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

On Thu, Feb 26, 2009 at 06:57:55PM +0300, Alexey Dobriyan wrote:

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On Thu, Feb 12, 2009 at 03:04:05PM -0800, Dave Hansen wrote:

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dave@nimitz:~/kernels/linux-2.6-openvz$ git diff v2.6.27.10... kernel/cpt/ | diffstat 

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 47 files changed, 20702 insertions(+)

One important thing that leaves out is the interaction that this code
has with the rest of the kernel.  That's critically important when
considering long-term maintenance, and I'd be curious how the OpenVZ
folks view it. 

OpenVZ as-is in some cases wants some functions to be made global
(and if C/R code will be modular, exported). Or probably several
iterators added.

But it's negligible amount of changes compared to main code.

Here is what C/R code wants from pid allocator.

With the introduction of hierarchical PID namespaces, struct pid can
have not one but many numbers -- tuple (pid_0, pid_1, ..., pid_N),
where pid_i is pid number in pid_ns which has level i.

Now root pid_ns of container has level n -- numbers from level n to N
inclusively should be dumped and restored.

During struct pid creation first n-1 numbers can be anything, because the're
outside of pid_ns, but the rest should be the same.

Code will be ifdeffed and commented, but anyhow, this is an example of
change C/R will require from the rest of the kernel.

--- a/kernel/pid.c
+++ b/kernel/pid.c

@@ -182,6 +182,34 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 	return -1;
 }
 
+static int set_pidmap(struct pid_namespace *pid_ns, pid_t pid)
+{
+	int offset;
+	struct pidmap *map;
+
+	offset = pid & BITS_PER_PAGE_MASK;
+	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
+	if (unlikely(!map->page)) {
+		void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
+		/*
+		 * Free the page if someone raced with us
+		 * installing it:
+		 */
+		spin_lock_irq(&pidmap_lock);
+		if (map->page)
+			kfree(page);
+		else
+			map->page = page;
+		spin_unlock_irq(&pidmap_lock);
+		if (unlikely(!map->page))
+			return -ENOMEM;
+	}
+	if (test_and_set_bit(offset, map->page))
+		return -EBUSY;
+	atomic_dec(&map->nr_free);
+	return pid;
+}
+
 int next_pidmap(struct pid_namespace *pid_ns, int last)
 {
 	int offset;

@@ -239,7 +267,7 @@ 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, int *cr_nr, unsigned int cr_level)
 {
 	struct pid *pid;
 	enum pid_type type;

@@ -253,7 +281,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 
 	tmp = ns;
 	for (i = ns->level; i >= 0; i--) {
-		nr = alloc_pidmap(tmp);
+		if (cr_nr && ns->level - i <= cr_level)
+			nr = set_pidmap(tmp, cr_nr[ns->level - i]);
+		else
+			nr = alloc_pidmap(tmp);
 		if (nr < 0)
 			goto out_free;