父进程fork子进程：
child = fork()
fork经过系统调用。来到了sys_fork。
asmlinkage int sys_fork(struct pt_regs regs)
{
return do_fork(SIGCHLD, regs.esp, ®s, 0);
}
int do_fork(unsigned long clone_flags, unsigned long stack_start, //stack_start为用户空间堆栈指针
struct pt_regs *regs, unsigned long stack_size)
{
int retval = -ENOMEM;
struct task_struct *p;
DECLARE_MUTEX_LOCKED(sem);
if (clone_flags & CLONE_PID) {
/* This is only allowed from the boot up thread */
if (current->pid)
return -EPERM;
}
current->vfork_sem = &sem;//假设clone_flags中CLONE_VFORK位置1，这个信号量用于up(&sem)。使父进程唤醒
p = alloc_task_struct();//为子进程分配两个连续的物理页面，低端用作子进程的task_struct结构，高端则用作其系统空间堆栈
if (!p)
goto fork_out;
*p = *current;//父进程的整个task_struct就被拷贝到了子进程的数据结构
retval = -EAGAIN;
if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur)
goto bad_fork_free;
atomic_inc(&p->user->__count);
atomic_inc(&p->user->processes);
/*
* Counter increases are protected by
* the kernel lock so nr_threads can't
* increase under us (but it may decrease).
*/
if (nr_threads >= max_threads)
goto bad_fork_cleanup_count;
get_exec_domain(p->exec_domain);
if (p->binfmt && p->binfmt->module)
__MOD_INC_USE_COUNT(p->binfmt->module);
p->did_exec = 0;
p->swappable = 0;
p->state = TASK_UNINTERRUPTIBLE;//不可中断等待状态
copy_flags(clone_flags, p);//将參数clone_flags中的标志位略加补充和变换，然后写入p->flags
p->pid = get_pid(clone_flags);//获取进程pid
p->run_list.next = NULL;
p->run_list.prev = NULL;
if ((clone_flags & CLONE_VFORK) || !(clone_flags & CLONE_PARENT)) {
p->p_opptr = current;
if (!(p->ptrace & PT_PTRACED))
p->p_pptr = current;
}
p->p_cptr = NULL;
init_waitqueue_head(&p->wait_chldexit);
p->vfork_sem = NULL;
spin_lock_init(&p->alloc_lock);
p->sigpending = 0;
init_sigpending(&p->pending);
p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
init_timer(&p->real_timer);
p->real_timer.data = (unsigned long) p;
p->leader = 0; /* session leadership doesn't inherit */
p->tty_old_pgrp = 0;
p->times.tms_utime = p->times.tms_stime = 0;
p->times.tms_cutime = p->times.tms_cstime = 0;
#ifdef CONFIG_SMP
{
int i;
p->has_cpu = 0;
p->processor = current->processor;
/* ?? should we just memset this ?? */
for(i = 0; i < smp_num_cpus; i++)
p->per_cpu_utime[i] = p->per_cpu_stime[i] = 0;
spin_lock_init(&p->sigmask_lock);
}
#endif
p->lock_depth = -1; /* -1 = no lock */
p->start_time = jiffies;
retval = -ENOMEM;
/* copy all the process information */
if (copy_files(clone_flags, p))//有条件地复制已打开文件的控制结构files_struct，这样的复制仅仅有在clone_flags中CLONE_FILES标志位为0时才真正进行。否则就仅仅是共享父进程的指针
goto bad_fork_cleanup;
if (copy_fs(clone_flags, p))//有条件地拷贝文件系统相关结构files_structfs_struct，这样的复制仅仅有在clone_flags中CLONE_FS标志位为0时才真正进行。否则就仅仅是共享父进程的指针
goto bad_fork_cleanup_files;
if (copy_sighand(clone_flags, p))//有条件地复制信号处理相关结构signal_struct。这样的复制仅仅有在clone_flags中CLONE_SIGHAND标志位为0时才真正进行。否则就仅仅是共享父进程的指针
goto bad_fork_cleanup_fs;
if (copy_mm(clone_flags, p))//有条件地复制内存管理相关结构mm_struct及其下属的vm_area_struct，这样的复制仅仅有在clone_flags中CLONE_VM标志位为0时才真正进行。否则就仅仅是共享父进程的指针
goto bad_fork_cleanup_sighand;
retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);//实际上却仅仅是复制父进程的系统空间堆栈
if (retval)
goto bad_fork_cleanup_sighand;
p->semundo = NULL;
/* Our parent execution domain becomes current domain
These must match for thread signalling to apply */
p->parent_exec_id = p->self_exec_id;
/* ok, now we should be set up.. */
p->swappable = 1;
p->exit_signal = clone_flags & CSIGNAL;//本进程运行exit()时应向父进程发出的信号,CSIGNAL
p->pdeath_signal = 0;
/*
* "share" dynamic priority between parent and child, thus the
* total amount of dynamic priorities in the system doesnt change,
* more scheduling fairness. This is only important in the first
* timeslice, on the long run the scheduling behaviour is unchanged.
