用户空间缺页异常pte_handle_fault()分析--(下)--写时复制【转】
转自：http://blog.csdn.net/vanbreaker/article/details/7955713版权声明：本文为博主原创文章，未经博主允许不得转载。
在pte_handle_fault()中，如果触发异常的页存在于主存中，那么该异常往往是由写了一个只读页触发的，此时需要进行COW(写时复制操作)。如当一个父进程通过fork()创建了一个子进程时，子进程将会共享父进程的页框。之后，无论是父进程还是子进程要对相应的内存进行写操作，都要进行COW，也就是为自己重新分配一个页框，并把之前的数据复制到页框中去，再写。
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[*]static inline int handle_pte_fault(struct mm_struct *mm,
[*] struct vm_area_struct *vma, unsigned long address,
[*] pte_t *pte, pmd_t *pmd, unsigned int flags)
[*]{
[*] pte_t entry;
[*] spinlock_t *ptl;
[*]
[*] entry = *pte;
[*]
[*] ...
[*] ...
[*] ...
[*] /********页在主存中的情况***********/
[*]
[*] ptl = pte_lockptr(mm, pmd);
[*] spin_lock(ptl);
[*] if (unlikely(!pte_same(*pte, entry)))
[*] goto unlock;
[*] if (flags & FAULT_FLAG_WRITE) {//异常由写访问触发
[*] if (!pte_write(entry))//而对应的页是不可写的
[*] return do_wp_page(mm, vma, address, //此时必须进行写时复制的操作
[*] pte, pmd, ptl, entry);
[*] entry = pte_mkdirty(entry);
[*] }
[*] entry = pte_mkyoung(entry);
[*] if (ptep_set_access_flags(vma, address, pte, entry, flags & FAULT_FLAG_WRITE)) {
[*] update_mmu_cache(vma, address, entry);
[*] } else {
[*] /*
[*] * This is needed only for protection faults but the arch code
[*] * is not yet telling us if this is a protection fault or not.
[*] * This still avoids useless tlb flushes for .text page faults
[*] * with threads.
[*] */
[*] if (flags & FAULT_FLAG_WRITE)
[*] flush_tlb_page(vma, address);
[*] }
[*]unlock:
[*] pte_unmap_unlock(pte, ptl);
[*] return 0;
[*]}
可以看到，hand_pte_fault()函数处理页存在于主存中的情况的关键操作都集中在do_wp_page()函数上。该函数是用来处理COW的，不过在COW之前先要做一些检查，比如说，如果对应的页只有一个进程使用，那么便可以直接修改页的权限为可读可写，而不进行COW。总之，不到不得以的情况下是不会进行COW的。
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[*]static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
[*] unsigned long address, pte_t *page_table, pmd_t *pmd,
[*] spinlock_t *ptl, pte_t orig_pte)
[*]{
[*] struct page *old_page, *new_page;
[*] pte_t entry;
[*] int reuse = 0, ret = 0;
[*] int page_mkwrite = 0;
[*] struct page *dirty_page = NULL;
[*]
[*] old_page = vm_normal_page(vma, address, orig_pte);//获取共享页
[*] if (!old_page) {//获取共享页失败
[*] /*
[*] * VM_MIXEDMAP !pfn_valid() case
[*] *
[*] * We should not cow pages in a shared writeable mapping.
[*] * Just mark the pages writable as we can't do any dirty
[*] * accounting on raw pfn maps.
[*] */
[*] /*如果vma的映射本来就是共享且可写的，则跳转至reuse直接使用orig_pte对应的页*/
[*] if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
[*] (VM_WRITE|VM_SHARED))
[*] goto reuse;
[*] /*否则跳转至gotten分配一个页*/
[*] goto gotten;
[*] }
[*]
[*] /*
[*] * Take out anonymous pages first, anonymous shared vmas are
[*] * not dirty accountable.
[*] */
[*] /*下面首先判断匿名页的情况，如果old_page是匿名页，并且只有一个进程使用它(reuse为1)，则
[*] 则直接使用该页*/
[*] if (PageAnon(old_page) && !PageKsm(old_page)) {
[*] /*这里先判断是否有其他进程竞争，修改了页表*/
[*] if (!trylock_page(old_page)) {
[*] page_cache_get(old_page);
[*] pte_unmap_unlock(page_table, ptl);
[*] lock_page(old_page);
[*] page_table = pte_offset_map_lock(mm, pmd, address,
[*] &ptl);
[*] if (!pte_same(*page_table, orig_pte)) {
[*] unlock_page(old_page);
[*] page_cache_release(old_page);
[*] goto unlock;
[*] }
[*] page_cache_release(old_page);
[*] }
[*] /*确定没有其他进程竞争，则进行reuse判断，通过reuse_swap_page()函数判断
[*] old_page的_mapcount字段是否为0，是的话则表明只有一个进程使用该匿名页*/
[*] reuse = reuse_swap_page(old_page);
[*] unlock_page(old_page);
[*] } else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
[*] (VM_WRITE|VM_SHARED))) {//如果vma的映射本来就是共享且可写的
[*] /*
[*] * Only catch write-faults on shared writable pages,
[*] * read-only shared pages can get COWed by
[*] * get_user_pages(.write=1, .force=1).
