[Devel] Re: [PATCH 3/3] Dynamic kmem cache allocator for pid namespaces

[Andrew Morton]

On Tue, 10 Jul 2007 18:28:10 +0400
Pavel Emelianov <xemul at openvz.org> wrote:

> Add kmem_cache to pid_namespace to allocate pids from.

I'm having trouble understanding this changelog.

> Since booth implementations expand the struct pid to carry
> more numerical values each namespace should have separate
> cache to store pids of different sizes.

Assuming "booth" means "both": you are referring to two different
implementations of <something>.  What are they?  Where are they?

I don't understand any of this :(

> Each kmem cache is names "pid_<NR>", where <NR> is the number
> of numerical ids on the pid.

What is a "numerical ID on a pid"?

> Different namespaces with same
> level of nesting will have same caches.

ok...

> This patch has two FIXMEs that are to be fixed after we reach
> the consensus about the struct pid itself.
> 
> The first one is that the namespace to free the pid from in
> free_pid() must be taken from pid. Now the init_pid_ns is
> used.

That looks like a fatal bug to me?  We free a slab object into a kmem_cache
which it was not obtained from?  slab will go BUG, surely?

> The second FIXME is about the cache allocation. When we do know
> how long the object will be then we'll have to calculate this
> size in create_pid_cachep. Right now the sizeof(struct pid)
> value is used.

hm, we already have code which is good at choosing an appropriate cache
based upon the requested size.  It's called kmalloc() ;)

Do we really expect that there will be so many of these objects that the
(modest) space-saving which a custom cache provides will be worthwhile?

> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index ddb9a4c..27cfad3 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -20,6 +20,7 @@ struct pid_namespace {
>  	struct pidmap pidmap[PIDMAP_ENTRIES];
>  	int last_pid;
>  	struct task_struct *child_reaper;
> +	struct kmem_cache_t *pid_cachep;
>  };
>  
>  extern struct pid_namespace init_pid_ns;
> diff --git a/kernel/pid.c b/kernel/pid.c
> index c6e3f9f..d3f0f91 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -32,7 +32,6 @@
>  #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
>  static struct hlist_head *pid_hash;
>  static int pidhash_shift;
> -static struct kmem_cache *pid_cachep;
>  struct pid init_struct_pid = INIT_STRUCT_PID;
>  
>  int pid_max = PID_MAX_DEFAULT;
> @@ -176,11 +175,16 @@ static int next_pidmap(struct pid_namesp
>  
>  fastcall void put_pid(struct pid *pid)
>  {
> +	struct pid_namespace *ns;
> +
>  	if (!pid)
>  		return;
> +
> +	/* FIXME - this must be the namespace this pid lives in */
> +	ns = &init_pid_ns;
>  	if ((atomic_read(&pid->count) == 1) ||
>  	     atomic_dec_and_test(&pid->count))
> -		kmem_cache_free(pid_cachep, pid);
> +		kmem_cache_free(ns->pid_cachep, pid);
>  }
>  EXPORT_SYMBOL_GPL(put_pid);
>  
> @@ -208,12 +212,14 @@ struct pid *alloc_pid(void)
>  	struct pid *pid;
>  	enum pid_type type;
>  	int nr = -1;
> +	struct pid_namespace *ns;
>  
> -	pid = kmem_cache_alloc(pid_cachep, GFP_KERNEL);
> +	ns = current->nsproxy->pid_ns;
> +	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
>  	if (!pid)
>  		goto out;
>  
> -	nr = alloc_pidmap(current->nsproxy->pid_ns);
> +	nr = alloc_pidmap(ns);
>  	if (nr < 0)
>  		goto out_free;
>  
> @@ -230,7 +236,7 @@ out:
>  	return pid;
>  
>  out_free:
> -	kmem_cache_free(pid_cachep, pid);
> +	kmem_cache_free(ns->pid_cachep, pid);
>  	pid = NULL;
>  	goto out;
>  }
> @@ -365,6 +371,56 @@ struct pid *find_ge_pid(int nr)
>  }
>  EXPORT_SYMBOL_GPL(find_get_pid);
>  
> +struct pid_cache {
> +	int nr_ids;
> +	char name[16];
> +	struct kmem_cache *cachep;
> +	struct list_head list;
> +};

Please put a lot of effort into documenting data structures.  They are key
to understanding the code and effort here really does pay off in
understandability and hence maintainability and quality.

I would contend that zero documentation simply does not meet kernel standards.

A good code comment would, amongst other things, explain to the reader why
this code exists at all.  That would (permanently) answer my kmalloc
question, above.

> +static LIST_HEAD(pid_caches_lh);
> +static DEFINE_MUTEX(pid_caches_mutex);
> +
> +/*
> + * creates the kmem cache to allocate pids from.
> + * @nr_ids: the number of numerical ids this pid will have to carry
> + */
> +
> +static struct kmem_cache *create_pid_cachep(int nr_ids)
> +{
> +	struct pid_cache *pcache;
> +	struct kmem_cache *cachep;
> +
> +	mutex_lock(&pid_caches_mutex);
> +	list_for_each_entry (pcache, &pid_caches_lh, list)
> +		if (pcache->nr_ids == nr_ids)
> +			goto out;
> +
> +	pcache = kmalloc(sizeof(struct pid_cache), GFP_KERNEL);
> +	if (pcache == NULL)
> +		goto err_alloc;
> +
> +	snprintf(pcache->name, sizeof(pcache->name), "pid_%d", nr_ids);
> +	cachep = kmem_cache_create(pcache->name,
> +			/* FIXME add numerical ids here */
> +			sizeof(struct pid), 0, SLAB_HWCACHE_ALIGN, NULL, NULL);
> +	if (cachep == NULL)
> +		goto err_cachep;
> +
> +	pcache->nr_ids = nr_ids;
> +	pcache->cachep = cachep;
> +	list_add(&pcache->list, &pid_caches_lh);
> +out:
> +	mutex_unlock(&pid_caches_mutex);
> +	return pcache->cachep;
> +
> +err_cachep:
> +	kfree(pcache);
> +err_alloc:
> +	mutex_unlock(&pid_caches_mutex);
> +	return NULL;
> +}
> +
>  struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *old_ns)
>  {
>  	BUG_ON(!old_ns);
> @@ -412,5 +468,7 @@ void __init pidmap_init(void)
>  	set_bit(0, init_pid_ns.pidmap[0].page);
>  	atomic_dec(&init_pid_ns.pidmap[0].nr_free);
>  
> -	pid_cachep = KMEM_CACHE(pid, SLAB_PANIC);
> +	init_pid_ns.pid_cachep = create_pid_cachep(1);
> +	if (init_pid_ns.pid_cachep == NULL)
> +		panic("Can't create pid_1 cachep\n");
>  }

hm, so that global list of pid_caches which we're carefully maintaining
doesn't actually get used for anything.

I assume there is some plan to use this list in the future, but this should
have been covered in the changelog, please.
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