linux-kernel - Re: [patch] ipv4: don't warn about skb ack allocation failures

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Date:	Thu, 18 Jun 2009 12:23:28 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	David Miller <davem@...emloft.net>
cc:	eric.dumazet@...il.com, jpiszcz@...idpixels.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] ipv4: don't warn about skb ack allocation failures

On Thu, 18 Jun 2009, David Miller wrote:

> > I disagree, page allocation failure messages show vital information about 
> > the state of the VM so that we can find bugs and GFP_ATOMIC allocations 
> > are the most common trigger for these diagnostic messages since 
> > __GFP_WAIT allocations can trigger direct reclaim (and __GFP_FS 
> > allocations can trigger the oom killer) to free memory and will retry the 
> > allocation if ~__GFP_NORETRY.
> 
> It's COMPLETELY and ABSOLUTELY normal for GFP_ATOMIC allocations to
> fail in the networking.
> 

__GFP_NOWARN exists for that reason.

> If you warn it will just spam the logs, and on a router forwarding
> millions of packets per second are you sure that can ever be sane?
> 

The spamming is ratelimited, but GFP_ATOMIC is really the only time we get 
such diagnostic information since __GFP_WAIT allocations can reclaim, 
__GFP_FS allocations can utilize the oom killer, and other order-0 
allocations are implicitly ~__GFP_NORETRY.

As previously mentioned, GFP_ATOMIC allocations that are not __GFP_NOWARN 
have been emitting these diagnostics since 2.5.53.  This has been on your 
TODO list for 6 1/2 years and now you insist all GFP_ATOMIC allocations 
change their default behavior?

I understand what you're trying to avoid, but I disagree with the approach 
of altering the default behavior of GFP_ATOMIC.  I may suggest that 
emitting the page allocation failures become a compile time option; 
CONFIG_DEBUG_VM would be my suggestion.

> Use statistics and tracing if necessary, but log spam no way...
> 

You need the meminfo that is emitted at the time of failure for it to be 
useful.
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