linux-kernel - Re: [PATCH 4.19 114/149] UAS: Revert commit 3ae62a42090f ("UAS: fix alignment of scatter/gather segments")

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Date:   Sun, 10 Nov 2019 10:34:02 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Oliver Neukum <oneukum@...e.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 4.19 114/149] UAS: Revert commit 3ae62a42090f ("UAS: fix
 alignment of scatter/gather segments")

On Sun, 10 Nov 2019, Oliver Neukum wrote:

> Am Freitag, den 08.11.2019, 10:35 -0500 schrieb Alan Stern:
> > On Fri, 8 Nov 2019, Greg Kroah-Hartman wrote:
> > 
> > > On Thu, Nov 07, 2019 at 12:32:45PM +0100, Oliver Neukum wrote:
> > > > Am Dienstag, den 05.11.2019, 17:38 +0100 schrieb Greg Kroah-Hartman:
> > > > > > > Given this information, perhaps you will decide that the revert is 
> > > > > > > worthwhile.
> > > > > > 
> > > > > > Damned if I do, damned if I do not.
> > > > > > Check for usbip and special case it?
> > > > > 
> > > > > We should be able to do that in the host controller driver for usbip,
> > > > > right?  What is the symptom if you use a UAS device with usbip and this
> > > > > commit?
> > > > 
> > > > Yes, that patch should then also be applied. Then it will work.
> > > > Without it, commands will fail, as transfers will end prematurely.
> > > 
> > > Ok, I'm confused now.  I just checked, and I really have no idea what
> > > needs to be backported anymore.  3ae62a42090f ("UAS: fix alignment of
> > > scatter/gather segments") was backported to all of the stable kernels,
> > > and now we reverted it.
> > > 
> > > So what else needs to be done here?
> > 
> > In one sense, nothing needs to be done.  3ae62a42090f was intended to
> > fix a long-standing problem with USBIP, but people reported a
> 
> OK, now I am a bit confused. AFAICT 3ae62a42090f actually did fix the
> issue. So if you simply revert it, the issue will reappear.

Correct.  I think.

> > regression in performance.  (Admittedly, the report was about the
> > correponding change to usb-storage, not the change to uas, but it's
> > reasonable to think the effect would be the same.)  So in line with the
> 
> Yes.
> 
> > no-regressions policy, we only need to revert the commit -- which you 
> > have already done.
> 
> But that breaks UAS over USBIP, doesn't it?

It was already broken to start with.  The attempted fix caused a
regression, so the fix must be reverted.

> > On the other hand, the long-standing problem in USBIP can be fixed by
> > back-porting commit ea44d190764b.  But since that commit isn't a
> > bug-fix (and since it's rather large), you may question whether it is
> > appropriate for the -stable kernel series.
> 
> It certainly is large. But without it UAS won't work over USBIP, will
> it? I think that is the central question we need to answer here.

You are right.  If Greg is okay with porting ea44d190764b to the stable 
kernels, I won't object.

Alan Stern