linux-kernel - Re: iov_iter

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Date:   Sun, 10 Sep 2017 22:19:07 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Dave Chinner <david@...morbit.com>
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 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...