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    SubjectRe: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user space data
    Rusty Russell, on 12/22/2008 03:43 AM wrote:
    > On Sunday 21 December 2008 06:09:18 Jeremy Fitzhardinge wrote:
    >> Evgeniy Polyakov wrote:
    >>> Things should work fine, since pskb_expand_head() copies whole shared
    >>> info structure (and thus will copy destructor), get all pages and then
    >>> copy all pointers into the new skb, and then release old skb's data.
    >>> So destructor for the pages should not rely on which skb it is called on
    >>> and check if pages are about to be really freed (i.e. check theirs
    >>> reference counter).
    >> OK.
    >>> __pskb_pull_tail() is tricky, it just puts some pages it does not want
    >>> to be present in the skb, but it could be possible to add there
    >>> destructor callback from the original skb with partial flag (or just
    >>> having destructor with two parameters: skb and page, and if page is not
    >>> NULL, then actually only given page is freed, otherwise the whole skb).
    >> Yes, that doesn't sound too bad.
    > That would be one approach. Actually, my patch solved this by keeping a
    > parent ref in various cases if the parent had a destructor: we only destroy
    > the parent when all the clones are gone.
    > Here's the patch for reference:
    > net: add destructor for skb data.
    > If we want to notify something when an skb is truly finished (such as
    > for tun vringfd support), we need a destructor on the data.
    > This turns out to be slightly non-trivial as fragments from one skb
    > get copied to another skb: if the first skb has a destructor (or its
    > parent does) we need to keep a reference to it and destroy it only
    > when (all the) children are destroyed. We add an 'orig' pointer to
    > the skb_shared_info to do this.
    > But there's currently no way to get from the shinfo to the head (to
    > kfree it), so we add a 'len' field. A better alternative to this
    > might be to move the skb_shared_info to before the head of the skb data.
    > Note that the destructor is responsible for calling kfree: for the tun
    > device, this is critical since the destructor can be called from any
    > context and it has to do a copy_to_user, so it queues the skb.


    Can you describe how one should use your patch, please? Maybe, there is
    some code you use to test it?


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