netdev - Re: [PATCH] net: decnet: Replace GFP_ATOMIC with GFP_KERNEL in dn_route

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Date:   Mon, 9 Apr 2018 22:48:38 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net,
        kafai@...com, weiwan@...gle.com, dsa@...ulusnetworks.com,
        johannes.berg@...el.com, fw@...len.de
Cc:     linux-decnet-user@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: decnet: Replace GFP_ATOMIC with GFP_KERNEL in
 dn_route_init



On 2018/4/9 22:44, Eric Dumazet wrote:
>
> On 04/09/2018 07:10 AM, Jia-Ju Bai wrote:
>> dn_route_init() is never called in atomic context.
>>
>> The call chain ending up at dn_route_init() is:
>> [1] dn_route_init() <- decnet_init()
>> decnet_init() is only set as a parameter of module_init().
>>
>> Despite never getting called from atomic context,
>> dn_route_init() calls __get_free_pages() with GFP_ATOMIC,
>> which waits busily for allocation.
>> GFP_ATOMIC is not necessary and can be replaced with GFP_KERNEL,
>> to avoid busy waiting and improve the possibility of sucessful allocation.
>>
>> This is found by a static analysis tool named DCNS written by myself.
>> And I also manually check it.
>>
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
>> ---
>>   net/decnet/dn_route.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/decnet/dn_route.c b/net/decnet/dn_route.c
>> index 0bd3afd..59ed12a 100644
>> --- a/net/decnet/dn_route.c
>> +++ b/net/decnet/dn_route.c
>> @@ -1898,7 +1898,7 @@ void __init dn_route_init(void)
>>   		while(dn_rt_hash_mask & (dn_rt_hash_mask - 1))
>>   			dn_rt_hash_mask--;
>>   		dn_rt_hash_table = (struct dn_rt_hash_bucket *)
>> -			__get_free_pages(GFP_ATOMIC, order);
>> +			__get_free_pages(GFP_KERNEL, order);
>>   	} while (dn_rt_hash_table == NULL && --order > 0);
>>   
>>   	if (!dn_rt_hash_table)
>>
> This might OOM under pressure.

Sorry, I do not understand this.
Could you please explain the reason?

> This would need __GFP_NOWARN | __GFP_NORETRY  I guess, and would target net-next
>

Do you mean
__get_free_pages(__GFP_NOWARN | __GFP_NORETRY) or
__get_free_pages(__GFP_NOWARN | __GFP_NORETRY | GFP_KERNEL)?


Best wishes,
Jia-Ju Bai