linux-kernel - Re: iov_iter

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Date:   Mon, 11 Sep 2017 10:31:13 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Dave Jones <davej@...emonkey.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org
Subject: Re: iov_iter_pipe warning.

On Mon, Sep 11, 2017 at 12:07:23AM +0100, Al Viro wrote:
> On Mon, Sep 11, 2017 at 08:08:14AM +1000, Dave Chinner wrote:
> > On Sun, Sep 10, 2017 at 10:19:07PM +0100, Al Viro wrote:
> > > On Mon, Sep 11, 2017 at 07:11:10AM +1000, Dave Chinner wrote:
> > > > On Sun, Sep 10, 2017 at 03:57:21AM +0100, Al Viro wrote:
> > > > > On Sat, Sep 09, 2017 at 09:07:56PM -0400, Dave Jones wrote:
> > > > > 
> > > > > > With this in place, I'm still seeing -EBUSY from invalidate_inode_pages2_range
> > > > > > which doesn't end well...
> > > > > 
> > > > > Different issue, and I'm not sure why that WARN_ON() is there in the
> > > > > first place.  Note that in a similar situation generic_file_direct_write()
> > > > > simply buggers off and lets the caller do buffered write...
> > > > 
> > > > XFS does not fall back to buffered IO when direct IO fails.  A
> > > > direct IO failure is indicative of a problem that needs to be fixed,
> > > > not use a "let's hope we can hide this" fallback path. Especially in
> > > > this case - EBUSY usually comes from the app is doing something we
> > > > /know/ is dangerous and it's occurrence to completely timing
> > > > dependent - if the timing is slightly different, we miss detection
> > > > and that can lead to silent data corruption.
> > > 
> > > In this case app is a fuzzer, which is bloody well supposed to poke
> > > into all kinds of odd usage patterns, though...
> > 
> > Yup, and we have quite a few tests in xfstests that specifically
> > exercise this same dark corner. We filter out these warnings from
> > the xfstests that exercise this case, though, because we know they
> > are going to be emitted and so aren't a sign of test failures...
> 
> BTW, another problem I see there is that iomap_dio_actor() should *NOT*
> assume that do-while loop in there will always manage to shove 'length'
> bytes out in case of success.  That is simply not true for pipe-backed
> destination.

splice does not go down the direct IO path, so iomap_dio_actor()
should never be handled a pipe as the destination for the IO data.
Indeed, splice read has to supply the pages to be put into the pipe,
which the DIO path does not do - it requires pages be supplied to
it. So I'm not sure why we'd care about pipe destination limitations
in the DIO path?

> And I'm not sure if outright failures halfway through
> are handled correctly.  What does it need a copy of dio->submit.iter for,
> anyway?  Why not work with dio->submit.iter directly?

No idea - that's a question for Christoph...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com