linux-kernel - Re: [PATCH v2] tcp: splice as many packets as possible at once

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Message-Id: <200902032238.08497.nickpiggin@yahoo.com.au>
Date:	Tue, 3 Feb 2009 22:38:06 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Miller <davem@...emloft.net>
Cc:	jarkao2@...il.com, zbr@...emap.net, herbert@...dor.apana.org.au,
	w@....eu, dada1@...mosbay.com, ben@...s.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH v2] tcp: splice as many packets as possible at once

On Saturday 31 January 2009 08:42:27 David Miller wrote:
> From: Jarek Poplawski <jarkao2@...il.com>
> Date: Tue, 27 Jan 2009 07:40:48 +0000
>
> > I think the main problem is to respect put_page() more, and maybe you
> > mean to add this to your allocator too, but using slab pages for this
> > looks a bit complex to me, but I can miss something.
>
> Hmmm, Jarek's comments here made me realize that we might be
> able to do some hack with cooperation with SLAB.
>
> Basically the idea is that if the page count of a SLAB page
> is greater than one, SLAB will not use that page for new
> allocations.

Wouldn't your caller need to know what objects are already
allocated in that page too?

> It's cheesy and the SLAB developers will likely barf at the
> idea, but it would certainly work.

It is nasty, yes. Using the page allocator directly seeems
like a better approach.

And btw. be careful of using page->_count for anything, due
to speculative page references... basically it is useful only
to test zero or non-zero refcount. If designing a new scheme
for the network layer, it would be nicer to begin by using
say _mapcount or private or some other field in there for a
refcount (and I have a patch to avoid the atomic
put_page_testzero in page freeing for a caller that does their
own refcounting, so don't fear that extra overhead too much :)).

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