netdev - Re: [PATCH bpf v6 03/12] bpf: sockmap, improved check for empty queue

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Message-ID: <87pm8agv8y.fsf@cloudflare.com>
Date:   Tue, 11 Apr 2023 19:53:51 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     daniel@...earbox.net, lmb@...valent.com, edumazet@...gle.com,
        bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...nel.org,
        andrii@...nel.org, will@...valent.com
Subject: Re: [PATCH bpf v6 03/12] bpf: sockmap, improved check for empty queue

On Fri, Apr 07, 2023 at 10:16 AM -07, John Fastabend wrote:
> We noticed some rare sk_buffs were stepping past the queue when system was
> under memory pressure. The general theory is to skip enqueueing
> sk_buffs when its not necessary which is the normal case with a system
> that is properly provisioned for the task, no memory pressure and enough
> cpu assigned.
>
> But, if we can't allocate memory due to an ENOMEM error when enqueueing
> the sk_buff into the sockmap receive queue we push it onto a delayed
> workqueue to retry later. When a new sk_buff is received we then check
> if that queue is empty. However, there is a problem with simply checking
> the queue length. When a sk_buff is being processed from the ingress queue
> but not yet on the sockmap msg receive queue its possible to also recv
> a sk_buff through normal path. It will check the ingress queue which is
> zero and then skip ahead of the pkt being processed.
>
> Previously we used sock lock from both contexts which made the problem
> harder to hit, but not impossible.
>
> To fix also check the 'state' variable where we would cache partially
> processed sk_buff. This catches the majority of cases. But, we also
> need to use the mutex lock around this check because we can't have both
> codes running and check sensibly. We could perhaps do this with atomic
> bit checks, but we are already here due to memory pressure so slowing
> things down a bit seems OK and simpler to just grab a lock.
>
> To reproduce issue we run NGINX compliance test with sockmap running and
> observe some flakes in our testing that we attributed to this issue.
>
> Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> Tested-by: William Findlay <will@...valent.com>
> Signed-off-by: John Fastabend <john.fastabend@...il.com>
> ---

Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>