用户空间缺页异常pte_handle_fault()分析--(下)--写时复制
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。
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。
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;}} elseVM_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);}} elseinc_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pteentry = 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;} elsemem_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;}