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    SubjectRe: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of user space data
    Hi Vladislav.

    On Wed, Dec 10, 2008 at 10:04:36PM +0300, Vladislav Bolkhovitin (vst@vlnb.net) wrote:
    > In the chosen approach new optional field void *net_priv was added to
    > struct page. It is enclosed by

    There is a huge no-no in networking land on increasing skb.
    Reason is simple every skb will carry potentially unneded data as long
    as given option is enabled, and most of the time it will.
    To break this barrier one has to have (I wanted to write ego, but then
    decided to replace it with mojo) so huge reason to do this, that it is
    almost impossible to have.

    Something tells me that increasing page structure with 8 bytes because
    of zero-copy iscsi transfer is not that great idea, since basically every
    user out there will have it enabled in the distro config and will waste
    noticeble amount of ram.

    The same problem of not sending any kind of notification to the user
    when his pages are 'acked' by receiving some packet or freeing the data
    exists long ago and was tried to be fixed several times.

    The most applicable to your case maybe DST experience. DST is a block
    layer device and all its pages starting from quite recent kernels are
    not allowed to be slab ones (xfs was the last one who provided slab
    pages in the bios), so each page has two 'unused' pointers in lru list
    entry, which you may reuse. If scsi layer may have slab pages from some
    place (although this does not sound like a good idea, ->sendpage() will
    bug on on them anyway), this hack will not work, otherwise you only need
    to have net_page_get/put stuff in and do not mess with increasing page.
    And this was tested 3-4 kernel releases ago, so things may be changed.

    Another appropach is to increase skb's shared data (at the end of the
    skb->data), and this approach was not frowned upon too much either, but
    it requires to mess with skb->destructor, which may not be appropriate
    in some cases. If iscsi does not use sockets (it does iirc), things are
    much simpler.

    Hope this helps.

    --
    Evgeniy Polyakov


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