RE: [PATCH] RDMA/siw: Fix the sendmsg byte count in siw_tcp_sendpages

[Bernard Metzler <BMT@zurich.ibm.com>]

> -----Original Message-----
> From: Pedro Falcato <pfalcato@suse.de>
> Sent: Wednesday, 23 July 2025 17:49
> To: Bernard Metzler <BMT@zurich.ibm.com>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>; Leon Romanovsky <leon@kernel.org>;
> Vlastimil Babka <vbabka@suse.cz>; Jakub Kicinski <kuba@kernel.org>; David
> Howells <dhowells@redhat.com>; Tom Talpey <tom@talpey.com>; linux-
> rdma@vger.kernel.org; linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> stable@vger.kernel.org; kernel test robot <oliver.sang@intel.com>
> Subject: [EXTERNAL] Re: [PATCH] RDMA/siw: Fix the sendmsg byte count in
> siw_tcp_sendpages
> 
> On Wed, Jul 23, 2025 at 02:52:12PM +0000, Bernard Metzler wrote:
> >
> >
> > > -----Original Message-----
> > > From: Pedro Falcato <pfalcato@suse.de>
> > > Sent: Wednesday, 23 July 2025 12:41
> > > To: Jason Gunthorpe <jgg@ziepe.ca>; Bernard Metzler
> <BMT@zurich.ibm.com>;
> > > Leon Romanovsky <leon@kernel.org>; Vlastimil Babka <vbabka@suse.cz>
> > > Cc: Jakub Kicinski <kuba@kernel.org>; David Howells
> <dhowells@redhat.com>;
> > > Tom Talpey <tom@talpey.com>; linux-rdma@vger.kernel.org; linux-
> > > kernel@vger.kernel.org; linux-mm@kvack.org; Pedro Falcato
> > > <pfalcato@suse.de>; stable@vger.kernel.org; kernel test robot
> > > <oliver.sang@intel.com>
> > [snip]
> > > ---
> > >  drivers/infiniband/sw/siw/siw_qp_tx.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/infiniband/sw/siw/siw_qp_tx.c
> > > b/drivers/infiniband/sw/siw/siw_qp_tx.c
> > > index 3a08f57d2211..9576a2b766c4 100644
> > > --- a/drivers/infiniband/sw/siw/siw_qp_tx.c
> > > +++ b/drivers/infiniband/sw/siw/siw_qp_tx.c
> > > @@ -340,11 +340,11 @@ static int siw_tcp_sendpages(struct socket *s,
> struct
> > > page **page, int offset,
> > >  		if (!sendpage_ok(page[i]))
> > >  			msg.msg_flags &= ~MSG_SPLICE_PAGES;
> > >  		bvec_set_page(&bvec, page[i], bytes, offset);
> > > -		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, size);
> > > +		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, bytes);
> > >
> > >  try_page_again:
> > >  		lock_sock(sk);
> > > -		rv = tcp_sendmsg_locked(sk, &msg, size);
> > > +		rv = tcp_sendmsg_locked(sk, &msg, bytes);
> > >  		release_sock(sk);
> > >
> >
> > Pedro, many thanks for catching this! I completely
> > missed it during my too sloppy review of that patch.
> > It's a serious bug which must be fixed asap.
> > BUT, looking closer, I do not see the offset being taken
> > into account when retrying a current segment. So,
> > resend attempts seem to send old data which are already
> > out. Shouldn't the try_page_again: label be above
> > bvec_set_page()??
> 
> This was raised off-list by Vlastimil - I think it's harmless to bump (but
> not use)
> the offset here, because by reusing the iov_iter we progressively consume
> the data
> (it keeps its own size and offset tracking internally). So the only thing
> we
> need to track is the size we pass to tcp_sendmsg_locked[1].
> 
Ah okay, I didn't know that. Are we sure? I am currently travelling and have
only limited possibilities to try out things. I just looked up other
use cases and found one in net/tls/tls_main.c#L197. Here the loop looks
very similar, but it works as I was suggesting (taking offset into account
and re-initializing new bvec in case of partial send).

> If desired (and if my logic is correct!) I can send a v2 deleting that bit.
> 

So yes if that's all save, please. We shall not have dead code.

Thanks!
Bernard.
> 
> [1] Assuming tcp_sendmsg_locked guarantees it will never consume something
> out
> of the iovec_iter without reporting it as bytes copied, which from a code
> reading
> it seems like it won't...
> 
> 
> --
> Pedro