Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user_pages loop

[Michal Hocko <mhocko@kernel.org>]

On Mon 29-07-19 20:55:09, Michal Hocko wrote:
> On Mon 29-07-19 11:49:52, Johannes Weiner wrote:
> > On Sun, Jul 28, 2019 at 03:29:38PM +0300, Konstantin Khlebnikov wrote:
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -847,8 +847,11 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> > >  			ret = -ERESTARTSYS;
> > >  			goto out;
> > >  		}
> > > -		cond_resched();
> > >  
> > > +		/* Reclaim memory over high limit before stocking too much */
> > > +		mem_cgroup_handle_over_high(true);
> > 
> > I'd rather this remained part of the try_charge() call. The code
> > comment in try_charge says this:
> > 
> > 	 * We can perform reclaim here if __GFP_RECLAIM but let's
> > 	 * always punt for simplicity and so that GFP_KERNEL can
> > 	 * consistently be used during reclaim.
> > 
> > The simplicity argument doesn't hold true anymore once we have to add
> > manual calls into allocation sites. We should instead fix try_charge()
> > to do synchronous reclaim for __GFP_RECLAIM and only punt to userspace
> > return when actually needed.
> 
> Agreed. If we want to do direct reclaim on the high limit breach then it
> should go into try_charge same way we do hard limit reclaim there. I am
> not yet sure about how/whether to scale the excess. The only reason to
> move reclaim to return-to-userspace path was GFP_NOWAIT charges. As you
> say, maybe we should start by always performing the reclaim for
> sleepable contexts first and only defer for non-sleeping requests.

In other words. Something like patch below (completely untested). Could
you give it a try Konstantin?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ba9138a4a1de..53a35c526e43 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2429,8 +2429,12 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 				schedule_work(&memcg->high_work);
 				break;
 			}
-			current->memcg_nr_pages_over_high += batch;
-			set_notify_resume(current);
+			if (gfpflags_allow_blocking(gfp_mask)) {
+				reclaim_high(memcg, nr_pages, GFP_KERNEL);
+			} else {
+				current->memcg_nr_pages_over_high += batch;
+				set_notify_resume(current);
+			}
 			break;
 		}
 	} while ((memcg = parent_mem_cgroup(memcg)));
-- 
Michal Hocko
SUSE Labs