netdev - Re: [PATCH net-next 2/7] rds: hold a sock ref from rds_message to the rds

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Message-ID: <CAF=yD-KqH=3X9R1AAYxdOfbb48Tikwo7uVGBQi8OpO9M6Baj1Q@mail.gmail.com>
Date:   Thu, 25 Jan 2018 15:44:53 +0100
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        santosh.shilimkar@...cle.com
Subject: Re: [PATCH net-next 2/7] rds: hold a sock ref from rds_message to the rds_sock

On Wed, Jan 24, 2018 at 12:45 PM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> The existing model holds a reference from the rds_sock to the
> rds_message, but the rds_message does not itself hold a sock_put()
> on the rds_sock. Instead the m_rs field in the rds_message is
> assigned when the message is queued on the sock, and nulled when
> the message is dequeued from the sock.
>
> We want to be able to notify userspace when the rds_message
> is actually freed (from rds_message_purge(), after the refcounts
> to the rds_message go to 0). At the time that rds_message_purge()
> is called, the message is no longer on the rds_sock retransmit
> queue. Thus the explicit reference for the m_rs is needed to
> send a notification that will signal to userspace that
> it is now safe to free/reuse any pages that may have
> been pinned down for zerocopy.
>
> This patch manages the m_rs assignment in the rds_message with
> the necessary refcount book-keeping.

You may alos be able to do the same as tcp zerocopy and
hold an sk reference on the notification skb.