结合fork.c文件分析进程创建的过程
本文为作业任务，只做浅显的分析，为大家提供一个分析的思路，很多细节都没有展示。如果想要更详细的分析请去搜索相关函数代码，博客园内有许多有用的信息供大家学习。
int nr_threads; int nr_running; int max_threads; unsigned long total_forks; /* Handle normal Linux uptimes. */ int last_pid; struct task_struct *pidhash[PIDHASH_SZ];
文件开头定义了线程数量，进程数量，最大线程数，创建的进程总个数，最新的pid号以及存放pid号的哈希表。
void add_wait_queue(wait_queue_head_t *q, wait_queue_t * wait) { unsigned long flags; wq_write_lock_irqsave(&q->lock, flags); wait->flags = 0; __add_wait_queue(q, wait); wq_write_unlock_irqrestore(&q->lock, flags); } void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t * wait) { unsigned long flags; wq_write_lock_irqsave(&q->lock, flags); wait->flags = WQ_FLAG_EXCLUSIVE; __add_wait_queue_tail(q, wait); wq_write_unlock_irqrestore(&q->lock, flags); } void remove_wait_queue(wait_queue_head_t *q, wait_queue_t * wait) { unsigned long flags; wq_write_lock_irqsave(&q->lock, flags); __remove_wait_queue(q, wait); wq_write_unlock_irqrestore(&q->lock, flags); }
这部分代码与进程的等待队列有关。Linux内核的等待队列是以双循环链表为基础数据结构，与进程调度机制紧密结合，能够用于实现核心的异步事件通知机制。等待队列在include/linux/wait.h中，这是一个通过list_head连接的典型双循环链表，在这个链表中，有两种数据结构：等待队列头（wait_queue_head_t）和等待队列项（wait_queue_t）。等待队列头和等待队列项中都包含一个list_head类型的域作为"连接件"。由于我们只需要对队列进行添加和删除操作，并不会修改其中的对象（等待队列项），因此，我们只需要提供一把保护整个基础设施和所有对象的锁，这把锁保存在等待队列头中，为wq_lock_t类型。在实现中，可以支持读写锁（rwlock）或自旋锁（spinlock）两种类型，通过一个宏定义来切换。如果使用读写锁，将wq_lock_t定义为rwlock_t类型；如果是自旋锁，将wq_lock_t定义为spinlock_t类型。无论哪种情况，分别相应设置wq_read_lock、wq_read_unlock、wq_read_lock_irqsave、wq_read_unlock_irqrestore、wq_write_lock_irq、wq_write_unlock、wq_write_lock_irqsave和wq_write_unlock_irqrestore等宏。在__wait_queue 中定义的WQ_FLAG_EXCLUSIVE表示节点对应的进程对临界资源具有排他性。remove_wait_queue函数用于将等待队列项wait从以q为等待队列头的等待队列中移除
void __init fork_init(unsigned long mempages) { /* * The default maximum number of threads is set to a safe * value: the thread structures can take up at most half * of memory. */ max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 2; init_task.rlim[RLIMIT_NPROC].rlim_cur = max_threads/2; init_task.rlim[RLIMIT_NPROC].rlim_max = max_threads/2; }
如注释所说，默认的最大线程数被设置为一个安全值:线程结构最多可以占用一半的内存。__init在include/linux/wait.h中，作用为将带有__init标识符的函数划分到.init.text段中，此段只在启动时做一次初始化载入。
/* Protects next_safe and last_pid. */ spinlock_t lastpid_lock = SPIN_LOCK_UNLOCKED; static int get_pid(unsigned long flags) { static int next_safe = PID_MAX; struct task_struct *p; if (flags & CLONE_PID) return current->pid; spin_lock(&lastpid_lock); if((++last_pid) & 0xffff8000) { last_pid = 300; /* Skip daemons etc. */ goto inside; } if(last_pid >= next_safe) { inside: next_safe = PID_MAX; read_lock(&tasklist_lock); repeat: for_each_task(p) { if(p->pid == last_pid || p->pgrp == last_pid || p->session == last_pid) { if(++last_pid >= next_safe) { if(last_pid & 0xffff8000) last_pid = 300; next_safe = PID_MAX; } goto repeat; } if(p->pid > last_pid && next_safe > p->pid) next_safe = p->pid; if(p->pgrp > last_pid && next_safe > p->pgrp) next_safe = p->pgrp; if(p->session > last_pid && next_safe > p->session) next_safe = p->session; } read_unlock(&tasklist_lock); } spin_unlock(&lastpid_lock); return last_pid; }
这部分代码用来给进程分配pid，对get_pid函数添加自旋锁保证函数的运行，对tasklist_lock添加读锁，确保pid数据安全。last_pid用于记录上一次分配给进程时的pid值。分配的pid一般而言是last_pid+1，如果超出进程个数的最大值（0xffff8000），那么进程pid值从300开始重新查找未用的。也就是说，一般用户进程的pid值范围[300，ffff8000]。（0~299,留给系统）。变量next_safe的含义是，在[last_pid,next_safe]之间，都是没有使用过的pid，一旦last_pid+1大于了next_safe，也就是说pid值进入了不可靠空间，有可能这个值被使用，这时需要遍历task来确认。这样遍历task找到一个没有用过的pid，同时确定next_safe，以保证next_safe到last_pid的区间中pid是空闲的，这样只要再次分配pid时，其值小于next_safe就可以直接分配，而不需要遍历task来查找空闲的pid。