*/
p->counter = (current->counter + 1) >> 1;
current->counter >>= 1;//task_struct结构中counter字段的值就是进程的运行时间配额，这里将父进程的时间配额分成两半，让父、子进程各有原值的一半。
if (!current->counter)
current->need_resched = 1;
/*
* Ok, add it to the run-queues and make it
* visible to the rest of the system.
*
* Let it rip!
*/
retval = p->pid;
p->tgid = retval;
INIT_LIST_HEAD(&p->thread_group);
write_lock_irq(&tasklist_lock);
if (clone_flags & CLONE_THREAD) {
p->tgid = current->tgid;
list_add(&p->thread_group, ¤t->thread_group);
}
SET_LINKS(p);//将子进程的task_struct结构链入内核的进程队列
hash_pid(p);//将其链入按其pid计算得的杂凑队列
nr_threads++;//进程数加1
write_unlock_irq(&tasklist_lock);
if (p->ptrace & PT_PTRACED)
send_sig(SIGSTOP, p, 1);
wake_up_process(p); //将子进程"唤醒"，也就是将其挂入可运行进程队列等待调用
++total_forks;
fork_out:
if ((clone_flags & CLONE_VFORK) && (retval > 0))//假设clone_flags中CLONE_VFORK位置1
down(&sem);//让父进程在一个信号量上运行一次down()操作。以达到扣留父进程的目的
return retval;//返回p->pid，也就是子进程的pid
bad_fork_cleanup_sighand:
exit_sighand(p);
bad_fork_cleanup_fs:
exit_fs(p); /* blocking */
bad_fork_cleanup_files:
exit_files(p); /* blocking */
bad_fork_cleanup:
put_exec_domain(p->exec_domain);
if (p->binfmt && p->binfmt->module)
__MOD_DEC_USE_COUNT(p->binfmt->module);
bad_fork_cleanup_count:
atomic_dec(&p->user->processes);
free_uid(p->user);
bad_fork_free:
free_task_struct(p);
goto fork_out;
}
当中regs对父进程系统堆栈的指针，stack_start为用户空间堆栈指针。
alloc_task_struct为子进程分配两个连续的物理页面，低端用作子进程的task_struct结构，高端则用作其系统空间堆栈，代码例如以下：
#define alloc_task_struct() ((struct task_struct *) __get_free_pages(GFP_KERNEL,1)
copy_flags，将參数clone_flags中的标志位略加补充和变换，然后写入p->flags。
static inline void copy_flags(unsigned long clone_flags, struct task_struct *p)
{
unsigned long new_flags = p->flags;
new_flags &= ~(PF_SUPERPRIV | PF_USEDFPU | PF_VFORK);
new_flags |= PF_FORKNOEXEC;
if (!(clone_flags & CLONE_PTRACE))
p->ptrace = 0;
if (clone_flags & CLONE_VFORK)
new_flags |= PF_VFORK;
p->flags = new_flags;
}
对于fork来说，clone_flags为SIGCHLD，copy_files，copy_fs，copy_sighand，copy_mm都是要真正复制。
copy_files。代码例如以下：
static int copy_files(unsigned long clone_flags, struct task_struct * tsk)
{
struct files_struct *oldf, *newf;
struct file **old_fds, **new_fds;
int open_files, nfds, size, i, error = 0;
/*
* A background process may not have any files ...
