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Date: Sat, 8 Oct 2016 19:29:03 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Dave Jones <davej@codemonkey.org.uk>, Chris Mason <clm@fb.com>,
        Josef Bacik <jbacik@fb.com>, David Sterba <dsterba@suse.com>,
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        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: btrfs_direct_IO oops
Message-ID: <20161008182903.GU19539@ZenIV.linux.org.uk>
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On Sat, Oct 08, 2016 at 02:08:06PM -0400, Dave Jones wrote:
> That code: matches this dissembly:
> 
>                 for (i = seg + 1; i < iter->nr_segs; i++) {

*whoa*

OK, that loop in check_direct_IO() should be done *ONLY* for
iovec iter - even for a bvec one it's completely bogus, and
for pipe ones it blows up immediately.

Sorry, I'd missed that bogosity - replace
        if (iov_iter_rw(iter) == WRITE)
		return 0;
with
	if (iov_iter_rw(iter) != READ || !iter_is_iovec(iter))
		return 0;
in there; that should fix the damn thing.