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

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Message-ID: <4A3957A1.5030407@gmail.com>
Date:	Wed, 17 Jun 2009 22:52:49 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] ipv4: don't warn about skb ack allocation failures

David Rientjes a écrit :
> On Wed, 17 Jun 2009, Eric Dumazet wrote:
> 
>>> ipv4: don't warn about skb ack allocation failures
>>>
>>> tcp_send_ack() will recover from alloc_skb() allocation failures, so avoid 
>>> emitting warnings.
>>>
>>> Signed-off-by: David Rientjes <rientjes@...gle.com>
>>> ---
>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>> --- a/net/ipv4/tcp_output.c
>>> +++ b/net/ipv4/tcp_output.c
>>> @@ -2442,7 +2442,7 @@ void tcp_send_ack(struct sock *sk)
>>>  	 * tcp_transmit_skb() will set the ownership to this
>>>  	 * sock.
>>>  	 */
>>> -	buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
>>> +	buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC | __GFP_NOWARN);
>>>  	if (buff == NULL) {
>>>  		inet_csk_schedule_ack(sk);
>>>  		inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
>> I count more than 800 GFP_ATOMIC allocations in net/ tree.
>>
>> Most (if not all) of them can recover in case of failures.
>>
>> Should we add __GFP_NOWARN to all of them ?
>>
> 
> Yes, if they are recoverable without any side effects.  Otherwise, they 
> will continue to emit page allocation failure messages which cause users 
> to waste their time when they recognize a problem of an unknown 
> seriousness level in both reporting the issue and looking for resulting 
> corruption.  The __GFP_NOWARN annotation suppresses such warnings for 
> those very reasons.

Then why emit the warning at first place ?

Once we patch all call sites to use GFP_ATOMIC | __GFP_NOWARN, I bet 99% 
GFP_ATOMIC allocations in kernel will use it, so we go back to silent mode.

If a GFP_ATOMIC call site *cannot* use __GFP_NOWARN, it will either :

- call panic()
- crash with a nice stack trace because caller was not aware NULL could be
returned by kmalloc()


Maybe GFP_ATOMIC should include __GFP_NOWARN

#define GFP_ATOMIC  (__GFP_HIGH)
->
#define GFP_ATOMIC  (__GFP_HIGH | __GFP_NOWARN)

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