Re: [PATCH 01/23] tref: Implement task references.

[Oleg Nesterov]

Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
>
> > I think there is another, much simpler solution. We can make a "reference" to
> > the
> > pid itself to protect it against free_pidmap(), so that this pid can't be
> > reused.
>
> However with my trivial hostile program I can with 32 or 33 living processes
> each with 1000 references to dead processes I can completely saturate the
> default pid map.  And it won't be obvious why alloc_pidmap is failing.

Yes, this is a problem. Please see the new version below. Instead of delaying
pid releasing, free_pidmap() just invalidates pid_ref. The code becomes even
simpler.

> Your resource consumption with the extra hash table is higher than
> mine at until very high process counts.

The size of ref_array[] could be arbitrary low (we can't use pid_hashfn() in
this case, of course). And tref adds 4 * sizeof(void*) to every task, and it
is much more complicated.

> In addition it doesn't really help with the problem that inspired
> this work.  That of having multiple pid spaces.  I could make it work
> by throwing a pspace reference in struct pid_ref, but without some
> fancy footwork it would prevent cleanup until all of the outside
> references are gone.
>
> So I kind of like it but I don't feel it does as good a job solving
> the problems I am solving.

Ok. I missed the virtualization/pspace discussion completely, so you are
very probably right.

Oleg.

struct pid_ref
{
	pid_t			pid;
	int			count;
	struct hlist_node	chain;
};

// allocated in pidhash_init()
static struct hlist_head *ref_hash;

static struct pid_ref *find_pid_ref(pid_t pid)
{
	struct hlist_node *elem;
	struct pid_ref *ref;

	hlist_for_each_entry(ref, elem, &ref_hash[pid_hashfn(pid)], chain)
		if (ref->pid == pid)
			return ref;

	return NULL;
}

// This is the only function modified.
fastcall void free_pidmap(int pid)
{
	pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
	int offset = pid & BITS_PER_PAGE_MASK;
	struct pid_ref *ref;

	clear_bit(offset, map->page);
	atomic_inc(&map->nr_free);

	ref = find_pid_ref(pid);
	if (unlikely(ref != NULL)) {
		hlist_del_init(&ref->chain);
		ref->pid = 0;
	}
}

static inline int pid_inuse(pid_t pid)
{
	pidmap_t *map = pidmap_array + pid / BITS_PER_PAGE;
	int offset = pid & BITS_PER_PAGE_MASK;

	return test_bit(offset, map->page);
}

struct pid_ref *alloc_pid_ref(pid_t pid)
{
	struct pid_ref *ref;

	write_lock_irq(&tasklist_lock);
	ref = find_pid_ref(pid);
	if (ref)
		ref->count++;
	else if (pid_inuse(pid)) {
		ref = kmalloc(sizeof(*ref), GFP_ATOMIC);
		if (ref) {
			ref->pid = pid;
			ref->count = 1;
			hlist_add_head(&ref->chain,
				&ref_hash[pid_hashfn(pid)]);
		}
	}
	write_unlock_irq(&tasklist_lock);

	return ref;
}

void free_pid_ref(struct pid_ref *ref)
{
	if (!ref)
		return;

	write_lock_irq(&tasklist_lock);
	if (!--ref->count) {
		hlist_del_init(&ref->chain);
		kfree(ref);
	}
	write_unlock_irq(&tasklist_lock);
}
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