netdev - RE: [PATCH v12 net-next 1/1] hv_sock: introduce Hyper-V Sockets

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Date:	Wed, 29 Jun 2016 09:17:11 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Rick Jones <rick.jones2@....com>,
	David Miller <davem@...emloft.net>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"cavery@...hat.com" <cavery@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"joe@...ches.com" <joe@...ches.com>
Subject: RE: [PATCH v12 net-next 1/1] hv_sock: introduce Hyper-V Sockets

> From: Rick Jones [mailto:rick.jones2@....com]
> Sent: Tuesday, June 28, 2016 23:43
> To: Dexuan Cui <decui@...rosoft.com>; David Miller <davem@...emloft.net>
> Cc: gregkh@...uxfoundation.org; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; jasowang@...hat.com; vkuznets@...hat.com;
> cavery@...hat.com; KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; joe@...ches.com
> Subject: Re: [PATCH v12 net-next 1/1] hv_sock: introduce Hyper-V Sockets
> 
> On 06/28/2016 02:59 AM, Dexuan Cui wrote:
> > The idea here is: IMO the syscalls sys_read()/write() shoudn't return
> > -ENOMEM, so I have to make sure the buffer allocation succeeds?
> >
> > I tried to use kmalloc with __GFP_NOFAIL, but I hit a warning in
> > in mm/page_alloc.c:
> > WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> >
> > What error code do you think I should return?
> > EAGAIN, ERESTARTSYS, or something else?
> >
> > May I have your suggestion? Thanks!
> 
> What happens as far as errno is concerned when an application makes a
> read() call against a (say TCP) socket associated with a connection
> which has been reset? 
I suppose it is ECONNRESET (Connection reset by peer).

>  Is it limited to those errno values listed in the
> read() manpage, or does it end-up getting an errno value from those
> listed in the recv() manpage?  Or, perhaps even one not (presently)
> listed in either?
> 
> rick jones

Actually "man read/write" says "Other errors may occur, depending on the
object connected to fd".

"man send/recv" indeed lists ENOMEM.

Considering AF_HYPERV is a new socket type, ENOMEM seems OK to me
and I'm going to post a new version of the patch.

In the long run, I think we should add a new API in the VMBus driver,
allowing data copy from VMBus ringbuffer into user mode buffer directly.
This way, we can even eliminate this temporary buffer.

Thanks,
-- Dexuan