linux-kernel - Re: [PATCH V3] Fix Committed

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Message-Id: <20090421102317.F113.A69D9226@jp.fujitsu.com>
Date:	Tue, 21 Apr 2009 10:41:20 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Eric B Munson <ebmunson@...ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mel@...ux.vnet.ibm.com,
	cl@...ux-foundation.org
Subject: Re: [PATCH V3] Fix Committed_AS underflow

> void vm_acct_memory(long pages)
> {
>         long *local;
> 	long local_min = -ACCT_THRESHOLD;
> 	long local_max = ACCT_THRESHOLD;
> 	long local_goal = 0;
> 
>         preempt_disable();
>         local = &__get_cpu_var(committed_space);
>         *local += pages;
>         if (*local > local_max || *local < local_min) {
>                 atomic_long_add(*local - local_goal, &vm_committed_space);
>                 *local = local_goal;
>         }
>         preempt_enable();
> }
> 
> But now consider if we changed the local_* variables a bit:
> 
> 	long local_min = -(ACCT_THRESHOLD*2);
> 	long local_max = 0
> 	long local_goal = -ACCT_THRESHOLD;
> 
> We'll get some possibly *large* numbers in meminfo, but it will at least
> never underflow.

if *local == -(ACCT_THRESHOLD*2), 
  *local - local_goal = -(ACCT_THRESHOLD*2) + ACCT_THRESHOLD = -ACCT_THRESHOLD

Then, we still pass negative value to atomic_long_add().
IOW, vm_committed_space still can be negative value.

Am I missing anything?



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