*/
oldf = current->files;
if (!oldf)
goto out;
if (clone_flags & CLONE_FILES) {//clone_flags中CLONE_FILES标志位为1
atomic_inc(&oldf->count);//仅仅是添加计数
goto out;
}
tsk->files = NULL;
error = -ENOMEM;
newf = kmem_cache_alloc(files_cachep, SLAB_KERNEL);
if (!newf)
goto out;
atomic_set(&newf->count, 1);
newf->file_lock = RW_LOCK_UNLOCKED;
newf->next_fd = 0;
newf->max_fds = NR_OPEN_DEFAULT;
newf->max_fdset = __FD_SETSIZE;
newf->close_on_exec = &newf->close_on_exec_init;
newf->open_fds = &newf->open_fds_init;
newf->fd = &newf->fd_array[0];
/* We don't yet have the oldf readlock, but even if the old
fdset gets grown now, we'll only copy up to "size" fds */
size = oldf->max_fdset;
if (size > __FD_SETSIZE) {
newf->max_fdset = 0;
write_lock(&newf->file_lock);
error = expand_fdset(newf, size);
write_unlock(&newf->file_lock);
if (error)
goto out_release;
}
read_lock(&oldf->file_lock);
open_files = count_open_files(oldf, size);
/*
* Check whether we need to allocate a larger fd array.
* Note: we're not a clone task, so the open count won't
* change.
*/
nfds = NR_OPEN_DEFAULT;
if (open_files > nfds) {
read_unlock(&oldf->file_lock);
newf->max_fds = 0;
write_lock(&newf->file_lock);
error = expand_fd_array(newf, open_files);
write_unlock(&newf->file_lock);
if (error)
goto out_release;
nfds = newf->max_fds;
read_lock(&oldf->file_lock);
}
old_fds = oldf->fd;
new_fds = newf->fd;
memcpy(newf->open_fds->fds_bits, oldf->open_fds->fds_bits, open_files/8);
memcpy(newf->close_on_exec->fds_bits, oldf->close_on_exec->fds_bits, open_files/8);
for (i = open_files; i != 0; i--) {
struct file *f = *old_fds++;
if (f)
get_file(f);
*new_fds++ = f;
}
read_unlock(&oldf->file_lock);
/* compute the remainder to be cleared */
size = (newf->max_fds - open_files) * sizeof(struct file *);
/* This is long word aligned thus could use a optimized version */
memset(new_fds, 0, size);
if (newf->max_fdset > open_files) {
int left = (newf->max_fdset-open_files)/8;
int start = open_files / (8 * sizeof(unsigned long));
memset(&newf->open_fds->fds_bits[start], 0, left);
memset(&newf->close_on_exec->fds_bits[start], 0, left);
}
tsk->files = newf;
error = 0;
out:
return error;
out_release:
free_fdset (newf->close_on_exec, newf->max_fdset);
free_fdset (newf->open_fds, newf->max_fdset);
kmem_cache_free(files_cachep, newf);
goto out;
}
待我们学习了文件系统后再细致分析。
copy_fs。代码例如以下：
static inline int copy_fs(unsigned long clone_flags, struct task_struct * tsk)
{
if (clone_flags & CLONE_FS) {//clone_flags中CLONE_FS标志位为1
atomic_inc(current->fs->count);//仅仅是添加计数
return 0;
}
tsk->fs = __copy_fs_struct(current->fs);
