Re: How much of a mess does OpenVZ make? ;) Was: What can OpenVZ do?

[Cedric Le Goater <legoater-GANU6spQydw@public.gmane.org>]

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Ying Han wrote:
> Hi Serge:
> I made a patch based on Oren's tree recently which implement a new
> syscall clone_with_pid. I tested with checkpoint/restart process tree
> and it works as expected.
> This patch has some hack in it which i made a copy of libc's clone and
> made modifications of passing one more argument(pid number). I will
> try to clean up the code and do more testing.

ok. 2 patches would also be interesting. one for the syscall and one
for the kernel support (which might be acceptable)

> New syscall clone_with_pid
> Implement a new syscall which clone a thread with a preselected pid number.

yes this definitely needed to restart a task/thread. we maintain an ugly 
hack which stores a pid in the current task that will be used by the next 
clone() call. 

> clone_with_pid(child_func, child_stack + CHILD_STACK - 16,
> 			CLONE_WITH_PID|SIGCHLD, pid, NULL);

since you're introducing a new syscall, I don't see why you need a 
CLONE_WITH_PID flag ?

(FYI, attached is my old attempt of clone_with_pid but that's too old)

[ ... ]

> +#define DEBUG
>  #include <linux/slab.h>
>  #include <linux/init.h>
>  #include <linux/unistd.h>
> @@ -959,10 +959,19 @@ static struct task_struct *copy_process(unsigned long cl
>  	int retval;
>  	struct task_struct *p;
>  	int cgroup_callbacks_done = 0;
> +	pid_t clone_pid = stack_size;
> 
>  	if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
>  		return ERR_PTR(-EINVAL);
> 
> +	/* We only allow the clone_with_pid when a new pid namespace is
> +	 * created. FIXME: how to restrict it.
> +	 */
> +	if ((clone_flags & CLONE_NEWPID) && (clone_flags & CLONE_WITH_PID))
> +		return ERR_PTR(-EINVAL);
> +	if ((clone_flags & CLONE_WITH_PID) && (clone_pid <= 1))
> +		return ERR_PTR(-EINVAL);

I would let alloc_pid() handle the error.

>  	/*
>  	 * Thread groups must share signals as well, and detached threads
>  	 * can only be started up within the thread group.
> @@ -1135,7 +1144,10 @@ static struct task_struct *copy_process(unsigned long c
> 
>  	if (pid != &init_struct_pid) {
>  		retval = -ENOMEM;
> -		pid = alloc_pid(task_active_pid_ns(p));
> +		if (clone_flags & CLONE_WITH_PID)
> +			pid = alloc_pid(task_active_pid_ns(p), clone_pid);
> +		else
> +			pid = alloc_pid(task_active_pid_ns(p), 0);

this is overkill IMO.

[ ... ]

> -static int alloc_pidmap(struct pid_namespace *pid_ns)
> +static int alloc_pidmap(struct pid_namespace *pid_ns, pid_t pid_nr)
>  {
>  	int i, offset, max_scan, pid, last = pid_ns->last_pid;
>  	struct pidmap *map;
> 
> -	pid = last + 1;
> +	if (pid_nr)
> +		pid = pid_nr;
> +	else
> +		pid = last + 1;
>
>  	if (pid >= pid_max)
>  		pid = RESERVED_PIDS;
>  	offset = pid & BITS_PER_PAGE_MASK;
> @@ -153,9 +156,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
>  			do {
>  				if (!test_and_set_bit(offset, map->page)) {
>  					atomic_dec(&map->nr_free);
> -					pid_ns->last_pid = pid;
> +					if (!pid_nr)
> +						pid_ns->last_pid = pid;
>  					return pid;
>  				}
> +				if (pid_nr)
> +					return -1;
>  				offset = find_next_offset(map, offset);
>  				pid = mk_pid(pid_ns, map, offset);
>  			/*
> @@ -239,21 +245,25 @@ void free_pid(struct pid *pid)
>  	call_rcu(&pid->rcu, delayed_put_pid);
>  }
> 
> -struct pid *alloc_pid(struct pid_namespace *ns)
> +struct pid *alloc_pid(struct pid_namespace *ns, pid_t pid_nr)
>  {
>  	struct pid *pid;
>  	enum pid_type type;
>  	int i, nr;
>  	struct pid_namespace *tmp;
>  	struct upid *upid;
> +	int level = ns->level;
> +
> +	if (pid_nr >= pid_max)
> +		return NULL;

let alloc_pidmap() handle it ? 

