linux-kernel - Re: [V9fs-developer] [PATCH] 9p: trans_fd, initialize recv fcall properly if not set

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Date:	Sun, 6 Sep 2015 08:55:40 +0200
From:	Dominique Martinet <dominique.martinet@....fr>
To:	Eric Van Hensbergen <ericvh@...il.com>
Cc:	Latchesar Ionkov <lucho@...kov.net>,
	V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
	netdev@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ron Minnich <rminnich@...dia.gov>
Subject: Re: [V9fs-developer] [PATCH] 9p: trans_fd, initialize recv fcall
 properly if not set

Eric Van Hensbergen wrote on Sat, Sep 05, 2015:
> On Thu, Sep 3, 2015 at 4:38 AM, Dominique Martinet
> <dominique.martinet@....fr> wrote:
> > To be honest, I think it might be better to just bail out if we get in
> > this switch (m->req->rc == NULL after p9_tag_lookup) and not try to
> > allocate more, because if we get there it's likely a race condition and
> > silently re-allocating will end up in more troubles than trying to
> > recover is worth.
> > Thoughts ?
> >
> 
> Hmmm...trying to rattle my brain and remember why I put it in there
> back in 2008.
> It might have just been over-defensive programming -- or more likely it just
> pre-dated all the zero copy infrastructure which pretty much guaranteed we had
> an rc allocated and what is there is vestigial.  I'm happy to accept a
> patch which
> makes this an assert, or perhaps just resets the connection because something
> has gone horribly wrong (similar to the ENOMEM path that is there now).

Yeah, it looks like the safety comes from the zero-copy stuff that came
much later.
Let's go with resetting the connection then. Hmm. EIO is a bit too
generic so would be good to avoid that if possible, but can't think of
anything better...

Speaking of zero-copy, I believe it should be fairly straight-forward to
implement for trans_fd now I've actually looked at it, since we do the
payload read after a p9_tag_lookup, would just need m->req to point to a
zc buffer. Write is similar, if there's a zc buffer just send it after
the header.
The cost is a couple more pointers in req and an extra if in both
workers, that seems pretty reasonable.

Well, I'm not using trans_fd much here (and unfortunately zero-copy
isn't possible at all given the transport protocol for RDMA, at least
for recv), but if anyone cares it probably could be done without too
much hassle for the fd workers.

-- 
Dominique
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