linux-kernel - Re: iov_iter

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Date:   Mon, 11 Sep 2017 07:11:10 +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 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. Hence if we detect it,
and our coherency guards can't fix up the coherency problem, we
issue a warning and fail the IO.

The warning is mostly there for us developers and it's been there
for years - it's so we don't end up chasing ghosts when we see that
warning in the logs. The usual vector is an app that mixes
concurrent DIO with mmap access to the same file, which we
explicitly say "don't do this because data corruption" in the
open(2) man page....

Cheers,

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