> 
>  	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
>  	if (!pid)
>  		goto out;
> 
> -	tmp = ns;
> -	for (i = ns->level; i >= 0; i--) {
> -		nr = alloc_pidmap(tmp);
> +	tmp = ns->parent;
> +	for (i = level-1; i >= 0; i--) {
> +		nr = alloc_pidmap(tmp, 0);
>  		if (nr < 0)
>  			goto out_free;
> 
> @@ -262,6 +272,14 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  		tmp = tmp->parent;
>  	}
> 
> +	nr = alloc_pidmap(ns, pid_nr);
> +	if (nr < 0)
> +		goto out_free;
> +	pid->numbers[level].nr = nr;
> +	pid->numbers[level].ns = ns;
> +	if (nr == pid_nr)
> +		pr_debug("nr == pid_nr == %d\n", nr);
> +
>  	get_pid_ns(ns);
>  	pid->level = ns->level;
>  	atomic_set(&pid->count, 1);
> 
> 
> 
> 
> 
> 
> 
> 
> On Thu, Mar 12, 2009 at 2:21 PM, Serge E. Hallyn <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> wrote:
>> Quoting Greg Kurz (gkurz-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org):
>>> On Thu, 2009-03-12 at 09:53 -0500, Serge E. Hallyn wrote:
>>>> Or are you suggesting that you'll do a dummy clone of (5594,2) so that
>>>> the next clone(CLONE_NEWPID) will be expected to be (5594,3,1)?
>>>>
>>> Of course not
>> Ok - someone *did* argue that at some point I think...
>>
>>> but one should be able to tell clone() to pick a specific
>>> pid.
>> Can you explain exactly how?  I must be missing something clever.
>>
>> -serge
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org"> email-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org </a>


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Subject: [RFC] forkpid() syscall

From: Cedric Le Goater <clg-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org>

let's the user specify a pid to fork and return EBUSY if the pid is
not available.

this patch includes a alloc_pid*() cleanup on the way errors are 
returned that could be pushed to mainline independently.

usage :

    #include <sys/syscall.h>

    #define __NR_forkpid 	324

    static inline int forkpid(int pid)
    {
	  return syscall(__NR_forkpid, pid);
    }
    
caveats : 
	fork oriented, should also cover clone
	i386 only
	does not cover 64 bits clone flags


Signed-off-by: Cedric Le Goater <clg-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org>
---
 arch/i386/kernel/process.c       |   15 +++++++++++----
 arch/i386/kernel/syscall_table.S |    1 +
 include/asm-i386/unistd.h        |    3 ++-
 include/linux/pid.h              |    2 +-
 include/linux/sched.h            |    2 +-
 kernel/fork.c                    |    9 +++++----
 kernel/pid.c                     |   28 +++++++++++++++-------------
 7 files changed, 36 insertions(+), 24 deletions(-)

Index: 2.6.22/kernel/pid.c
===================================================================
--- 2.6.22.orig/kernel/pid.c
+++ 2.6.22/kernel/pid.c
@@ -96,12 +96,12 @@ static fastcall void free_pidmap(struct 
 	atomic_inc(&map->nr_free);
 }
 
-static int alloc_pidmap(struct pid_namespace *pid_ns)
+static int alloc_pidmap(struct pid_namespace *pid_ns, pid_t upid)
 {
 	int i, offset, max_scan, pid, last = pid_ns->last_pid;
 	struct pidmap *map;
 
-	pid = last + 1;
+	pid = upid ? upid : last + 1;
 	if (pid >= pid_max)
 		pid = RESERVED_PIDS;
 	offset = pid & BITS_PER_PAGE_MASK;
@@ -130,6 +130,8 @@ static int alloc_pidmap(struct pid_names
 					pid_ns->last_pid = pid;
 					return pid;
 				}
+				if (upid)
+					return -EBUSY;
 				offset = find_next_offset(map, offset);
 				pid = mk_pid(pid_ns, map, offset);
 			/*
@@ -153,7 +155,7 @@ static int alloc_pidmap(struct pid_names
 		}
 		pid = mk_pid(pid_ns, map, offset);
 	}
-	return -1;
+	return -EAGAIN;
 }
 
 static int next_pidmap(struct pid_namespace *pid_ns, int last)
@@ -203,19 +205,24 @@ fastcall void free_pid(struct pid *pid)
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
-struct pid *alloc_pid(void)
+struct pid *alloc_pid(pid_t upid)
 {
 	struct pid *pid;
 	enum pid_type type;
 	int nr = -1;
 
 	pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
-	if (!pid)
+	if (!pid) {
+		pid = ERR_PTR(-ENOMEM);
 		goto out;
+	}
 
-	nr = alloc_pidmap(current->nsproxy->pid_ns);
-	if (nr < 0)
-		goto out_free;
+	nr = alloc_pidmap(current->nsproxy->pid_ns, upid);
+	if (nr < 0) {
+		kmem_cache_free(pid_cachep, pid);
+		pid = ERR_PTR(nr);
+		goto out;
+	}
 
 	atomic_set(&pid->count, 1);
 	pid->nr = nr;
@@ -228,11 +235,6 @@ struct pid *alloc_pid(void)
 
 out:
 	return pid;
-
-out_free:
-	kmem_cache_free(pid_cachep, pid);
-	pid = NULL;
-	goto out;
 }
 
 struct pid * fastcall find_pid(int nr)
Index: 2.6.22/arch/i386/kernel/process.c
===================================================================
--- 2.6.22.orig/arch/i386/kernel/process.c
+++ 2.6.22/arch/i386/kernel/process.c
@@ -355,7 +355,7 @@ int kernel_thread(int (*fn)(void *), voi
 	regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;
 
