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

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Tuesday, December 18, 2012, 4:26:38 PM, you wrote:

> 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?

Hi Ian,

It ran overnight and i haven't seen the warn_once trigger.
(but i also didn't with the previous patch)

--
Sander

> Ian.




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