linux-kernel - Re: [PATCH V3] RDMA/siw: Convert siw_tx_hdt() to kmap_local

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Message-ID: <20210624173316.GA2799309@iweiny-DESK2.sc.intel.com>
Date:   Thu, 24 Jun 2021 10:33:16 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Bernard Metzler <BMT@...ich.ibm.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Mike Marciniszyn <mike.marciniszyn@...nelisnetworks.com>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Doug Ledford <dledford@...hat.com>,
        Faisal Latif <faisal.latif@...el.com>,
        Shiraz Saleem <shiraz.saleem@...el.com>,
        Kamal Heib <kheib@...hat.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3] RDMA/siw: Convert siw_tx_hdt() to kmap_local_page()

On Thu, Jun 24, 2021 at 03:45:55PM +0000, Bernard Metzler wrote:
> 
> >@@ -593,7 +601,7 @@ static int siw_tx_hdt(struct siw_iwarp_tx *c_tx,
> >struct socket *s)
> > 	} else {
> > 		rv = kernel_sendmsg(s, &msg, iov, seg + 1,
> > 				    hdr_len + data_len + trl_len);
> >-		siw_unmap_pages(page_array, kmap_mask);
> >+		siw_unmap_pages(iov, kmap_mask, seg+1);
> 
> seg+1 is one to many, since the last segment references the iWarp
> trailer (CRC). There are 2 reason for this multi-segment processing
> in the transmit path. (1) efficiency and (2) MTU based packet framing.
> The iov contains the complete iWarp frame with header, (potentially
> multiple) data fragments, and the CRC. It gets pushed to TCP in one
> go, praying for iWarp framing stays intact (which most time works).
> So the code can collect data form multiple SGE's of a WRITE or
> SEND and tries putting those into one frame, if MTU allows, and
> adds header and trailer. 
>
> The last segment (seg + 1) references the CRC, which is never kmap'ed.

siw_unmap_pages() take a length and seg is the index...

But ok so a further optimization...

Fair enough.

> 
> I'll try the code next days, but it looks good otherwise!
 
I believe this will work though.

Ira

> Thanks very much!
> > 	}
> > 	if (rv < (int)hdr_len) {
> > 		/* Not even complete hdr pushed or negative rv */
> >-- 
> >2.28.0.rc0.12.gb6a658bd00c9
> >
> >