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

[Michal Hocko]

On Wed 17-05-17 09:48:25, Cristopher Lameter wrote:
> On Wed, 17 May 2017, Michal Hocko wrote:
> 
> > > > So how are you going to distinguish VM_FAULT_OOM from an empty mempolicy
> > > > case in a raceless way?
> > >
> > > You dont have to do that if you do not create an empty mempolicy in the
> > > first place. The current kernel code avoids that by first allowing access
> > > to the new set of nodes and removing the old ones from the set when done.
> >
> > which is racy and as Vlastimil pointed out. If we simply fail such an
> > allocation the failure will go up the call chain until we hit the OOM
> > killer due to VM_FAULT_OOM. How would you want to handle that?
> 
> The race is where? If you expand the node set during the move of the
> application then you are safe in terms of the legacy apps that did not
> include static bindings.

I am pretty sure it is describe in those changelogs and I won't repeat
it here.

> If you have screwy things like static mbinds in there then you are
> hopelessly lost anyways. You may have moved the process to another set
> of nodes but the static bindings may refer to a node no longer
> available. Thus the OOM is legitimate.

The point is that you do _not_ want such a process to trigger the OOM
because it can cause other processes being killed.

> At least a user space app could inspect
> the situation and come up with custom ways of dealing with the mess.

I do not really see how would this help to prevent a malicious user from
playing tricks.

-- 
Michal Hocko
SUSE Labs