netdev - Re: [PATCH 21/62] net: remove the second argument of k[un]map

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Date:	Mon, 28 Nov 2011 19:26:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Cong Wang <amwang@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Bruce Allan <bruce.w.allan@...el.com>,
	Carolyn Wyborny <carolyn.wyborny@...el.com>,
	Don Skidmore <donald.c.skidmore@...el.com>,
	Greg Rose <gregory.v.rose@...el.com>,
	Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@...el.com>,
	John Ronciak <john.ronciak@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Dean Nelson <dnelson@...hat.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Jiri Pirko <jpirko@...hat.com>,
	e1000-devel@...ts.sourceforge.net, netdev@...r.kernel.org
Subject: Re: [PATCH 21/62] net: remove the second argument of
 k[un]map_atomic()

Le lundi 28 novembre 2011 à 10:06 -0800, Alexander Duyck a écrit :
> On 11/27/2011 12:07 AM, Eric Dumazet wrote:
> > But why are these drivers using kmap_atomic() in first place, since
> > their fragments are allocated in regular zone (GFP_ATOMIC or
> > GFP_KERNEL) ?
> 
> I was asking the same thing myself recently when I started working on
> some copy-break like code for the ixgbe driver.  I believe the main
> reason is a lack of documentation.  This code is based loosely on the
> skb_copy_bits code which will use kmap_skb_frag over all of the paged
> portions of the sk_buff.  As such it was decided to map things via
> kmap_atomic in order to guarantee the pages had a valid virtual address.
> 
> If I understand things correctly, what you are brining up is that pages
> allocated with either GFP_ATOMIC or GFP_KERNEL will always be allocated
> from the lowmem pool and as such page_address should always succeed.  Is
> that correct?
> 

Yes.

Either you could :

A) allocate high memory pages to lower pressure on LOWMEM area on 32bit
kernels.

B) avoid using kmap_atomic() & kunmap_atomic() and be faster on 32bit
kernels.

	skb_put(skb, length);
	memcpy(skb->data, page_address(buffer_info->page), length);

 



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