Re: [PATCH] Fix guest memory leak and panic

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 19 octobre 2011 à 09:55 +0100, Ian Campbell a écrit :
> On Tue, 2011-10-18 at 14:20 +0100, Eric Dumazet wrote:
> > But I suggest using get_page(pkt_dev->page), this seems more obvious to
> > me [ This was how I wrote the thing ;) ]
> 
> I guess it depends on whether you consider the reference to be on the
> page or to be on the frag (which contains the page). The distinction
> would only matter if pktgen were to transition to using the forthcoming
> destructors though.
> 

Yes, but get_page() is now a clear sign we dirty a cache line, and I am
trying to explain to driver authors this is a contention point they
should address to get best performance from sub-page frags devices.

The hidden __skb_frag_ref() in skb_frag_set_page() was misleading
them ;)

> Here's a version like you describe:
> 
> 8<---------------------------------------------------------------
> 
> From 369022220db31efb9c261cbabcb890a4d216a176 Mon Sep 17 00:00:00 2001
> From: Ian Campbell <ian.campbell@citrix.com>
> Date: Tue, 18 Oct 2011 09:59:37 +0100
> Subject: [PATCH] net: do not take an additional reference in skb_frag_set_page
> 
> I audited all of the callers in the tree and only one of them (pktgen) expects
> it to do so. Taking this reference is pretty obviously confusing and error
> prone.
> 
> In particular I looked at the following commits which switched callers of
> (__)skb_frag_set_page to the skb paged fragment api:
> 
> 6a930b9f163d7e6d9ef692e05616c4ede65038ec cxgb3: convert to SKB paged frag API.
> 5dc3e196ea21e833128d51eb5b788a070fea1f28 myri10ge: convert to SKB paged frag API.
> 0e0634d20dd670a89af19af2a686a6cce943ac14 vmxnet3: convert to SKB paged frag API.
> 86ee8130a46769f73f8f423f99dbf782a09f9233 virtionet: convert to SKB paged frag API.
> 4a22c4c919c201c2a7f4ee09e672435a3072d875 sfc: convert to SKB paged frag API.
> 18324d690d6a5028e3c174fc1921447aedead2b8 cassini: convert to SKB paged frag API.
> b061b39e3ae18ad75466258cf2116e18fa5bbd80 benet: convert to SKB paged frag API.
> b7b6a688d217936459ff5cf1087b2361db952509 bnx2: convert to SKB paged frag API.
> 804cf14ea5ceca46554d5801e2817bba8116b7e5 net: xfrm: convert to SKB frag APIs
> ea2ab69379a941c6f8884e290fdd28c93936a778 net: convert core to skb paged frag APIs
> 
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> ---
>  include/linux/skbuff.h |    1 -
>  net/core/pktgen.c      |    1 +
>  2 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 64f8695..78741da 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1765,7 +1765,6 @@ static inline void *skb_frag_address_safe(const skb_frag_t *frag)
>  static inline void __skb_frag_set_page(skb_frag_t *frag, struct page *page)
>  {
>  	frag->page = page;
> -	__skb_frag_ref(frag);
>  }
>  
>  /**
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 796044a..e81f5d0 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2602,6 +2602,7 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
>  				if (!pkt_dev->page)
>  					break;
>  			}
> +			get_page(pkt_dev->page);
>  			skb_frag_set_page(skb, i, pkt_dev->page);
>  			skb_shinfo(skb)->frags[i].page_offset = 0;
>  			/*last fragment, fill rest of data*/

Thanks !

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>