Re: [PATCH 2/3] mm, oom: do not enfore OOM killer for __GFP_NOFAIL automatically

[Hillf Danton]

On Tuesday, January 24, 2017 8:41 PM Michal Hocko wrote: 
> On Fri 20-01-17 16:33:36, Hillf Danton wrote:
> >
> > On Tuesday, December 20, 2016 9:49 PM Michal Hocko wrote:
> > >
> > > @@ -1013,7 +1013,7 @@ bool out_of_memory(struct oom_control *oc)
> > >  	 * make sure exclude 0 mask - all other users should have at least
> > >  	 * ___GFP_DIRECT_RECLAIM to get here.
> > >  	 */
> > > -	if (oc->gfp_mask && !(oc->gfp_mask & (__GFP_FS|__GFP_NOFAIL)))
> > > +	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
> > >  		return true;
> > >
> > As to GFP_NOFS|__GFP_NOFAIL request, can we check gfp mask
> > one bit after another?
> >
> > 	if (oc->gfp_mask) {
> > 		if (!(oc->gfp_mask & __GFP_FS))
> > 			return false;
> >
> > 		/* No service for request that can handle fail result itself */
> > 		if (!(oc->gfp_mask & __GFP_NOFAIL))
> > 			return false;
> > 	}
> 
> I really do not understand this request. 

It's a request of both NOFS and NOFAIL, and I think we can keep it from
hitting oom killer by shuffling the current gfp checks.
I hope it can make nit sense to your work.

> This patch is removing the __GFP_NOFAIL part... 

Yes, and I don't stick to handling NOFAIL requests inside oom.
 
> Besides that why should they return false?

It's feedback to page allocator that no kill is issued, and 
extra attention is needed.

thanks
Hillf