Re: [Xen-devel] [PATCH] xen/netfront: improve truesize tracking

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-12-18 at 15:12 +0000, Eric Dumazet wrote:
> On Tue, 2012-12-18 at 13:51 +0000, Ian Campbell wrote:
> > Using RX_COPY_THRESHOLD is incorrect if the SKB is actually smaller
> > than that. We have already accounted for this in
> > NETFRONT_SKB_CB(skb)->pull_to so use that instead.
> > 
> > Fixes WARN_ON from skb_try_coalesce.
> > 
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: annie li <annie.li@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Cc: netdev@xxxxxxxxxxxxxxx
> > Cc: stable@xxxxxxxxxx # 3.7.x only
> > ---
> >  drivers/net/xen-netfront.c |   15 +++++----------
> >  1 files changed, 5 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> > index caa0110..b06ef81 100644
> > --- a/drivers/net/xen-netfront.c
> > +++ b/drivers/net/xen-netfront.c
> > @@ -971,17 +971,12 @@ err:
> >              * overheads. Here, we add the size of the data pulled
> >              * in xennet_fill_frags().
> >              *
> > -            * We also adjust for any unused space in the main
> > -            * data area by subtracting (RX_COPY_THRESHOLD -
> > -            * len). This is especially important with drivers
> > -            * which split incoming packets into header and data,
> > -            * using only 66 bytes of the main data area (see the
> > -            * e1000 driver for example.)  On such systems,
> > -            * without this last adjustement, our achievable
> > -            * receive throughout using the standard receive
> > -            * buffer size was cut by 25%(!!!).
> > +            * We also adjust for the __pskb_pull_tail done in
> > +            * handle_incoming_queue which pulls data from the
> > +            * frags into the head area, which is already
> > +            * accounted in RX_COPY_THRESHOLD.
> >              */
> > -           skb->truesize += skb->data_len - RX_COPY_THRESHOLD;
> > +           skb->truesize += skb->data_len - NETFRONT_SKB_CB(skb)->pull_to;
> >             skb->len += skb->data_len;
> >  
> >             if (rx->flags & XEN_NETRXF_csum_blank)
> 
> 
> But skb truesize is not what you think.

Indeed, it seems I was completely backwards about what it means!

> You must account the exact memory used by this skb, not only the used
> part of it.
> 
> At the very minimum, it should be
> 
> skb->truesize += skb->data_len;
> 
> But it really should be the allocated size of the fragment.
> 
> If its a page, then its a page, even if you use one single byte in it.

So actually we want += PAGE_SIZE * skb_shinfo(skb)->nr_frags ?

Sander, can you try that change?

Ian.


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