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

[Michal Hocko]

On Wed 25-01-17 15:00:51, Hillf Danton wrote:
> 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.
> 

I still do not understand. The whole point we are doing the late
__GFP_FS check is explained in 3da88fb3bacf ("mm, oom: move GFP_NOFS
check to out_of_memory"). And the reason why I am _removing_
__GFP_NOFAIL is explained in the changelog of this patch.

> > 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.

Be careful, the semantic of out_of_memory is different. Returning false
means that the oom killer has been disabled and so the allocation should
fail rather than loop for ever.

-- 
Michal Hocko
SUSE Labs