linux-kernel - Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit policy

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Date:   Fri, 29 May 2020 23:43:15 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Kleen, Andi" <andi.kleen@...el.com>, Qian Cai <cai@....pw>,
        Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit
 policy

On Thu, May 28, 2020 at 11:21:36PM +0800, Kleen, Andi wrote:
> 
> 
> >If it's true, then there could be 2 solutions, one is to skip the WARN_ONCE as it has no practical value, as the real >check is the following code, the other is to rectify the percpu counter when the policy is changing to >OVERCOMMIT_NEVER.
> 
> I think it's better to fix it up when the policy changes. That's the right place. The WARN_ON might be useful to catch other bugs.

If we keep the WARN_ON, then the draft fix patch I can think of looks like:

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index a66595b..02d87fc 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -98,6 +98,20 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
 }
 EXPORT_SYMBOL(percpu_counter_add_batch);
 
+void percpu_counter_sync(struct percpu_counter *fbc)
+{
+	unsigned long flags;
+	s64 count;
+
+	raw_spin_lock_irqsave(&fbc->lock, flags);
+	count = __this_cpu_read(*fbc->counters);
+	fbc->count += count;
+	__this_cpu_sub(*fbc->counters, count);
+	raw_spin_unlock_irqrestore(&fbc->lock, flags);
+}
+EXPORT_SYMBOL(percpu_counter_sync);
+
+
 /*
  * Add up all the per-cpu counts, return the result.  This is a more accurate
  * but much slower version of percpu_counter_read_positive()
diff --git a/mm/util.c b/mm/util.c
index 580d268..24322da 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -746,14 +746,24 @@ int overcommit_ratio_handler(struct ctl_table *table, int write, void *buffer,
 	return ret;
 }
 
+static  void sync_overcommit_as(struct work_struct *dummy)
+{
+	percpu_counter_sync(&vm_committed_as);
+}
+
 int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer,
 		size_t *lenp, loff_t *ppos)
 {
 	int ret;
 
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
-	if (ret == 0 && write)
+	if (ret == 0 && write) {
+		if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
+			schedule_on_each_cpu(sync_overcommit_as);
+
 		mm_compute_batch();
+	}
 
 	return ret;
 }

Any comments?

Thanks,
Feng

> -Andi
>