Re: [PATCH 4/5] radix-tree: account radix_tree_node to memory cgroup

[Johannes Weiner <hannes@cmpxchg.org>]

On Sun, Feb 07, 2016 at 08:27:34PM +0300, Vladimir Davydov wrote:
> Allocation of radix_tree_node objects can be easily triggered from
> userspace, so we should account them to memory cgroup. Besides, we need
> them accounted for making shadow node shrinker per memcg (see
> mm/workingset.c).
> 
> A tricky thing about accounting radix_tree_node objects is that they are
> mostly allocated through radix_tree_preload(), so we can't just set
> SLAB_ACCOUNT for radix_tree_node_cachep - that would likely result in a
> lot of unrelated cgroups using objects from each other's caches.
> 
> One way to overcome this would be making radix tree preloads per memcg,
> but that would probably look cumbersome and overcomplicated.
> 
> Instead, we make radix_tree_node_alloc() first try to allocate from the
> cache with __GFP_ACCOUNT, no matter if the caller has preloaded or not,
> and only if it fails fall back on using per cpu preloads. This should
> make most allocations accounted.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

I'm not too stoked about the extra slab call. But the preload call
allocates nodes for the worst-case insertion, so you are absolutely
right that charging there would not make sense for cgroup ownership.
And I can't think of anything better to do here.

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