[*] */
[*] if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
[*] struct vm_fault vmf;
[*] int tmp;
[*]
[*] vmf.virtual_address = (void __user *)(address &
[*] PAGE_MASK);
[*] vmf.pgoff = old_page->index;
[*] vmf.flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
[*] vmf.page = old_page;
[*]
[*] /*
[*] * Notify the address space that the page is about to
[*] * become writable so that it can prohibit this or wait
[*] * for the page to get into an appropriate state.
[*] *
[*] * We do this without the lock held, so that it can
[*] * sleep if it needs to.
[*] */
[*] page_cache_get(old_page);//增加old_page的引用计数作为保护
[*] pte_unmap_unlock(page_table, ptl);
[*]
[*] /*这里通知即将修改页的权限*/
[*] tmp = vma->vm_ops->page_mkwrite(vma, &vmf);
[*]
[*] /*如果无法修改的话，则跳转到unwritable_page*/
[*] if (unlikely(tmp &
[*] (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) {
[*] ret = tmp;
[*] goto unwritable_page;
[*] }
[*] if (unlikely(!(tmp & VM_FAULT_LOCKED))) {
[*] lock_page(old_page);
[*] if (!old_page->mapping) {
[*] ret = 0; /* retry the fault */
[*] unlock_page(old_page);
[*] goto unwritable_page;
[*] }
[*] } else
[*] VM_BUG_ON(!PageLocked(old_page));
[*]
[*] /*
[*] * Since we dropped the lock we need to revalidate
[*] * the PTE as someone else may have changed it.If
[*] * they did, we just return, as we can count on the
[*] * MMU to tell us if they didn't also make it writable.
[*] */
[*] /*走到这里表示已经成功修改了页的权限了，这里同样重新获取页表，判断是否和之前一致*/
[*] page_table = pte_offset_map_lock(mm, pmd, address,
[*] &ptl);
[*] if (!pte_same(*page_table, orig_pte)) {
[*] unlock_page(old_page);
[*] page_cache_release(old_page);
[*] goto unlock;
[*] }
[*]
[*] page_mkwrite = 1;
[*] }
[*] dirty_page = old_page;
[*] get_page(dirty_page);
[*] reuse = 1;
[*] }
[*]
[*] if (reuse) {//reuse处理，也就是说不进行COW，可以直接在old_page上进行写操作
[*]reuse:
[*] flush_cache_page(vma, address, pte_pfn(orig_pte));
[*] entry = pte_mkyoung(orig_pte);//标记_PAGE_ACCESSED位
[*] entry = maybe_mkwrite(pte_mkdirty(entry), vma);//将页的权限修改为可读可写，并且标记为脏页
[*] if (ptep_set_access_flags(vma, address, page_table, entry,1))
[*] update_mmu_cache(vma, address, entry);
[*] ret |= VM_FAULT_WRITE;
[*] goto unlock;
[*] }
[*]
[*] /*
[*] * Ok, we need to copy. Oh, well..
[*] */
[*] /***************终于走到了不得已的一步了，下面只好进行COW了********************/
[*] page_cache_get(old_page);
[*]gotten:
[*] pte_unmap_unlock(page_table, ptl);
[*]
[*] if (unlikely(anon_vma_prepare(vma)))
[*] goto oom;
[*]
[*] if (is_zero_pfn(pte_pfn(orig_pte))) {
[*] new_page = alloc_zeroed_user_highpage_movable(vma, address);//分配一个零页面
[*] if (!new_page)
[*] goto oom;
[*] } else {
[*] new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);//分配一个非零页面
[*] if (!new_page)
[*] goto oom;
[*] cow_user_page(new_page, old_page, address, vma);//将old_page中的数据拷贝到new_page
[*] }
[*] __SetPageUptodate(new_page);
[*]
[*] /*
[*] * Don't let another task, with possibly unlocked vma,
[*] * keep the mlocked page.
