((*pte & PTE_V) == 0 || (*pte & PTE_U) == 0) {\n    return -1;\n  }\n\n  if ((*pte & PTE_C) == 0) {\n    return -1;\n  }\n\n  uint flags;\n  uint64 pa0, pa1;\n\n  flags = PTE_FLAGS(*pte);\n  flags &= ~PTE_C;\n  flags |= PTE_W;\n  pa0 = PTE2PA(*pte);\n  if ((pa1 = (uint64)kalloc()) == 0) {\n    return -1;\n  }\n\n  memmove((void *)pa1, (void *)pa0, PGSIZE);\n\n  kfree((void *)pa0);\n\n  *pte = PA2PTE(pa1) | flags;\n\n  return 0;\n}<\/code><\/pre>\n
    \u4e0a\u8ff0\u5b8c\u6210\u4e4b\u540e\uff0c\u5e94\u8be5\u53ef\u4ee5\u901a\u8fc7\u4e24\u4e2asimple\u6d4b\u8bd5\uff0c\u548c\u4e09\u4e2athree\u6d4b\u8bd5\uff0c\u4f46\u662ffile\u6d4b\u8bd5\u4f1a\u5931\u8d25\uff0c\u56e0\u4e3a\u6211\u4eec\u6ca1\u6709\u4fee\u6539copyout<\/code>\u51fd\u6570\u4f7f\u4e4b\u9002\u5e94cow\u673a\u5236\u3002<\/p>\n
    4 copyout<\/h2>\n
    \u4e4b\u524d\u7684copyout<\/code>\u51fd\u6570\u76f4\u63a5\u5c06\u7269\u7406\u9875\u9762\u76f4\u63a5\u5199\u5230\u4e86\u7528\u6237\u7684\u7269\u7406\u9875\u9762\uff0c\u6ca1\u6709\u8003\u8651\u8be5\u9875\u9762\u662f\u4e0d\u662f\u53ef\u5199\u7684\u3002\u5728\u8fd9\u91cc\uff0c\u6211\u4eec\u6dfb\u52a0\u4e24\u6761\u5bf9\u865a\u62df\u5730\u5740\u7684\u5904\u7406\u3002<\/p>\n
    4.1 PTE\u68c0\u67e5<\/h3>\n
    \u901a\u8fc7walk<\/code>\u51fd\u6570\u67e5\u627e\u8fd9\u4e2a\u865a\u62df\u5730\u5740\u7684PTE\uff0c\u9996\u5148\u68c0\u67e5\u8fd9\u4e2aPTE\u662f\u5426\u5b58\u5728\uff0c\u5982\u679c\u4e0d\u5b58\u5728\u5219\u8fd4\u56de\u9519\u8bef\u3002<\/p>\n
    \u5982\u679c\u8be5PTE\u6ca1\u6709\u5199\u6807\u8bb0\uff0c\u6211\u4eec\u68c0\u67e5\u6ee1\u4e0d\u6ee1\u8db3cow\u673a\u5236\uff0c\u5982\u679c\u4e0d\u6ee1\u8db3\uff0c\u8fd4\u56de\u9519\u8bef\u3002<\/p>\n
    4.2 \u4ee3\u7801<\/h3>\n
    while (len > 0) {\n    va0 = PGROUNDDOWN(dstva);\n\n    \/\/ add code below\n    pte_t  *pte = walk(pagetable, va0, 0);\n\n    if (pte == 0) {\n      return -1;\n    }\n\n    if ((*pte & PTE_W) == 0) {\n      if (cowpagefault(pagetable, va0) != 0) {\n        return -1;\n      }\n    }\n\n    ......\n}<\/code><\/pre>\n
    \u5230\u8fd9\u4e00\u6b65\uff0c\u57fa\u672c\u5b9e\u73b0\u4e86cow\u673a\u5236\uff0c\u522b\u5fd8\u8bb0\u5c06\u65b0\u5199\u7684\u51fd\u6570\u5728kernel\/defs.h<\/code>\u4e2d\u58f0\u660e<\/strong>\uff0c\u7f16\u8bd1\u8fd0\u884ccowtest<\/code>\uff0c\u5e94\u8be5\u53ef\u4ee5\u901a\u8fc7\u6240\u6709\u6d4b\u8bd5\u3002<\/p>\n
    5 \u9057\u7559\u95ee\u9898<\/h2>\n
