Re: [PATCH] Fix guest memory leak and panic

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

Le mardi 18 octobre 2011 à 10:47 +0100, Ian Campbell a écrit :
> On Tue, 2011-10-18 at 09:36 +0100, Ian Campbell wrote:
> > I think the best thing might be to remove the additional ref taking from
> > the setter function and audit the previous changes to ensure they
> > conform. I'll do that right away and post a fixup patch ASAP.
> 
> Sigh, only one out of the ten callers of (__)skb_frag_set_page expects
> skb_frag_set_page to take a new reference. I think that's pretty
> comprehensive evidence that the current behaviour is unexpected and
> wrong. 
> 
> Sorry about this.
> 
> Ian.
> 
> 8<---------------------------------------------------------
> 
> From 42c26b7ca640bd5cb6f9c3bc76db96c92ed8ff82 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..c4effd4 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2603,6 +2603,7 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
>  					break;
>  			}
>  			skb_frag_set_page(skb, i, pkt_dev->page);
> +			skb_frag_ref(skb, i);
>  			skb_shinfo(skb)->frags[i].page_offset = 0;
>  			/*last fragment, fill rest of data*/
>  			if (i == (frags - 1))

I am ok with this patch, since it makes sense to let driver do the
page->_count change itself (it can use a batched add() in case a page is
splitted in many frags)

But I suggest using get_page(pkt_dev->page), this seems more obvious to
me [ This was how I wrote the thing ;) ]