netdev - RE: [patch v1, kernel version 3.2.1] rtnetlink workaround around the skb buff size issue

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Date:	Tue, 7 Feb 2012 18:59:25 +0000
From:	"Rose, Gregory V" <gregory.v.rose@...el.com>
To:	David Miller <davem@...emloft.net>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>
CC:	"steweg@...t.sk" <steweg@...t.sk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [patch v1, kernel version 3.2.1] rtnetlink workaround around
 the skb buff size issue

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org]
> On Behalf Of David Miller
> Sent: Tuesday, February 07, 2012 10:32 AM
> To: bhutchings@...arflare.com
> Cc: Rose, Gregory V; steweg@...t.sk; linux-kernel@...r.kernel.org;
> netdev@...r.kernel.org
> Subject: Re: [patch v1, kernel version 3.2.1] rtnetlink workaround around
> the skb buff size issue
> 
> From: Ben Hutchings <bhutchings@...arflare.com>
> Date: Tue, 7 Feb 2012 18:26:32 +0000
> 
> > On Tue, 2012-02-07 at 00:13 +0000, Rose, Gregory V wrote:
> >> > -----Original Message-----
> >> > From: Ben Hutchings [mailto:bhutchings@...arflare.com]
> >> > Sent: Monday, February 06, 2012 3:51 PM
> >> > To: Rose, Gregory V
> >> > Cc: David Miller; steweg@...t.sk; linux-kernel@...r.kernel.org;
> >> > netdev@...r.kernel.org
> >> > Subject: RE: [patch v1, kernel version 3.2.1] rtnetlink workaround
> around
> >> > the skb buff size issue
> >> >
> >> > On Mon, 2012-02-06 at 04:41 +0000, Rose, Gregory V wrote:
> >> > [...]
> >> > > > This is not how we're going to fix this.  I already stated the
> desired
> >> > > > way to fix this, which is to make the existing dump request have
> a way
> >> > > > for the requestor to enable extended parts of the device dump.
> >> > > >
> >> > > > This is just like netlink diag socket dumps, where the dump
> request
> >> > > > specifies what the user wants to see.
> >> > > >
> >> > > > In this case we'd add a netlink attribute to the dump request
> which
> >> > > > is just a u32 bitmask or similar.
> >> > > >
> >> > > > The Intel engineer who added the VF dump support said he would
> work on
> >> > > > this fix so why don't you just wait patiently for him to do the
> work?
> >> > >
> >> > > The patch below is what I've got so far.  Right now the bit mask
> array
> >> > > is global so if you enable display of VF (n) on one interface it
> will
> >> > > enable display of the same VF on other interfaces.  I intend to
> move
> >> > > the bit mask array into the net_device structure so we can set the
> >> > > display mask for each interface independently.
> >> >
> >> > I don't think this can work.  Using an application that requests VF
> >> > information and uses large buffers (e.g. the updated 'ip') will break
> >> > another application that doesn't (e.g. current Network Manager),
> won't
> >> > it?
> >>
> >> It's my understanding that the problem isn't with the application
> >> buffer size but with the kernel buffer size, which is limited to a
> >> page.
> > [...]
> >
> > Then it's no wonder you're going about this the wrong way.
> 
> It is the userland buffer size that is the problem, userland will allocate
> max(8192, PAGE_SIZE) for it's recvmsg() call, that's why we have to only
> dump VF's and other potentially large objects when the user enables it in
> the request.

Ah well, when that was stated before I was under the impression it was in the kernel.  My bad...

Anyway, if Ben would like to do this work it's fine by me.  He seems to have some very definite opinions about this and I don't feel like going through another several weeks of fighting about it him.  I've got a patch ready to submit today that goes along the lines of what I've already posted but with a few clean ups.  I'll post it, if it gets shot down then fine, I'll get on with my other work.

- Greg

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