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) { struct file *file; retval = -ENOMEM; if(mpnt->vm_flags & VM_DONTCOPY) continue; tmp = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL); if (!tmp) goto fail_nomem; *tmp = *mpnt; tmp->vm_flags &= ~VM_LOCKED; tmp->vm_mm = mm; mm->map_count++; tmp->vm_next = NULL; file = tmp->vm_file; if (file) { 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); 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; pprev = &tmp->vm_next; if (retval) goto fail_nomem; } retval = 0; if (mm->map_count >= AVL_MIN_MAP_COUNT) build_mmap_avl(mm); fail_nomem: flush_tlb_mm(current->mm); return retval; } spinlock_t mmlist_lock __cacheline_aligned = SPIN_LOCK_UNLOCKED; #define allocate_mm() (kmem_cache_alloc(mm_cachep, SLAB_KERNEL)) #define free_mm(mm) (kmem_cache_free(mm_cachep, (mm))) 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(); if (mm->pgd) return mm; free_mm(mm); return NULL; } /* * Allocate and initialize an mm_struct. */ struct mm_struct * mm_alloc(void) { struct mm_struct * mm; mm = allocate_mm(); if (mm) { memset(mm, 0, sizeof(*mm)); return mm_init(mm); } return NULL; } /* * Called when the last reference to the mm * is dropped: either by a lazy thread or by * mmput. Free the page directory and the mm. */ inline void __mmdrop(struct mm_struct *mm) { if (mm == &init_mm) BUG(); pgd_free(mm->pgd); destroy_context(mm); free_mm(mm); } /* * Decrement the use count and release all resources for an mm. */ void mmput(struct mm_struct *mm) { if (atomic_dec_and_lock(&mm->mm_users, &mmlist_lock)) { list_del(&mm->mmlist); spin_unlock(&mmlist_lock); exit_mmap(mm); mmdrop(mm); } } 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); } }
这部分代码为内存管理部分，代码中的注释向我们大致说明了本段代码的功能。
Linux内核通过一个被称为进程描述符的task_struct结构体来管理进程，这个结构体包含了一个进程所需的所有信息。它定义在include/linux/sched.h文件中。每一个进程都会有自己独立的mm_struct，这样每一个进程都会有自己独立的地址空间，这样才能互不干扰。在地址空间中，mmap为地址空间的内存区域（用vm_area_struct结构来表示）链表，表示起来更加方便。mm_struct的结构描述了进程的用户空间的结构，定义了用户空间的段分布：数据段，代码段，堆栈段。其中pgd_t是该进程用户空间地址映射到物理地址时使用vm_area_struct是进程用户空间已映射到物理空间的虚拟地址区间，定义在/include/linux/mm.h。mmap是该空间区块组成的链表。vm_flag是描述对虚拟区间的操作的标志。
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) return 0; if (clone_flags & CLONE_VM) { atomic_inc(&oldmm->mm_users); mm = oldmm; goto good_mm; } retval = -ENOMEM; mm = allocate_mm(); if (!mm) goto fail_nomem; /* Copy the current MM stuff.. */ memcpy(mm, oldmm, sizeof(*mm)); if (!mm_init(mm)) goto fail_nomem; down(&oldmm->mmap_sem); retval = dup_mmap(mm); 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); if (init_new_context(tsk,mm)) goto free_pt; good_mm: tsk->mm = mm; tsk->active_mm = mm; return 0; free_pt: mmput(mm); fail_nomem: return retval; } 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; } struct fs_struct *copy_fs_struct(struct fs_struct *old) { return __copy_fs_struct(old); } static inline int copy_fs(unsigned long clone_flags, struct task_struct * tsk) { if (clone_flags & CLONE_FS) { atomic_inc(¤t->fs->count); return 0; } tsk->fs = __copy_fs_struct(current->fs); if (!tsk->fs) return -1; return 0; } static int count_open_files(struct files_struct *files, int size) { int i; /* Find the last open fd */ for (i = size/(8*sizeof(long)); i > 0; ) { if (files->open_fds->fds_bits[--i]) break; } i = (i+1) * 8 * sizeof(long); return i; } 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) { atomic_inc(&oldf->count); 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; } static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk) { struct signal_struct *sig; if (clone_flags & CLONE_SIGHAND) { atomic_inc(¤t->sig->count); 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; } 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; }
