[Devel] Re: [PATCH v5 06/14] memcg: kmem controller infrastructure

[David Rientjes]

On Fri, 19 Oct 2012, Glauber Costa wrote:

> >>> What about gfp & __GFP_FS?
> >>>
> >>
> >> Do you intend to prevent or allow OOM under that flag? I personally
> >> think that anything that accepts to be OOM-killed should have GFP_WAIT
> >> set, so that ought to be enough.
> >>
> > 
> > The oom killer in the page allocator cannot trigger without __GFP_FS 
> > because direct reclaim has little chance of being very successful and 
> > thus we end up needlessly killing processes, and that tends to happen 
> > quite a bit if we dont check for it.  Seems like this would also happen 
> > with memcg if mem_cgroup_reclaim() has a large probability of failing?
> > 
> 
> I can indeed see tests for GFP_FS in some key locations in mm/ before
> calling the OOM Killer.
> 
> Should I test for GFP_IO as well?

It's not really necessary, if __GFP_IO isn't set then it wouldn't make 
sense for __GFP_FS to be set.

> If the idea is preventing OOM to
> trigger for allocations that can write their pages back, how would you
> feel about the following test:
> 
> may_oom = (gfp & GFP_KERNEL) && !(gfp & __GFP_NORETRY) ?
> 

I would simply copy the logic from the page allocator and only trigger oom 
for __GFP_FS and !__GFP_NORETRY.