[RFC]: Support for zero-copy TCP transmit of user space data

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Hello linux-mm,

Recently I submitted a new SCSI target framework (SCST) and 4 target drivers for it for the first iteration of review and comments. See http://lkml.org/lkml/2008/12/10/245 for details.

An iSCSI target driver iSCSI-SCST was a part of the patchset (http://lkml.org/lkml/2008/12/10/293). For it a nice optimization to have TCP zero-copy transmit of user space data was implemented. Patch, implementing this optimization was also sent in the patchset, see http://lkml.org/lkml/2008/12/10/296.

I would like to ask, if the approach used in this patch can be acceptable from your point of view? I understand, that extending struct page is a very much undesirable, but, from other side:

- This approach is very simple and straightforward. The patch is only 309 lines long, including comments. All other alternative implementations would be at least an order of magnitude more complicated.

- Related kernel config option TCP_ZERO_COPY_TRANSFER_COMPLETION_NOTIFICATION should be disabled by default in general distro kernels, so the would be no harm at all from this patch. ISCSI-SCST can work without this patch or with TCP_ZERO_COPY_TRANSFER_COMPLETION_NOTIFICATION option disabled, although with user space device handlers it will work considerably worse. Only few distro kernels users need an iSCSI target and only few among such users need to use user space device handlers. People who need both iSCSI target *and* fast working user space device handlers would simply enable that option and rebuild the kernel. Rejecting this patch provides much worse alternative: those people would also have to *patch* the kernel at first, only then enable that option, then rebuild the kernel.

- Although usage of struct page to keep network related pointer might look as a layering violation, it isn't. I wrote in http://lkml.org/lkml/2008/12/15/190 why.

Thanks,
Vlad
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