if (!tsk->fs)
return -1;
return 0;
}
static inline struct fs_struct *__copy_fs_struct(struct fs_struct *old)
{
struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);
/* We don't need to lock fs - think why ;-) */
if (fs) {
atomic_set(&fs->count, 1);
fs->lock = RW_LOCK_UNLOCKED;
fs->umask = old->umask;
read_lock(&old->lock);
fs->rootmnt = mntget(old->rootmnt);
fs->root = dget(old->root);
fs->pwdmnt = mntget(old->pwdmnt);
fs->pwd = dget(old->pwd);
if (old->altroot) {
fs->altrootmnt = mntget(old->altrootmnt);
fs->altroot = dget(old->altroot);
} else {
fs->altrootmnt = NULL;
fs->altroot = NULL;
}
read_unlock(&old->lock);
}
return fs;
}
我们看到，在这里要复制的是fs_struct数据结构，而不复制更深层的数据结构。对于更深层的数据结构通过mntget()和dget()递增响应数据结构中共享计数。
copy_sighand。代码例如以下：
static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)
{
struct signal_struct *sig;
if (clone_flags & CLONE_SIGHAND) {//假设clone_flags中CLONE_SIGHAND标志位为1
atomic_inc(current->sig->count);//添加计数
return 0;
}
sig = kmem_cache_alloc(sigact_cachep, GFP_KERNEL);
tsk->sig = sig;
if (!sig)
return -1;
spin_lock_init(&sig->siglock);
atomic_set(&sig->count, 1);
memcpy(tsk->sig->action, current->sig->action, sizeof(tsk->sig->action));
return 0;
}
struct signal_struct {
atomic_t count;
struct k_sigaction action[_NSIG];
spinlock_t siglock;
};copy_mm，代码例如以下：
static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)
{
struct mm_struct * mm, *oldmm;
int retval;
tsk->min_flt = tsk->maj_flt = 0;
tsk->cmin_flt = tsk->cmaj_flt = 0;
tsk->nswap = tsk->cnswap = 0;
tsk->mm = NULL;
tsk->active_mm = NULL;
/*
* Are we cloning a kernel thread?
*
* We need to steal a active VM for that..
*/
oldmm = current->mm;
if (!oldmm)//假设是内核线程，那么oldmm为null，直接返回
return 0;
if (clone_flags & CLONE_VM) {//假设clone_flags中CLONE_VM标志位为1
atomic_inc(&oldmm->mm_users);//添加mm_users计数
mm = oldmm;
goto good_mm;
}
retval = -ENOMEM;//clone_flags中CLONE_VM标志位为0
mm = allocate_mm();//分配mm_struct
if (!mm)
goto fail_nomem;
/* Copy the current MM stuff.. */
memcpy(mm, oldmm, sizeof(*mm));
if (!mm_init(mm))//初始化mm_struct
goto fail_nomem;
down(&oldmm->mmap_sem);
retval = dup_mmap(mm);//vm_area_struct数据结构和页面映射表的复制
up(&oldmm->mmap_sem);
/*
* Add it to the mmlist after the parent.
*
* Doing it this way means that we can order
* the list, and fork() won't mess up the
* ordering significantly.
*/
spin_lock(&mmlist_lock);
list_add(&mm->mmlist, &oldmm->mmlist);
spin_unlock(&mmlist_lock);
if (retval)
goto free_pt;
/*
* child gets a private LDT (if there was an LDT in the parent)
*/
copy_segments(tsk, mm);//对ldt来说。我们不关心
if (init_new_context(tsk,mm))//空语句
