linux-kernel - Re: [PATCH] RDMA/siw: Fix the sendmsg byte count in siw_tcp

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Message-ID: <l5cavrkmzvebjqz62ttdajqc24q3fksxogiibv4tiee7c3j2lk@skxdyrnsqgmm>
Date: Fri, 25 Jul 2025 17:09:57 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Bernard Metzler <BMT@...ich.ibm.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Jakub Kicinski <kuba@...nel.org>, 
	David Howells <dhowells@...hat.com>, Tom Talpey <tom@...pey.com>, 
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "stable@...r.kernel.org" <stable@...r.kernel.org>, 
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] RDMA/siw: Fix the sendmsg byte count in siw_tcp_sendpages

On Wed, Jul 23, 2025 at 04:49:30PM +0000, Bernard Metzler wrote:
> 
> 
> > -----Original Message-----
> > From: Pedro Falcato <pfalcato@...e.de>
> > Sent: Wednesday, 23 July 2025 17:49
> > To: Bernard Metzler <BMT@...ich.ibm.com>
> > Cc: Jason Gunthorpe <jgg@...pe.ca>; Leon Romanovsky <leon@...nel.org>;
> > Vlastimil Babka <vbabka@...e.cz>; Jakub Kicinski <kuba@...nel.org>; David
> > Howells <dhowells@...hat.com>; Tom Talpey <tom@...pey.com>; linux-
> > rdma@...r.kernel.org; linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> > stable@...r.kernel.org; kernel test robot <oliver.sang@...el.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@...e.de>
> > > > Sent: Wednesday, 23 July 2025 12:41
> > > > To: Jason Gunthorpe <jgg@...pe.ca>; Bernard Metzler
> > <BMT@...ich.ibm.com>;
> > > > Leon Romanovsky <leon@...nel.org>; Vlastimil Babka <vbabka@...e.cz>
> > > > Cc: Jakub Kicinski <kuba@...nel.org>; David Howells
> > <dhowells@...hat.com>;
> > > > Tom Talpey <tom@...pey.com>; linux-rdma@...r.kernel.org; linux-
> > > > kernel@...r.kernel.org; linux-mm@...ck.org; Pedro Falcato
> > > > <pfalcato@...e.de>; stable@...r.kernel.org; kernel test robot
> > > > <oliver.sang@...el.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].
> > 

Hi,

Sorry for the delay.

> 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

I'm not 100% sure, and if some more authoritative voice (David, or Jakub for
the net side) could confirm my analysis, it would be great.

> 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.

Understood. I'll send a v2 resetting the bvec and iov_iter if we get no further
feedback in the meanwhile.

> 
> 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

-- 
Pedro