Re: iov_iter_pipe warning.

[Al Viro]

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.  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?