goto free_pt;
good_mm:
tsk->mm = mm;
tsk->active_mm = mm;
return 0;
free_pt:
mmput(mm);
fail_nomem:
return retval;
}
显然，对mm_struct的复制也仅仅是在clone_flags中CLONE_VM标志位为0时才真正进行，否则就仅仅是通过已经复制的指针共享父进程的用户空间。
对mm_struct的复制就不仅仅是局限于这个数据结构本身了，也包含了对更深层数据结构的复制。
当中最重要的是vm_area_struct数据结构和页面映射表的复制，这是由dup_mmap()复制的。
allocate_mm，分配mm_struct。代码例如以下：
#define allocate_mm() (kmem_cache_alloc(mm_cachep, SLAB_KERNEL))
mm_init。初始化mm_struct。代码例如以下：
static struct mm_struct * mm_init(struct mm_struct * mm)
{
atomic_set(&mm->mm_users, 1);
atomic_set(&mm->mm_count, 1);
init_MUTEX(&mm->mmap_sem);
mm->page_table_lock = SPIN_LOCK_UNLOCKED;
mm->pgd = pgd_alloc();//指向新分配的页文件夹表
if (mm->pgd)
return mm;
free_mm(mm);
return NULL;
}
dup_mmap是vm_area_struct数据结构和页面映射表的复制。代码例如以下：
static inline int dup_mmap(struct mm_struct * mm)
{
struct vm_area_struct * mpnt, *tmp, **pprev;
int retval;
flush_cache_mm(current->mm);
mm->locked_vm = 0;
mm->mmap = NULL;
mm->mmap_avl = NULL;
mm->mmap_cache = NULL;
mm->map_count = 0;
mm->cpu_vm_mask = 0;
mm->swap_cnt = 0;
mm->swap_address = 0;
pprev = &mm->mmap;
for (mpnt = current->mm->mmap ; mpnt ; mpnt = mpnt->vm_next) {//对于父进程的全部虚拟空间进行轮询
struct file *file;
retval = -ENOMEM;
if(mpnt->vm_flags & VM_DONTCOPY)
continue;
tmp = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);//分配子进程的vm_struct
if (!tmp)
goto fail_nomem;
*tmp = *mpnt;//父进程的vm_struct复制给子进程vm_struct
tmp->vm_flags &= ~VM_LOCKED;
tmp->vm_mm = mm;
mm->map_count++;//虚拟空间数加1
tmp->vm_next = NULL;
file = tmp->vm_file;
if (file) {//假设为null
struct inode *inode = file->f_dentry->d_inode;
get_file(file);
if (tmp->vm_flags & VM_DENYWRITE)
atomic_dec(&inode->i_writecount);
/* insert tmp into the share list, just after mpnt */
spin_lock(&inode->i_mapping->i_shared_lock);
if((tmp->vm_next_share = mpnt->vm_next_share) != NULL)
mpnt->vm_next_share->vm_pprev_share =
&tmp->vm_next_share;
mpnt->vm_next_share = tmp;
tmp->vm_pprev_share = &mpnt->vm_next_share;
spin_unlock(&inode->i_mapping->i_shared_lock);
}
/* Copy the pages, but defer checking for errors */
retval = copy_page_range(mm, current->mm, tmp);//复制虚拟空间对应的页文件夹表项和页表项
if (!retval && tmp->vm_ops && tmp->vm_ops->open)
tmp->vm_ops->open(tmp);
/*
* Link in the new vma even if an error occurred,
* so that exit_mmap() can clean up the mess.
*/
*pprev = tmp;//下一个虚拟空间
pprev = &tmp->vm_next;
if (retval)
goto fail_nomem;
}
retval = 0;
if (mm->map_count >= AVL_MIN_MAP_COUNT)//当虚拟空间数大于AVL_MIN_MAP_COUNT
build_mmap_avl(mm);//形成avl树，方便查找
fail_nomem:
flush_tlb_mm(current->mm);
return retval;
}
copy_page_range。代码例如以下：
int copy_page_range(struct mm_struct *dst, struct mm_struct *src,
