netdev - Re: [PATCH net 1/4] Revert "net/smc: don't wait for send buffer space when data was already sent"

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Message-ID: <20211027084710.1f4a4ff1@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 27 Oct 2021 08:47:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Karsten Graul <kgraul@...ux.ibm.com>
Cc:     Tony Lu <tonylu@...ux.alibaba.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-rdma@...r.kernel.org, jacob.qi@...ux.alibaba.com,
        xuanzhuo@...ux.alibaba.com, guwen@...ux.alibaba.com,
        dust.li@...ux.alibaba.com
Subject: Re: [PATCH net 1/4] Revert "net/smc: don't wait for send buffer
 space when data was already sent"

On Wed, 27 Oct 2021 17:38:27 +0200 Karsten Graul wrote:
> What we found out was that applications called sendmsg() with large data
> buffers using blocking sockets. This led to the described situation, were the
> solution was to early return to user space even if not all data were sent yet.
> Userspace applications should not have a problem with the fact that sendmsg()
> returns a smaller byte count than requested.
> 
> Reverting this patch would bring back the stalled connection problem.

I'm not sure. The man page for send says:

       When the message does not fit into  the  send  buffer  of  the  socket,
       send()  normally blocks, unless the socket has been placed in nonblock‐
       ing I/O mode.  In nonblocking mode it would fail with the error  EAGAIN
       or  EWOULDBLOCK in this case.

dunno if that's required by POSIX or just a best practice.