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Date:   Fri, 17 Jan 2020 13:38:27 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net 3/3] udp: avoid bulk memory scheduling on memory pressure.

On Fri, Jan 17, 2020 at 12:51 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 1/17/20 9:27 AM, Paolo Abeni wrote:
> > Williem reported that after commit 0d4a6608f68c ("udp: do rmem bulk
> > free even if the rx sk queue is empty") the memory allocated by
> > an almost idle system with many UDP sockets can grow a lot.
> >
> > This change addresses the issue enabling memory pressure tracking
> > for UDP and flushing the fwd allocated memory on dequeue if the
> > UDP protocol is under memory pressure.
> >
> > Note that with this patch applied, the system allocates more
> > liberally memory for UDP sockets while the total memory usage is
> > below udp_mem[1], while the vanilla kernel would allow at most a
> > single page per socket when UDP memory usage goes above udp_mem[0]
> > - see __sk_mem_raise_allocated().
> >
> > Reported-and-diagnosed-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> > Fixes: commit 0d4a6608f68c ("udp: do rmem bulk free even if the rx sk queue is empty")

Thanks a lot for the quick follow-up, Paolo!

> Not a proper Fixes: tag

And to give credit where it's due: Eric diagnosed the issue.

> Frankly I would rather revert this patch, unless you show how much things were improved.

In response to your question in the cover letter, I also think that
for net the three-line revert patch is more obviously correct.

Memory pressure might be helpful in net-next with some iteration, of course.

> Where in the UDP code the forward allocations will be released while udp_memory_pressure
> is hit ?
>
> TCP has many calls to sk_mem_reclaim() and sk_mem_reclaim_partial() to try
> to gracefully exit memory pressure.
>

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