 	/* Ok, create the new process.. */
-	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
+	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL, 0);
 }
 EXPORT_SYMBOL(kernel_thread);
 
@@ -722,9 +722,16 @@ struct task_struct fastcall * __switch_t
 	return prev_p;
 }
 
+asmlinkage int sys_forkpid(struct pt_regs regs)
+{
+	pid_t pid = regs.ebx;
+
+	return do_fork(SIGCHLD, regs.esp, &regs, 0, NULL, NULL, pid);
+}
+
 asmlinkage int sys_fork(struct pt_regs regs)
 {
-	return do_fork(SIGCHLD, regs.esp, &regs, 0, NULL, NULL);
+	return do_fork(SIGCHLD, regs.esp, &regs, 0, NULL, NULL, 0);
 }
 
 asmlinkage int sys_clone(struct pt_regs regs)
@@ -739,7 +746,7 @@ asmlinkage int sys_clone(struct pt_regs 
 	child_tidptr = (int __user *)regs.edi;
 	if (!newsp)
 		newsp = regs.esp;
-	return do_fork(clone_flags, newsp, &regs, 0, parent_tidptr, child_tidptr);
+	return do_fork(clone_flags, newsp, &regs, 0, parent_tidptr, child_tidptr, 0);
 }
 
 /*
@@ -754,7 +761,7 @@ asmlinkage int sys_clone(struct pt_regs 
  */
 asmlinkage int sys_vfork(struct pt_regs regs)
 {
-	return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0, NULL, NULL);
+	return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0, NULL, NULL, 0);
 }
 
 /*
Index: 2.6.22/arch/i386/kernel/syscall_table.S
===================================================================
--- 2.6.22.orig/arch/i386/kernel/syscall_table.S
+++ 2.6.22/arch/i386/kernel/syscall_table.S
@@ -323,3 +323,4 @@ ENTRY(sys_call_table)
 	.long sys_signalfd
 	.long sys_timerfd
 	.long sys_eventfd
+	.long sys_forkpid
Index: 2.6.22/include/asm-i386/unistd.h
===================================================================
--- 2.6.22.orig/include/asm-i386/unistd.h
+++ 2.6.22/include/asm-i386/unistd.h
@@ -329,10 +329,11 @@
 #define __NR_signalfd		321
 #define __NR_timerfd		322
 #define __NR_eventfd		323
+#define __NR_forkpid		324
 
 #ifdef __KERNEL__
 
-#define NR_syscalls 324
+#define NR_syscalls 325
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
Index: 2.6.22/kernel/fork.c
===================================================================
--- 2.6.22.orig/kernel/fork.c
+++ 2.6.22/kernel/fork.c
@@ -1358,15 +1358,16 @@ long do_fork(unsigned long clone_flags,
 	      struct pt_regs *regs,
 	      unsigned long stack_size,
 	      int __user *parent_tidptr,
-	      int __user *child_tidptr)
+	      int __user *child_tidptr,
+	      pid_t upid)
 {
 	struct task_struct *p;
 	int trace = 0;
-	struct pid *pid = alloc_pid();
+	struct pid *pid = alloc_pid(upid);
 	long nr;
 
-	if (!pid)
-		return -EAGAIN;
+	if (IS_ERR(pid))
+		return PTR_ERR(pid);
 	nr = pid->nr;
 	if (unlikely(current->ptrace)) {
 		trace = fork_traceflag (clone_flags);
Index: 2.6.22/include/linux/sched.h
===================================================================
--- 2.6.22.orig/include/linux/sched.h
+++ 2.6.22/include/linux/sched.h
@@ -1433,7 +1433,7 @@ extern int allow_signal(int);
 extern int disallow_signal(int);
 
 extern int do_execve(char *, char __user * __user *, char __user * __user *, struct pt_regs *);
-extern long do_fork(unsigned long, unsigned long, struct pt_regs *, unsigned long, int __user *, int __user *);
+extern long do_fork(unsigned long, unsigned long, struct pt_regs *, unsigned long, int __user *, int __user *, pid_t);
 struct task_struct *fork_idle(int);
 
 extern void set_task_comm(struct task_struct *tsk, char *from);
Index: 2.6.22/include/linux/pid.h
===================================================================
--- 2.6.22.orig/include/linux/pid.h
+++ 2.6.22/include/linux/pid.h
@@ -95,7 +95,7 @@ extern struct pid *FASTCALL(find_pid(int
 extern struct pid *find_get_pid(int nr);
 extern struct pid *find_ge_pid(int nr);
 
-extern struct pid *alloc_pid(void);
+extern struct pid *alloc_pid(pid_t upid);
 extern void FASTCALL(free_pid(struct pid *pid));
 
 static inline pid_t pid_nr(struct pid *pid)