Re: [RFC 1/6] mm, page_alloc: fix more premature OOM due to race with cpuset update

From: Michal Hocko
Date: Thu May 18 2017 - 13:24:38 EST

On Thu 18-05-17 11:57:55, Cristopher Lameter wrote:
> On Thu, 18 May 2017, Michal Hocko wrote:
> > > Nope. The OOM in a cpuset gets the process doing the alloc killed. Or what
> > > that changed?
> !!!!!
> > >
> > > At this point you have messed up royally and nothing is going to rescue
> > > you anyways. OOM or not does not matter anymore. The app will fail.
> >
> > Not really. If you can trick the system to _think_ that the intersection
> > between mempolicy and the cpuset is empty then the OOM killer might
> > trigger an innocent task rather than the one which tricked it into that
> > situation.
> See above. OOM Kill in a cpuset does not kill an innocent task but a task
> that does an allocation in that specific context meaning a task in that
> cpuset that also has a memory policty.

No, the oom killer will chose the largest task in the specific NUMA
domain. If you just fail such an allocation then a page fault would get
VM_FAULT_OOM and pagefault_out_of_memory would kill a task regardless of
the cpusets.

> Regardless of that the point earlier was that the moving logic can avoid
> creating temporary situations of empty sets of nodes by analysing the
> memory policies etc and only performing moves when doing so is safe.

How are you going to do that in a raceless way? Moreover the whole
discussion is about _failing_ allocations on an empty cpuset and
mempolicy intersection.

Michal Hocko