netdev - Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the addresss returned by vmalloc()

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Message-ID: <20101130143720.GA6017@hmsreliant.think-freely.org>
Date:	Tue, 30 Nov 2010 09:37:20 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Changli Gao <xiaosuo@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jpirko@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] af_packet: use vmalloc_to_page() instead for the
 addresss returned by vmalloc()

On Tue, Nov 30, 2010 at 03:12:21PM +0100, Eric Dumazet wrote:
> Le mardi 30 novembre 2010 à 21:56 +0800, Changli Gao a écrit :
> > The following commit causes the pgv->buffer may point to the memory
> > returned by vmalloc(). And we can't use virt_to_page() for the vmalloc
> > address.
> > 
> > This patch introduces a new inline function pgv_to_page(), which calls
> > vmalloc_to_page() for the vmalloc address, and virt_to_page() for the
> > __get_free_pages address.
> > 
> >     commit 0e3125c755445664f00ad036e4fc2cd32fd52877
> >     Author: Neil Horman <nhorman@...driver.com>
> >     Date:   Tue Nov 16 10:26:47 2010 -0800
> > 
> >     packet: Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v4)
> > 
> 
> nice catch.
> 
Ouch, yes, thanks.

> > Signed-off-by: Changli Gao <xiaosuo@...il.com>
> > ---
> >  net/packet/af_packet.c |   21 ++++++++++++++-------
> >  1 file changed, 14 insertions(+), 7 deletions(-)
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index 422705d..0171b20 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -224,6 +224,13 @@ struct packet_skb_cb {
> >  
> >  #define PACKET_SKB_CB(__skb)	((struct packet_skb_cb *)((__skb)->cb))
> >  
> > +static inline struct page *pgv_to_page(void *addr)
> > +{
> > +	if (is_vmalloc_addr(addr))
> > +		return vmalloc_to_page(addr);
> 
> Hmm, I am wondering if calling vmalloc_to_page(addr) several times for
> each packet is not too expensive ? I believe it is.
> 
> What about caching "struct page *" pointer somewhere ?
> 
> Then later we have :
> 
> > -		p_start = virt_to_page(h.raw);
> > -		p_end = virt_to_page(h_end);
> > +		p_start = pgv_to_page(h.raw);
> > +		p_end = pgv_to_page(h_end);
> >  		while (p_start <= p_end) {
> >  			flush_dcache_page(p_start);
> >  			p_start++;
> 
> This was OK before Neil patch... after vmalloc(), assumption that
> p_start can be incremented is completely wrong.
> 
> To fix this, we need something else than your patch.
> 
Off the top of my head, I would think that pgv_to_page could be prototyped such
that it could accept addr, offset and struct page ** arguments.  That way we can
track the current page that we're mapped to, lowering the number of calls to
vmalloc_to_page, and we can still use an increment like we do above (as long as
its wrapped in a subsequent call to pgv_to_page)
Neil

> 
> 
> 
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