    \u975e\u5e38\u6fc0\u52a8\u5730\u901a\u8fc7\u4e86cowtest<\/code>\uff0c\u4f46\u662f\u9057\u61beusertests<\/code>\u5728copyout<\/code>\u4e0a\u5c31\u9519\u4e86\uff0c\u4ea7\u751f\u4e86scause = 0x2<\/code>\u7684\u9519\u8bef\uff0c\u5e76\u89e6\u53d1\u4e86panic(\u201cwalk\u201d)\uff0c\u68c0\u67e5walk<\/code>\u51fd\u6570\uff0c\u53d1\u73b0\u865a\u62df\u5730\u5740\u8d85\u8fc7\u4e86\u865a\u62df\u5730\u5740\u7684\u6700\u5927\u503c\u3002\uff08\u603b\u6709\u4eba\u60f3\u89e6\u78b0\u865a\u62df\u5730\u5740\u7684\u5e95\u7ebf\uff09<\/p>\n
    \u90a3\u4e48\u6211\u4eec\u8981\u5728copyout<\/code>\u4e2d\u5bf9\u865a\u62df\u5730\u5740\u8fdb\u884c\u68c0\u67e5\uff0c\u662f\u5426\u8d8a\u754c\u3002\u57284.2\u7684\u57fa\u7840\u4e0a\u6dfb\u52a0\u4ee5\u4e0b\u4ee3\u7801\uff1a<\/p>\n
    if (va0 >= MAXVA) {\n    return -1;\n}<\/code><\/pre>\n
    \u6b64\u65f6\u6d4b\u8bd5usertests<\/code>\uff0c\u54ce\u5440\uff0c\u53c8\u5bc4\u5728\u4e86MAXVAplus<\/code>\u6d4b\u8bd5\u4e0a\uff0c\u9519\u8bef\u539f\u56e0\u4e5f\u662fpainc("walk")<\/code>\uff0c\u901a\u8fc7\u81ea\u5df1\u5728\u4e0a\u4e2a\u5b9e\u9a8c\u5199\u7684backtrace<\/code>\u68c0\u67e5\uff0c\u53d1\u73b0\u9519\u8bef\u662f\u5728cowpagefault<\/code>\u4e2d\u5f15\u8d77\u7684\uff0c\u56e0\u4e3acowpagefault<\/code>\u4e5f\u6ca1\u68c0\u67e5\u865a\u62df\u5730\u5740\u662f\u5426\u8d8a\u754c\u3002\u57283.4\u7684\u4ee3\u7801\u57fa\u7840\u4e0a\u52a0\u4e0a\u4ee5\u4e0b\u4ee3\u7801\uff1a<\/p>\n
    int\ncowpagefault(pagetable_t pagetable, uint64 va)\n{\n  if (va >= MAXVA) {\n    return -1;\n  }\n    ......\n}<\/code><\/pre>\n
    \u81f3\u6b64\uff0c\u4e5f\u5c31\u901a\u8fc7\u4e86\u6240\u6709usertests<\/code>\uff0c\u5b9e\u9a8c\u7ed3\u675f\u3002<\/p>\n
    6 END<\/h2>\n
    \u8fd9\u4e2a\u5b9e\u9a8c\u7684\u4ee3\u7801\uff0c\u6211\u5199\u4e86\u4e09\u4e2a\u7248\u672c\uff0c\u8fd9\u4e2a\u901a\u8fc7\u7248\u672c\uff08\u501f\u9274\u4e86\u7f51\u4e0a\u7684\u666e\u904d\u7684\u601d\u8def\uff09\u548c\u6211\u7684\u601d\u8def\u6709\u4e09\u70b9\u4e0d\u540c\uff1a\uff08\u4e3b\u8bed\uff1a\u6211\u7684\u4ee3\u7801\uff09<\/p>\n
      \n
    • \u6ca1\u6709\u5bf9cow<\/code>\u7684\u7f3a\u9875\u9519\u8bef\u8fdb\u884c\u5c01\u88c5\u5904\u7406<\/li>\n
    • \u5bf9\u7269\u7406\u5185\u5b58\u7684\u5f15\u7528\u8ba1\u6570\u6ca1\u6709\u8003\u8651\u8fdb\u7a0b\u5185\u6838\u90e8\u5206(KERNELBASE\u4e4b\u4e0b)\uff0c\u8ba4\u4e3a\u8fd9\u4e9b\u90e8\u5206\u662f\u4e0d\u80fd\u4fee\u6539\u7684\uff0c\u53ea\u6709\u7cfb\u7edf\u8fdb\u7a0b\u7ed3\u675f\u65f6\u624d\u91ca\u653e\u7a7a\u95f4\u3002\u800c\u4e14\u81ea\u5df1\u5b9e\u73b0\u4e86\u5f15\u7528\u8ba1\u6570\u589e\u52a0\uff0c\u548c\u5f15\u7528\u8ba1\u6570\u51cf\u5c11\u7684\u51fd\u6570\uff0c\u5e76\u8ba4\u4e3a\u6240\u6709\u7684\u91ca\u653e\u5185\u5b58\u7684\u64cd\u4f5c\u90fd\u4f1a\u7ecf\u8fc7uvmunmap<\/code>\u51fd\u6570\uff0c\u6240\u4ee5\u5728\u8fd9\u4e2a\u51fd\u6570\u4e2d\u4f7f\u7528\u4e86\u5f15\u7528\u51cf\u5c11\u51fd\u6570\uff0c\u800c\u5728kfree<\/code>\u51fd\u6570\u4e2d\u4ec5\u8003\u8651\u5f15\u7528\u8ba1\u6570\u662f\u5426\u4e3a0\u5373\u53ef<\/li>\n