struct vm_area_struct *vma)
{
pgd_t * src_pgd, * dst_pgd;
unsigned long address = vma->vm_start;
unsigned long end = vma->vm_end;
unsigned long cow = (vma->vm_flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;//可写，而又不是共享
src_pgd = pgd_offset(src, address)-1;
dst_pgd = pgd_offset(dst, address)-1;
for (;;) {
pmd_t * src_pmd, * dst_pmd;
src_pgd++; dst_pgd++;
/* copy_pmd_range */
if (pgd_none(*src_pgd))
goto skip_copy_pmd_range;
if (pgd_bad(*src_pgd)) {
pgd_ERROR(*src_pgd);
pgd_clear(src_pgd);
skip_copy_pmd_range: address = (address + PGDIR_SIZE) & PGDIR_MASK;
if (!address || (address >= end))
goto out;
continue;
}
if (pgd_none(*dst_pgd)) {
if (!pmd_alloc(dst_pgd, 0))
goto nomem;
}
src_pmd = pmd_offset(src_pgd, address);
dst_pmd = pmd_offset(dst_pgd, address);
do {
pte_t * src_pte, * dst_pte;
/* copy_pte_range */
if (pmd_none(*src_pmd))
goto skip_copy_pte_range;
if (pmd_bad(*src_pmd)) {
pmd_ERROR(*src_pmd);
pmd_clear(src_pmd);
skip_copy_pte_range: address = (address + PMD_SIZE) & PMD_MASK;
if (address >= end)
goto out;
goto cont_copy_pmd_range;
}
if (pmd_none(*dst_pmd)) {
if (!pte_alloc(dst_pmd, 0))
goto nomem;
}
src_pte = pte_offset(src_pmd, address);
dst_pte = pte_offset(dst_pmd, address);
do {
pte_t pte = *src_pte;
struct page *ptepage;
/* copy_one_pte */
if (pte_none(pte)) //第一种情况
goto cont_copy_pte_range_noset;
if (!pte_present(pte)) { //另外一种情况
swap_duplicate(pte_to_swp_entry(pte));
goto cont_copy_pte_range;
}
ptepage = pte_page(pte);//得到页表项所指的页面
if ((!VALID_PAGE(ptepage)) || //第三种情况
PageReserved(ptepage))
goto cont_copy_pte_range;
/* If it's a COW mapping, write protect it both in the parent and the child */
if (cow) {//第四种情况
ptep_set_wrprotect(src_pte);//改成仅仅读
pte = *src_pte;
}
/* If it's a shared mapping, mark it clean in the child */
if (vma->vm_flags & VM_SHARED)
pte = pte_mkclean(pte);
pte = pte_mkold(pte);
get_page(ptepage);//添加页面使用计数
//cow为0时，仅仅读页面。第五种情况
cont_copy_pte_range: set_pte(dst_pte, pte);//将此表项拷贝到子进程的页表项
cont_copy_pte_range_noset: address += PAGE_SIZE;
if (address >= end)
goto out;
src_pte++;
dst_pte++;
} while ((unsigned long)src_pte & PTE_TABLE_MASK);
cont_copy_pmd_range: src_pmd++;
dst_pmd++;
} while ((unsigned long)src_pmd & PMD_TABLE_MASK);
}
out:
return 0;
nomem:
return -ENOMEM;
}
开头是对页文件夹表项的循环，中间是对中间文件夹项的循环。最后是对页表项的循环，我们把注意力放在最后一层循环。也就是对页表项的循环。
循环中检查父进程一个页表中的每个表项，依据表项的内容决定具体的操作。
而表项的内容，则无非是以下这么一些可能：
1、表项的内容为全0。所以pte_none()返回1。说明该页面的映射尚未建立，或者说是个“空洞”，因此不须要做不论什么事。
2、表项的最低位，即_PAGE_PRESENT标志位为0，所以pte_present返回1。
