netdev - Re: [PATCH] tcp: splice as many packets as possible at once

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Date:	Sun, 18 Jan 2009 22:16:03 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	w@....eu, jarkao2@...il.com, zbr@...emap.net, 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] tcp: splice as many packets as possible at once

From: David Miller <davem@...emloft.net>
Date: Thu, 15 Jan 2009 15:34:49 -0800 (PST)

> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Fri, 16 Jan 2009 10:32:05 +1100
> 
> > On Thu, Jan 15, 2009 at 03:26:08PM -0800, David Miller wrote:
> > > +static inline struct page *linear_to_page(struct page *page, unsigned int len,
> > > +					  unsigned int offset)
> > > +{
> > > +	struct page *p = alloc_pages(GFP_KERNEL, 0);
> > > +
> > > +	if (!p)
> > > +		return NULL;
> > > +	memcpy(page_address(p) + offset, page_address(page) + offset, len);
> > 
> > This won't work very well if skb->head is longer than a page.
> > 
> > We'll need to divide it up into individual pages.
> 
> Oh yes the same bug I pointed out the other day.
> 
> But Willy can test this patch as-is, since he is not using
> jumbo frames in linear SKBs.

Actually, Herbert, it turns out this case should be OK.

Look a level or two higher, at __splice_segment(), it even has a
comment :-)

--------------------
	do {
		unsigned int flen = min(*len, plen);

		/* the linear region may spread across several pages  */
		flen = min_t(unsigned int, flen, PAGE_SIZE - poff);

		if (spd_fill_page(spd, page, flen, poff, skb, linear))
			return 1;

		__segment_seek(&page, &poff, &plen, flen);
		*len -= flen;

	} while (*len && plen);
--------------------

That code should work and do what we need.
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