    • \u6ca1\u6709\u5904\u7406\u865a\u62df\u5730\u5740\u4e0d\u5408\u6cd5\u7684\u60c5\u51b5<\/li>\n<\/ul>\n
      \u6240\u4ee5\u4e0a\u8ff0\u63cf\u8ff0\u7684\u4e0d\u540c\u5730\u65b9\uff0c\u4f7f\u6211\u7684\u4ee3\u7801\u4ec5\u901a\u8fc7\u4e86sample\uff0cthree\u7684\u4e24\u4e2a\u6d4b\u8bd5\u70b9\uff0c\u7b2c\u4e09\u4e2akalloc<\/code>\u5931\u8d25\u3002\u5982\u679c\u6211\u4fee\u6539\u6d4b\u8bd5\u7a0b\u5e8f\uff0c\u8ba9\u5b50\u8fdb\u7a0b\u591a\u591await(0)<\/code>\u4e00\u4e0b\uff0c\u5c31\u53ef\u4ee5\u901a\u8fc7cowtest<\/code>\u7684\u6240\u6709\u6d4b\u8bd5\uff0c\u867d\u7136\u4e0d\u662f\u5f88\u7406\u89e3\u8fd9\u4e2a\u9519\u8bef\u539f\u56e0\u3002\u4f46\u662fusertests<\/code>\u7684copyout<\/code>\u7684\u6d4b\u8bd5\u5c31\u5d29\u6e83\u4e86\uff0c\u4e0d\u77e5\u9053\u662f\u4e0d\u662f\u56e0\u4e3a\u6ca1\u6709\u5904\u7406\u975e\u6cd5\u7684\u865a\u62df\u5730\u5740\u3002<\/p>\n
      \u4e0b\u5217\u662f\u672c\u9898\u89e3\u63cf\u8ff0\u7684\u4ee3\u7801\u7684\u6d4b\u8bd5\u7ed3\u679c\u3002<\/p>\n
      == Test running cowtest == \n$ make qemu-gdb\n(20.9s) \n== Test   simple == \n  simple: OK \n== Test   three == \n  three: OK \n== Test   file == \n  file: OK \n== Test usertests == \n$ make qemu-gdb\n(166.3s) \n== Test   usertests: copyin == \n  usertests: copyin: OK \n== Test   usertests: copyout == \n  usertests: copyout: OK \n== Test   usertests: all tests == \n  usertests: all tests: OK \n== Test time == \ntime: OK \nScore: 110\/110<\/code><\/pre>\n
      7 \u524d\u4e24\u7248\u9519\u8bef\u539f\u56e0\u5206\u6790<\/h2>\n
      \u6309\u71676 END<\/code>\u4e2d\u7684\u4e09\u4e2a\u89e3\u91ca\uff0c\u6211\u518d\u5206\u6210\u56db\u4e2a\uff1a<\/p>\n
      7.1 \u6ca1\u6709\u5bf9cowpage\u9519\u8bef\u5c01\u88c5<\/h3>\n
      \u8fd9\u4e2a\u5bf9\u5b9e\u9a8c\u7684\u6b63\u786e\u6027\u5f71\u54cd\u4e0d\u5927<\/p>\n
      7.2 \u6ca1\u6709\u5bf9KERNELBASE\u4e4b\u4e0b\u7684\u7a7a\u95f4\u8fdb\u884c\u5f15\u7528\u8ba1\u6570<\/h3>\n
      \u901a\u8fc7\u540e\u9762\u7684\u6d4b\u8bd5\uff0c\u53ef\u4ee5\u4e0d\u5bf9KERNELBASE\u5730\u5740\u4e4b\u4e0b\u8fdb\u884c\u5f15\u7528\u8ba1\u6570\uff0c\u56e0\u4e3a\u8fd9\u4e9b\u90e8\u5206\u4e0d\u548c\u5185\u6838\u6216\u8005\u7528\u6237\u8fdb\u7a0b\u6709\u5173\u3002<\/p>\n
      \"image-20221107105130680\"<\/div><\/p>\n
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