说明映射已建立，可是该页面眼下不在内存中，已经被调出到交换设备上。此时表项的内容指向"盘面页面"的地点，而如今该盘上页面多了一个"用户"。所以要通过swap_duplicate()递增它的共享计数。就转到cont_copy_pte_range将此表项拷贝到子进程的页表项。
3、映射已建立。可是物理页面不是一个有效的内存页面。所以VALID_PAGE()返回0。
读者能够回想一下。我们曾经讲过有些物理页面在外设接口卡上，对应的地址为“总线地址”。而并非内存页面。
这样的页面，就转到cont_copy_pte_range将此表项拷贝到子进程的页表项。
4、须要从父进程复制的可写页面。
本来，此时应该分配一个空暇的内存页面。再从父进程的页面把内容复制过来，并为之建立映射。
显然，这个代价是不小的。然后，对这么辛辛苦苦复制下来的页面，子进程是否一定会用呢？特别是会有写訪问么？假设仅仅是读訪问。则仅仅要父进程从此不再写这个页面。就全然能够通过复制指针来共享这个页面，那不知要省事多少了。所以，Linux内核採用了一种称为"copy on write"的技术，先通过复制页表项临时共享这个页面。到子进程真的要写着个页面时再次分配页面和复制。
变量cow是"copy on write"的缩写。可写。而又不是共享。
实际上。对于绝大多数的可写虚拟区间，cow都是1。在通过复制页表项临时共享一个页表项时要做两件重要的事情，首先将父进程的页表项改成写保护(仅仅读)。然后把已经改成写保护的表项设置到子进程的页表项。
这样一来，响应的页面在两个进程中都变成"仅仅读"了。当无论是父进程或是子进程企图写入该页面时，都会引起一次页面异常。而页面异常处理程序对此的反应则是另行分配一个物理页面。并把内容真正地拷贝到新的物理页面中，让父、子进程各自拥有自己的物理页面，然后将两个页表项中对应的表项改成可写。可是copy_on_write仅仅有在父、子进程各自拥有自己的页表时才干实现。当CLONE_VM标志位为1时。由于父、子进程通过指针共享用户空间，copy_on_write就用不上了。
此时，父、子进程是在真正的意义上共享用户空间。父进程写入其用户空间的内容同一时候也“写入”子进程的用户空间。
5、父进程的仅仅读页面。这样的页面本来就不须要复制。因而能够复制页表项共享物理页面。
返回到do_fork。继续运行copy_thread。代码例如以下：
int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
unsigned long unused,
struct task_struct * p, struct pt_regs * regs)
{
struct pt_regs * childregs;
childregs = ((struct pt_regs *) (THREAD_SIZE + (unsigned long) p)) - 1;//指向了子进程系统空间堆栈中的pt_regs结构
struct_cpy(childregs, regs);//把当前进程系统空间堆栈中的pt_regs结构复制过去
childregs->eax = 0;//子进程系统空间堆栈中的pt_regs结构eax置成0
childregs->esp = esp;//子进程系统空间堆栈中的pt_regs结构esp置成这里的參数esp，在fork中，则来自调用do_fork()前夕的regs.esp,所以实际上并没有改变
p->thread.esp = (unsigned long) childregs;//子进程系统空间堆栈中pt_regs结构的起始地址
p->thread.esp0 = (unsigned long) (childregs+1);//指向子进程的系统空间堆栈的顶端
p->thread.eip = (unsigned long) ret_from_fork;
savesegment(fs,p->thread.fs);
savesegment(gs,p->thread.gs);
unlazy_fpu(current);
struct_cpy(&p->thread.i387, ¤t->thread.i387);
return 0;
}
最后形成例如以下图：
二、clone和vfork
clone的用户态接口是：int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg)。
我们看下这clone、fork、vfork几个系统调用的差别：
asmlinkage int sys_fork(struct pt_regs regs)
{
return do_fork(SIGCHLD, regs.esp, ®s, 0);
}
asmlinkage int sys_clone(struct pt_regs regs)
{
unsigned long clone_flags;
unsigned long newsp;
clone_flags = regs.ebx;//就是用户态的flags
newsp = regs.ecx;//就是用户态的child_stack
if (!newsp)
newsp = regs.esp;
return do_fork(clone_flags, newsp, ®s, 0);
}
/*
* This is trivial, and on the face of it looks like it
* could equally well be done in user mode.
*
* Not so, for quite unobvious reasons - register pressure.
* In user mode vfork() cannot have a stack frame, and if
* done by calling the "clone()" system call directly, you
* do not have enough call-clobbered registers to hold all
* the information you need.
