netdev - Re: [PATCH] TX_RING and packet mmap

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Date:	Sun, 12 Apr 2009 21:27:37 +0200
From:	Johann Baudy <johann.baudy@...-log.net>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>, jamal <hadi@...erus.ca>
Subject: Re: [PATCH] TX_RING and packet mmap

I've seen that during skb_linearize(), skb_shinfo(skb)->frags[i].page
is not lost even if pages has been put. So I can get back original
frag address from this info in skb destructor. Can I rely on
skb_shinfo(skb)->frags[i].page on more complicated sending paths ... ?

Thanks in advance,
Johann





On Sun, Apr 12, 2009 at 4:24 PM, Evgeniy Polyakov <zbr@...emap.net> wrote:
> On Sun, Apr 12, 2009 at 01:23:30PM +0200, Johann Baudy (johann.baudy@...-log.net) wrote:
>> > Page itself will live until destruction time, when skb is linearized
>> > page is freed, so your own destructor will find that given page is no
>> > longer used, and appropriate memory area can be overwritten.
>> Ok, so if my understanding is correct, fragment data is copied, frags
>> info (page, offset, size ...) of  skb_shinfo(skb) remain available and
>> skb->data_len is equal to 0 (to tag it as linear).
>> Then I can use address of skb_shinfo(n)->frags[0].page to derive my
>> header whatever sending path.
>
> No, I was wrong, linearization may silently put pages without calling a
> destructor.
>
> --
>        Evgeniy Polyakov
>



-- 
Johann Baudy
johaahn@...il.com
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