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

[Feng Tang]

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
>