On Thu 24-05-18 21:58:49, TSUKADA Koutaro wrote: > On 2018/05/24 17:20, Michal Hocko wrote: > > 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. > > I really can not understand what's wrong with this. That page is obviously > released before being added to the persistent pool, and at that time it is > uncharged from memcg to which the task belongs(This assumes my patch-set). > After that, the same page obtained from the pool is not surplus hugepage > so it will not be charged to memcg again. I do not see anything like that. adjust_pool_surplus is simply and accounting thing. At least the last time I've checked. Maybe your patchset handles that? > >> 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. > > Absolutely you are saying the right thing, but, for example, can mlock(2)ed > pages be swapped out by reclaim?(What is the difference between mlock(2)ed > pages and hugetlb page?) No mlocked pages cannot be reclaimed and that is why we restrict them to a relatively small amount. > >> 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. > > Simply kill any of the tasks belonging to that memcg. Maybe, no one wants > reclaim at the time of account of with surplus hugepages. But that will not release the hugetlb memory, does it? > [...] > >> 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 you change the hugetlb pool size, migrate pages > > between per-numa pools (have a look at adjust_pool_surplus). > > As I looked at, what kind of fatal problem is caused by charging surplus > hugepages to memcg by just manipulating counter of statistical information? Fatal? Not sure. It simply tries to add an alien memory to the memcg concept so I would pressume an unexpected behavior (e.g. not being able to reclaim memcg or, over reclaim, trashing etc.). -- Michal Hocko SUSE Labs
