Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

[Michal Hocko <mhocko@kernel.org>]

On Wed 22-01-20 19:15:47, David Hildenbrand wrote:
> On 22.01.20 17:46, Michal Hocko wrote:
> > On Wed 22-01-20 12:58:16, David Hildenbrand wrote:
[...]
> >> Especially interesting for IBM z Systems, whereby memory
> >> onlining/offlining will trigger the actual population of memory in the
> >> hypervisor. So if an admin wants to offline some memory (to give it back
> >> to the hypervisor), it would use lsmem to identify such blocks first,
> >> instead of trying random blocks until one offlining request succeeds.
> > 
> > I am sorry for being dense here but I still do not understand why s390
> 
> It's good that we talk about it :) It's hard to reconstruct actual use
> cases from tools and some documentation only ...
> 
> Side note (just FYI): One difference on s390x compared to other
> architectures (AFAIKS) is that once memory is offline, you might not be
> allowed (by the hypervisor) to online it again - because it was
> effectively unplugged. Such memory is not removed via remove_memory(),
> it's simply kept offline.

I have a very vague understanding of s390 specialities but this is not
really relevant to the discussion AFAICS because this happens _after_
offlining.
 
> > and the way how it does the hotremove matters here. Afterall there are
> > no arch specific operations done until the memory is offlined. Also
> > randomly checking memory blocks and then hoping that the offline will
> > succeed is not way much different from just trying the offline the
> > block. Both have to crawl through the pfn range and bail out on the
> > unmovable memory.
> 
> I think in general we have to approaches to memory unplugging.
> 
> 1. Know explicitly what you want to unplug (e.g., a DIMM spanning
> multiple memory blocks).
> 
> 2. Find random memory blocks you can offline/unplug.
> 
> 
> For 1, I think we both agree that we don't need this. Just try to
> offline and you know if it worked.
> 
> Now of course, for 2 you can try random blocks until you succeeded. From
> a sysadmin point of view that's very inefficient. From a powerpc-utils
> point of view, that's inefficient.

How exactly is check + offline more optimal then offline which makes
check as its first step? I will get to your later points after this is
clarified.
-- 
Michal Hocko
SUSE Labs