Re: [PATCH] slub: add missing kmem cgroup support to kmem_cache_free_bulk

[Vladimir Davydov <vdavydov@virtuozzo.com>]

On Thu, Nov 05, 2015 at 02:06:21PM +0900, Joonsoo Kim wrote:
> On Thu, Oct 29, 2015 at 02:05:31PM +0100, Jesper Dangaard Brouer wrote:
> > Initial implementation missed support for kmem cgroup support
> > in kmem_cache_free_bulk() call, add this.
> > 
> > If CONFIG_MEMCG_KMEM is not enabled, the compiler should
> > be smart enough to not add any asm code.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > ---
> >  mm/slub.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 9be12ffae9fc..9875864ad7b8 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2845,6 +2845,9 @@ static int build_detached_freelist(struct kmem_cache *s, size_t size,
> >  	if (!object)
> >  		return 0;
> >  
> > +	/* Support for kmemcg */
> > +	s = cache_from_obj(s, object);
> > +
> >  	/* Start new detached freelist */
> >  	set_freepointer(s, object, NULL);
> >  	df->page = virt_to_head_page(object);
> 
> Hello,
> 
> It'd better to add this 's = cache_from_obj()' on kmem_cache_free_bulk().
> Not only build_detached_freelist() but also slab_free() need proper
> cache.

Yeah, Joonsoo is right. Besides, there's a bug in kmem_cache_alloc_bulk:

> /* Note that interrupts must be enabled when calling this function. */
> bool kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> 			   void **p)
> {
> 	struct kmem_cache_cpu *c;
> 	int i;
> 
> 	/*
> 	 * Drain objects in the per cpu slab, while disabling local
> 	 * IRQs, which protects against PREEMPT and interrupts
> 	 * handlers invoking normal fastpath.
> 	 */
> 	local_irq_disable();
> 	c = this_cpu_ptr(s->cpu_slab);
> 
> 	for (i = 0; i < size; i++) {
> 		void *object = c->freelist;
> 
> 		if (unlikely(!object)) {
> 			/*
> 			 * Invoking slow path likely have side-effect
> 			 * of re-populating per CPU c->freelist
> 			 */
> 			p[i] = ___slab_alloc(s, flags, NUMA_NO_NODE,
> 					    _RET_IP_, c);
> 			if (unlikely(!p[i]))
> 				goto error;
> 
> 			c = this_cpu_ptr(s->cpu_slab);
> 			continue; /* goto for-loop */
> 		}
> 
> 		/* kmem_cache debug support */
> 		s = slab_pre_alloc_hook(s, flags);

slab_pre_alloc_hook expects a global cache and returns per memcg one, so
calling this function from inside a kmemcg will result in hitting the
VM_BUG_ON in __memcg_kmem_get_cache, not saying about mis-accounting of
__slab_alloc.

memcg_kmem_get_cache should be called once, in the very beginning of
kmem_cache_alloc_bulk, and it should be matched by memcg_kmem_put_cache
when we are done.

Thanks,
Vladimir

> 		if (unlikely(!s))
> 			goto error;
> 
> 		c->freelist = get_freepointer(s, object);
> 		p[i] = object;
> 
> 		/* kmem_cache debug support */
> 		slab_post_alloc_hook(s, flags, object);
> 	}
> 	c->tid = next_tid(c->tid);
> 	local_irq_enable();
> 
> 	/* Clear memory outside IRQ disabled fastpath loop */
> 	if (unlikely(flags & __GFP_ZERO)) {
> 		int j;
> 
> 		for (j = 0; j < i; j++)
> 			memset(p[j], 0, s->object_size);
> 	}
> 
> 	return true;
> 
> error:
> 	__kmem_cache_free_bulk(s, i, p);
> 	local_irq_enable();
> 	return false;
> }

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