netdev - Re: [PATCH net-next] mlx4: support __GFP

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Date:   Wed, 18 Jan 2017 07:24:31 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH net-next] mlx4: support __GFP_MEMALLOC for rx

On Wed, 2017-01-18 at 18:11 +0300, Konstantin Khlebnikov wrote:
> On 18.01.2017 17:23, Eric Dumazet wrote:

> >
> > Take a look at sk_filter_trim_cap(), where the RX packets received on a
> > socket which does not have SOCK_MEMALLOC is dropped.
> >
> >         /*
> >          * If the skb was allocated from pfmemalloc reserves, only
> >          * allow SOCK_MEMALLOC sockets to use it as this socket is
> >          * helping free memory
> >          */
> >         if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
> >                 return -ENOMEM;
> 
> I suppose this happens in BH context right after receiving packet?
> 
> Potentially any ACK could free memory in TCP send queue,
> so using reserves here makes sense.

Yes, but only sockets with SOCK_MEMALLOC have this contract with the mm
layer.

For 'other' sockets, one possible trick would be that if only the page
fragment attached to skb had the pfmemalloc bit, and not the sk_buff
itself, we could attempt a skb_condense() operation [1], but it is not
really easy to properly recompute skb->pfmemalloc.

Pure TCP ACK packets can usually be trimmed by skb_condense().
Since they have no payload, we have a guarantee they wont sit in a queue
and hold memory.