netdev - Re: [PATCH] tcp_sendpage(): fix broken page iteration

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Message-ID: <20070318224009.GA15407@localdomain>
Date:	Mon, 19 Mar 2007 00:40:10 +0200
From:	Dan Aloni <da-x@...atomic.org>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp_sendpage(): fix broken page iteration

On Sun, Mar 18, 2007 at 02:49:27PM -0700, David Miller wrote:
> From: Dan Aloni <da-x@...atomic.org>
> Date: Sun, 18 Mar 2007 14:43:46 +0200
> 
> > do_tcp_sendpages() should not iterate 'pages' as an array since 
> > it is not an array of 'struct page *', but a pointer to a single 
> > entity of 'struct page *' passed on the stack as a parameter to 
> > tcp_send_page() (hence it would crash if poffset + psize > PAGE_SIZE,
> > because pages[1] and beyond most probably not constitutes a valid 
> > 'struct page *').
> 
> do_tcp_sendpages() should never get passed poffset+psize>PAGE_SIZE,
> that would be a bug.

Oh, then the name of that function was quite misleading...

Anyway, I thought it would make a valid case for a situation where
you have a kmalloc'ed buffer that happens to cross a page boundery 
and you want to call ->sendpage() to send it over using network
DMA. 

As I see it, with this constraint you either call sendpage twice or
you use kernel_sendmsg(), I am not sure which would me more 
efficient - I guess it depends on psize. I wish there was a better
interface than sendpage that would have factored it in...

Thanks anyway for the heads up.

-- 
Dan Aloni
XIV LTD, http://www.xivstorage.com
da-x (at) monatomic.org, dan (at) xiv.co.il
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