Re: [PATCH v2 0/7] mm: pages for hugetlb's overcommit may be able to charge to memcg

[Michal Hocko]

On Thu 24-05-18 13:39:59, TSUKADA Koutaro wrote:
> On 2018/05/23 3:54, Michal Hocko wrote:
[...]
> > I am also quite confused why you keep distinguishing surplus hugetlb
> > pages from regular preallocated ones. Being a surplus page is an
> > implementation detail that we use for an internal accounting rather than
> > something to exhibit to the userspace even more than we do currently.
> 
> I apologize for having confused.
> 
> The hugetlb pages obtained from the pool do not waste the buddy pool.

Because they have already allocated from the buddy allocator so the end
result is very same.

> On
> the other hand, surplus hugetlb pages waste the buddy pool. Due to this
> difference in property, I thought it could be distinguished.

But this is simply not correct. Surplus pages are fluid. If you increase
the hugetlb size they will become regular persistent hugetlb pages.
 
> Although my memcg knowledge is extremely limited, memcg is accounting for
> various kinds of pages obtained from the buddy pool by the task belonging
> to it. I would like to argue that surplus hugepage has specificity in
> terms of obtaining from the buddy pool, and that it is specially permitted
> charge requirements for memcg.

Not really. Memcg accounts primarily for reclaimable memory. We do
account for some non-reclaimable slabs but the life time should be at
least bound to a process life time. Otherwise the memcg oom killer
behavior is not guaranteed to unclutter the situation. Hugetlb pages are
simply persistent. Well, to be completely honest tmpfs pages have a
similar problem but lacking the swap space for them is kinda
configuration bug.

> It seems very strange that charge hugetlb page to memcg, but essentially
> it only charges the usage of the compound page obtained from the buddy pool,
> and even if that page is used as hugetlb page after that, memcg is not
> interested in that.

Ohh, it is very much interested. The primary goal of memcg is to enforce
the limit. How are you going to do that in an absence of the reclaimable
memory? And quite a lot of it because hugetlb pages usually consume a
lot of memory.

> I will completely apologize if my way of thinking is wrong. It would be
> greatly appreciated if you could mention why we can not charge surplus
> hugepages to memcg.
> 
> > Just look at what [sw]hould when you need to adjust accounting - e.g.
> > due to the pool resize. Are you going to uncharge those surplus pages
> > ffrom memcg to reflect their persistence?
> > 
> 
> I could not understand the intention of this question, sorry. When resize
> the pool, I think that the number of surplus hugepages in use does not
> change. Could you explain what you were concerned about?

It does change when ou change the hugetlb pool size, migrate pages
between per-numa pools (have a look at adjust_pool_surplus).
-- 
Michal Hocko
SUSE Labs