Messages in this thread | | | Subject | Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2 | From | Kamezawa Hiroyuki <> | Date | Tue, 15 Dec 2015 18:29:59 +0900 |
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On 2015/12/15 17:30, Vladimir Davydov wrote: > On Tue, Dec 15, 2015 at 12:12:40PM +0900, Kamezawa Hiroyuki wrote: >> On 2015/12/15 0:30, Michal Hocko wrote: >>> On Thu 10-12-15 14:39:14, Vladimir Davydov wrote: >>>> In the legacy hierarchy we charge memsw, which is dubious, because: >>>> >>>> - memsw.limit must be >= memory.limit, so it is impossible to limit >>>> swap usage less than memory usage. Taking into account the fact that >>>> the primary limiting mechanism in the unified hierarchy is >>>> memory.high while memory.limit is either left unset or set to a very >>>> large value, moving memsw.limit knob to the unified hierarchy would >>>> effectively make it impossible to limit swap usage according to the >>>> user preference. >>>> >>>> - memsw.usage != memory.usage + swap.usage, because a page occupying >>>> both swap entry and a swap cache page is charged only once to memsw >>>> counter. As a result, it is possible to effectively eat up to >>>> memory.limit of memory pages *and* memsw.limit of swap entries, which >>>> looks unexpected. >>>> >>>> That said, we should provide a different swap limiting mechanism for >>>> cgroup2. >>>> This patch adds mem_cgroup->swap counter, which charges the actual >>>> number of swap entries used by a cgroup. It is only charged in the >>>> unified hierarchy, while the legacy hierarchy memsw logic is left >>>> intact. >>> >>> I agree that the previous semantic was awkward. The problem I can see >>> with this approach is that once the swap limit is reached the anon >>> memory pressure might spill over to other and unrelated memcgs during >>> the global memory pressure. I guess this is what Kame referred to as >>> anon would become mlocked basically. This would be even more of an issue >>> with resource delegation to sub-hierarchies because nobody will prevent >>> setting the swap amount to a small value and use that as an anon memory >>> protection. >>> >>> I guess this was the reason why this approach hasn't been chosen before >> >> Yes. At that age, "never break global VM" was the policy. And "mlock" can be >> used for attacking system. > > If we are talking about "attacking system" from inside a container, > there are much easier and disruptive ways, e.g. running a fork-bomb or > creating pipes - such memory can't be reclaimed and global OOM killer > won't help.
You're right. We just wanted to avoid affecting global memory reclaim by each cgroup settings.
Thanks, -Kame
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