*/
asmlinkage int sys_vfork(struct pt_regs regs)
{
return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, ®s, 0);//主要差别是有两个标志位CLONE_VFORK，CLONE_VM
}
假设全然没实用户空间，就称为"内核线程"；而假设共享用户空间则就是为”用户线程“。
那么vfork出来的是用户线程，共享用户空间。copy_mm中代码例如以下：
if (clone_flags & CLONE_VM) {//假设clone_flags中CLONE_VM标志位为1
atomic_inc(&oldmm->mm_users);//添加mm_users计数
mm = oldmm;
goto good_mm;
}
vfork和fork另一个差别就是CLONE_VFORK标志位，体如今代码中，do_fork的最后：
fork_out:
if ((clone_flags & CLONE_VFORK) && (retval > 0))//假设clone_flags中CLONE_VFORK位置1
down(&sem);//让父进程在一个信号量上运行一次down()操作，以达到扣留父进程的目的
return retval;
当调用do_fork的參数中CLONE_VFORK标志位为1时，一定要保证让子进程先运行。一直到子进程通过系统调用execve运行一个新的可运行程序或者通过系统调用exit()退出系统时，才干够恢复父进程的运行。为什么呢？在创建子进程时，假设CLONE_VM为1，仅仅是简单地复制父进程的task_struct结构中指向其mm_struct结构的指针来共享。
此时。父、子进程是在真正的意义上共享用户空间，父进程写入其用户空间的内容同一时候也“写入”子进程的用户空间。绝不能让两个进程都回到用户空间并发地运行；否则，必定是两个进程终于都乱来一气后者因非法越界訪问而死亡。解决的办法仅仅能是”扣留“当中一个进程，而仅仅让一个进程回到用户空间，直到两个进程不再共享它们的用户空间后者当中一个进程消亡为至。
所以才有了上面的操作。让父进程在一个信号量上运行一次down()操作，以达到扣留父进程的目的。
那么谁来运行up操作呢？
子进程在通过execve运行一个新的可运行程序时会做这件事，此外，子进程在通过exit()退出系统时也会做这件事。
代码例如以下：
void mm_release(void)
{
struct task_struct *tsk = current;
/* notify parent sleeping on vfork() */
if (tsk->flags & PF_VFORK) {
tsk->flags &= ~PF_VFORK;
up(tsk->p_opptr->vfork_sem);
}
}
三、内核线程
int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
{
long retval, d0;
__asm__ __volatile__(
"movl %%esp,%%esi\n\t" //系统调用前的堆栈指针赋值给esi
"int $0x80\n\t"
"cmpl %%esp,%%esi\n\t" //系统调用后的堆栈指针和系统调用前的堆栈指针相比，假设不同就是子进程，假设同样就是父进程
"je 1f\n\t" //跳到父进程
"movl %4,%%eax\n\t"//把參数arg压入堆栈，作为參数
"pushl %%eax\n\t"
"call *%5\n\t" //call fn
"movl %3,%0\n\t" //eax为_NR_exit
"int $0x80\n" //运行exit系统调用
"1:\t"
:"=&a" (retval), "=&S" (d0)
:"0" (__NR_clone), "i" (__NR_exit),//eax为_NR_clone
"r" (arg), "r" (fn),
"b" (flags | CLONE_VM)//ebx为flags | CLONE_VM
: "memory");
return retval;
}
刚開始eax为_NR_clone。ebx为flags | CLONE_VM，然后调用int 0x80系统调用。那么就进入了sys_clone，代码例如以下：
asmlinkage int sys_clone(struct pt_regs regs)
{
unsigned long clone_flags;
unsigned long newsp;
clone_flags = regs.ebx;//就是用户态的flags | CLONE_VM
newsp = regs.ecx;//newsp为null
if (!newsp)
newsp = regs.esp;
return do_fork(clone_flags, newsp, ®s, 0);
}
那么kernel_thread出来的是内核线程，mm指针为null，copy_mm中代码例如以下：
oldmm = current->mm;
if (!oldmm)//假设是内核线程，那么oldmm为null，直接返回
return 0;
最后附上，全部标志位的作用：
#define CSIGNAL 0x000000ff /* signal mask to be sent at exit */
#define CLONE_VM 0x00000100 /* set if VM shared between processes */
#define CLONE_FS 0x00000200 /* set if fs info shared between processes */
#define CLONE_FILES 0x00000400 /* set if open files shared between processes */
#define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */
#define CLONE_PID 0x00001000 /* set if pid shared */
#define CLONE_PTRACE 0x00002000 /* set if we want to let tracing continue on the child too */
#define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_PARENT 0x00008000 /* set if we want to have the same parent as the cloner */
#define CLONE_THREAD 0x00010000 /* Same thread group? */
#define CLONE_SIGNAL (CLONE_SIGHAND | CLONE_THREAD)
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