linux-kernel - Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user

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Date:   Mon, 29 Jul 2019 11:49:52 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit
 in get_user_pages loop

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.