父进程中在调用fork()派生新进程，实际上相当于创建了进程的一个拷贝；复制出来的子进程有自己的 task_struct结构和系统空间堆栈，但与父进程共享其他所有的资源。Linux为此提供了两个系统调用，一个是fork()，另一个是clone()。我们现在主要讨论fork()。fork()是全部复制，父进程所需的资源全部通过数据结构的复制传递给子进程，而完成这一操作的函数定义就是上方所写的代码段。调用fork时，内核会在copy_mm函数中处理子进程的mm_struct,在copy_files函数中处理拷贝父进程打开的文件的相关事宜,在copy_fs中记录进程所在文件系统的根目录和当前目录信息， copy_sighand中复制进程对信号的处理方式。
/* * Ok, this is the main fork-routine. It copies the system process * information (task[nr]) and sets up the necessary registers. It also * copies the data segment in its entirety. The "stack_start" and * "stack_top" arguments are simply passed along to the platform * specific copy_thread() routine. Most platforms ignore stack_top. * For an example that's using stack_top, see * arch/ia64/kernel/process.c. */ int do_fork(unsigned long clone_flags, unsigned long stack_start,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; p = alloc_task_struct(); if (!p) goto fork_out; *p = *current; 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; copy_flags(clone_flags, p); p->pid = get_pid(clone_flags); 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)) goto bad_fork_cleanup; if (copy_fs(clone_flags, p)) goto bad_fork_cleanup_files; if (copy_sighand(clone_flags, p)) goto bad_fork_cleanup_fs; if (copy_mm(clone_flags, p)) goto bad_fork_cleanup_sighand; retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs); 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; 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; 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); hash_pid(p); nr_threads++; write_unlock_irq(&tasklist_lock); if (p->ptrace & PT_PTRACED) send_sig(SIGSTOP, p, 1); wake_up_process(p); /* do this last */ ++total_forks; fork_out: if ((clone_flags & CLONE_VFORK) && (retval > 0)) down(&sem); return retval; 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; }
如开头注释第一句所说，这部分代码是fork.c中最主要的函数。
do_fork首先进行一些参数及权限的检查,仅允许从线程启动。之后进行内存的分配，复制父进程的task_struct。判断进程数量，将从父进程中继承的task_struct初始化，获取新的pid，分配CPU，解锁后设定运行时间。将子进程的pid放入pidhash表中，就可以唤醒子进程了。代码中间部分有设置进程判断，若发现非法进程会直接清理掉。清理函数在代码尾部定义。
/* SLAB cache for signal_struct structures (tsk->sig) */ kmem_cache_t *sigact_cachep; /* SLAB cache for files_struct structures (tsk->files) */ kmem_cache_t *files_cachep; /* SLAB cache for fs_struct structures (tsk->fs) */ kmem_cache_t *fs_cachep; /* SLAB cache for vm_area_struct structures */ kmem_cache_t *vm_area_cachep; /* SLAB cache for mm_struct structures (tsk->mm) */ kmem_cache_t *mm_cachep; void __init proc_caches_init(void) { sigact_cachep = kmem_cache_create("signal_act", sizeof(struct signal_struct), 0, SLAB_HWCACHE_ALIGN, NULL, NULL); if (!sigact_cachep) panic("Cannot create signal action SLAB cache"); files_cachep = kmem_cache_create("files_cache", sizeof(struct files_struct), 0, SLAB_HWCACHE_ALIGN, NULL, NULL); if (!files_cachep) panic("Cannot create files SLAB cache"); fs_cachep = kmem_cache_create("fs_cache", sizeof(struct fs_struct), 0, SLAB_HWCACHE_ALIGN, NULL, NULL); if (!fs_cachep) panic("Cannot create fs_struct SLAB cache"); vm_area_cachep = kmem_cache_create("vm_area_struct", sizeof(struct vm_area_struct), 0, SLAB_HWCACHE_ALIGN, NULL, NULL); if(!vm_area_cachep) panic("vma_init: Cannot alloc vm_area_struct SLAB cache"); mm_cachep = kmem_cache_create("mm_struct", sizeof(struct mm_struct), 0, SLAB_HWCACHE_ALIGN, NULL, NULL); if(!mm_cachep) panic("vma_init: Cannot alloc mm_struct SLAB cache"); }
最后这部分代码作用是处理进程的缓存，为proc文件系统创建高速缓冲。
从文件开头的宏定义，到等待队列的处理，到线程数的安全处理，到pid的分配，到进程的内存管理，到父进程复制出子进程。fork()函数中对进程的创建大致是以上步骤。主要在于copy部分对task_struct复制和复制后的初始化。
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