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Date:   Wed, 27 May 2020 13:03:13 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
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        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 06/19] mm: memcg/slab: obj_cgroup API
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On Wed, May 27, 2020 at 03:56:14PM -0400, Johannes Weiner wrote:
> On Tue, May 26, 2020 at 02:42:14PM -0700, Roman Gushchin wrote:
> > @@ -257,6 +257,98 @@ struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr)
> >  }
> >  
> >  #ifdef CONFIG_MEMCG_KMEM
> > +extern spinlock_t css_set_lock;
> > +
> > +static void obj_cgroup_release(struct percpu_ref *ref)
> > +{
> > +	struct obj_cgroup *objcg = container_of(ref, struct obj_cgroup, refcnt);
> > +	struct mem_cgroup *memcg;
> > +	unsigned int nr_bytes;
> > +	unsigned int nr_pages;
> > +	unsigned long flags;
> > +
> > +	/*
> > +	 * At this point all allocated objects are freed, and
> > +	 * objcg->nr_charged_bytes can't have an arbitrary byte value.
> > +	 * However, it can be PAGE_SIZE or (x * PAGE_SIZE).
> > +	 *
> > +	 * The following sequence can lead to it:
> > +	 * 1) CPU0: objcg == stock->cached_objcg
> > +	 * 2) CPU1: we do a small allocation (e.g. 92 bytes),
> > +	 *          PAGE_SIZE bytes are charged
> > +	 * 3) CPU1: a process from another memcg is allocating something,
> > +	 *          the stock if flushed,
> > +	 *          objcg->nr_charged_bytes = PAGE_SIZE - 92
> > +	 * 5) CPU0: we do release this object,
> > +	 *          92 bytes are added to stock->nr_bytes
> > +	 * 6) CPU0: stock is flushed,
> > +	 *          92 bytes are added to objcg->nr_charged_bytes
> > +	 *
> > +	 * In the result, nr_charged_bytes == PAGE_SIZE.
> > +	 * This page will be uncharged in obj_cgroup_release().
> > +	 */
> 
> Thanks for adding this.
> 
> > +int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
> > +{
> > +	struct mem_cgroup *memcg;
> > +	unsigned int nr_pages, nr_bytes;
> > +	int ret;
> > +
> > +	if (consume_obj_stock(objcg, size))
> > +		return 0;
> > +
> > +	rcu_read_lock();
> > +	memcg = obj_cgroup_memcg(objcg);
> > +	css_get(&memcg->css);
> > +	rcu_read_unlock();
> 
> Can you please also add the comment here I mentioned last time? To
> explain why we're not checking objcg->nr_charged_bytes if we have
> already pre-allocated bytes that could satisfy the allocation.

I've added a comment into drain_obj_stock() where nr_charged_bytes is bumped.
But I can add another on here, np.

> 
> Otherwise, looks good to me.

Thanks!