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

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Date:   Mon, 9 Apr 2018 07:44:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jia-Ju Bai <baijiaju1990@...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 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.

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