[*] */
[*] if ((vma->vm_flags & VM_LOCKED) && old_page) {
[*] lock_page(old_page); /* for LRU manipulation */
[*] clear_page_mlock(old_page);
[*] unlock_page(old_page);
[*] }
[*]
[*] if (mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))
[*] goto oom_free_new;
[*]
[*] /*
[*] * Re-check the pte - we dropped the lock
[*] */
[*] page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
[*] if (likely(pte_same(*page_table, orig_pte))) {
[*] if (old_page) {
[*] if (!PageAnon(old_page)) {
[*] dec_mm_counter(mm, file_rss);
[*] inc_mm_counter(mm, anon_rss);
[*] }
[*] } else
[*] inc_mm_counter(mm, anon_rss);
[*] flush_cache_page(vma, address, pte_pfn(orig_pte));
[*] entry = mk_pte(new_page, vma->vm_page_prot);//获取new_page的pte
[*] entry = maybe_mkwrite(pte_mkdirty(entry), vma);//修改new_page的权限
[*] /*
[*] * Clear the pte entry and flush it first, before updating the
[*] * pte with the new entry. This will avoid a race condition
[*] * seen in the presence of one thread doing SMC and another
[*] * thread doing COW.
[*] */
[*] ptep_clear_flush(vma, address, page_table);
[*] page_add_new_anon_rmap(new_page, vma, address);
[*] /*
[*] * We call the notify macro here because, when using secondary
[*] * mmu page tables (such as kvm shadow page tables), we want the
[*] * new page to be mapped directly into the secondary page table.
[*] */
[*] set_pte_at_notify(mm, address, page_table, entry);
[*] update_mmu_cache(vma, address, entry);
[*] if (old_page) {
[*] /*
[*] * Only after switching the pte to the new page may
[*] * we remove the mapcount here. Otherwise another
[*] * process may come and find the rmap count decremented
[*] * before the pte is switched to the new page, and
[*] * "reuse" the old page writing into it while our pte
[*] * here still points into it and can be read by other
[*] * threads.
[*] *
[*] * The critical issue is to order this
[*] * page_remove_rmap with the ptp_clear_flush above.
[*] * Those stores are ordered by (if nothing else,)
[*] * the barrier present in the atomic_add_negative
[*] * in page_remove_rmap.
[*] *
[*] * Then the TLB flush in ptep_clear_flush ensures that
[*] * no process can access the old page before the
[*] * decremented mapcount is visible. And the old page
[*] * cannot be reused until after the decremented
[*] * mapcount is visible. So transitively, TLBs to
[*] * old page will be flushed before it can be reused.
[*] */
[*] page_remove_rmap(old_page);
[*] }
[*]
[*] /* Free the old page.. */
[*] new_page = old_page;
[*] ret |= VM_FAULT_WRITE;
[*] } else
[*] mem_cgroup_uncharge_page(new_page);
[*]
[*] if (new_page)
[*] page_cache_release(new_page);
[*] if (old_page)
[*] page_cache_release(old_page);
[*]unlock:
[*] pte_unmap_unlock(page_table, ptl);
[*] if (dirty_page) {
[*] /*
[*] * Yes, Virginia, this is actually required to prevent a race
[*] * with clear_page_dirty_for_io() from clearing the page dirty
[*] * bit after it clear all dirty ptes, but before a racing
[*] * do_wp_page installs a dirty pte.
[*] *
[*] * do_no_page is protected similarly.
[*] */
[*] if (!page_mkwrite) {
[*] wait_on_page_locked(dirty_page);
[*] set_page_dirty_balance(dirty_page, page_mkwrite);
[*] }
[*] put_page(dirty_page);
[*] if (page_mkwrite) {
[*] struct address_space *mapping = dirty_page->mapping;
[*]
[*] set_page_dirty(dirty_page);
[*] unlock_page(dirty_page);
[*] page_cache_release(dirty_page);
[*] if (mapping) {
[*] /*
[*] * Some device drivers do not set page.mapping
[*] * but still dirty their pages
[*] */
[*] balance_dirty_pages_ratelimited(mapping);
[*] }
[*] }
[*]
[*] /* file_update_time outside page_lock */
[*] if (vma->vm_file)
[*] file_update_time(vma->vm_file);
[*] }
[*] return ret;
[*]oom_free_new:
[*] page_cache_release(new_page);
[*]oom:
[*] if (old_page) {
[*] if (page_mkwrite) {
[*] unlock_page(old_page);
[*] page_cache_release(old_page);
[*] }
[*] page_cache_release(old_page);
[*] }
[*] return VM_FAULT_OOM;
[*]
[*]unwritable_page:
[*] page_cache_release(old_page);
[*] return ret;
[*]}
[*]
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