linux-kernel - Re: [PATCH 1/1] mm: avoid re-using pfmemalloc page in page_frag

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Date:   Wed, 4 Nov 2020 09:50:30 +0100
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Rama Nichanamatlu <rama.nichanamatlu@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Dongli Zhang <dongli.zhang@...cle.com>, linux-mm@...ck.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, davem@...emloft.net, kuba@...nel.org,
        aruna.ramakrishna@...cle.com, bert.barbe@...cle.com,
        venkat.x.venkatsubra@...cle.com, manjunath.b.patil@...cle.com,
        joe.jin@...cle.com, srinivas.eeda@...cle.com
Subject: Re: [PATCH 1/1] mm: avoid re-using pfmemalloc page in
 page_frag_alloc()

On 11/4/20 2:16 AM, Rama Nichanamatlu wrote:
>> Thanks for providing the numbers.  Do you think that dropping (up to)
>> 7 packets is acceptable?
> 
> net.ipv4.tcp_syn_retries = 6
> 
> tcp clients wouldn't even get that far leading to connect establish issues.

This does not really matter. If host was under memory pressure,
dropping a few packets is really not an issue.

Please do not add expensive checks in fast path, just to "not drop a packet"
even if the world is collapsing.

Also consider that NIC typically have thousands of pre-allocated page/frags
for their RX ring buffers, they might all have pfmemalloc set, so we are speaking
of thousands of packet drops before the RX-ring can be refilled with normal (non pfmemalloc) page/frags.

If we want to solve this issue more generically, we would have to try
to copy data into a non pfmemalloc frag instead of dropping skb that
had frags allocated minutes ago under memory pressure.

This copy could happen in core networking stack, but this seems adding
more pressure to mm layer under pressure.