Re: [PATCH] radix-tree: account nodes to memcg only if explicitly requested

[Johannes Weiner]

On Mon, Aug 01, 2016 at 07:06:05PM +0300, Vladimir Davydov wrote:
> On Mon, Aug 01, 2016 at 11:24:09AM -0400, Johannes Weiner wrote:
> > On Mon, Aug 01, 2016 at 04:13:08PM +0300, Vladimir Davydov wrote:
> > > @@ -351,6 +351,12 @@ static int __radix_tree_preload(gfp_t gfp_mask, int nr)
> > >  	struct radix_tree_node *node;
> > >  	int ret = -ENOMEM;
> > >  
> > > +	/*
> > > +	 * Nodes preloaded by one cgroup can be be used by another cgroup, so
> > > +	 * they should never be accounted to any particular memory cgroup.
> > > +	 */
> > > +	gfp_mask &= ~__GFP_ACCOUNT;
> > 
> > But *all* page cache radix tree nodes are allocated from inside the
> > preload code, since the tree insertions need mapping->tree_lock. So
> > this would effectively disable accounting of the biggest radix tree
> > consumer in the kernel, no?
> 
> No, that's not how accounting of radix tree nodes works. We never
> account preloaded nodes, because this could result in a node accounted
> to one cgroup used by an unrelated cgroup. Instead we always try to
> kmalloc a node on insertion falling back on preloads only if kmalloc
> fails - see commit 58e698af4c634 ("radix-tree: account radix_tree_node
> to memory cgroup").

You are right, I forgot we are doing this. The patch makes sense then.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>