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

From: Konstantin Khlebnikov
Date: Fri Aug 02 2019 - 06:01:20 EST

On 02.08.2019 12:40, Michal Hocko wrote:
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,
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?

This should work but also eliminate all benefits from deferred reclaim:
bigger batching and running without of any locks.

After that gap between high and max will work just as reserve for atomic allocations.

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,
- 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);
+ }
} while ((memcg = parent_mem_cgroup(memcg)));