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Date:   Fri, 15 Nov 2019 09:10:29 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 20/25] jbd2: Reserve space for revoke descriptor blocks

On Fri, Nov 15, 2019 at 09:20:33AM -0500, Theodore Y. Ts'o wrote:
> On Fri, Nov 15, 2019 at 11:02:22AM +0100, Jan Kara wrote:
> > On Thu 14-11-19 23:52:23, Eric Biggers wrote:
> > > On Tue, Nov 05, 2019 at 05:44:26PM +0100, Jan Kara wrote:
> > > >  static inline int jbd2_handle_buffer_credits(handle_t *handle)
> > > >  {
> > > > -	return handle->h_buffer_credits;
> > > > +	journal_t *journal = handle->h_transaction->t_journal;
> > > > +
> > > > +	return handle->h_buffer_credits -
> > > > +		DIV_ROUND_UP(handle->h_revoke_credits_requested,
> > > > +			     journal->j_revoke_records_per_block);
> > > >  }
> > > 
> > > This patch is causing a crash with 'kvm-xfstests -c dioread_nolock ext4/024'.
> > > Looks like this code incorrectly assumes that h_transaction is always valid
> > > rather than the other member of the union, h_journal.
> > 
> > Right, thanks for the report! Just out of curiosity: You have to have that
> > tracepoint enabled for the crash to trigger, don't you? Because I'm pretty
> > sure I did dioread_nolock runs...
> 
> I've been *definitely* been doing dioread_nolock runs (including two
> last night), with no failures.
> 
> ext4/dioread_nolock: 485 tests, 40 skipped, 5142 seconds
> 

No I didn't enable the tracepoint.  I think the difference is that I had
CONFIG_UBSAN enabled.  I get the crash if I use the following kconfig:

	curl -o .config 'https://git.kernel.org/pub/scm/fs/ext2/xfstests-bld.git/plain/kernel-configs/x86_64-config-5.4'
	echo CONFIG_UBSAN=y >> .config
	make olddefconfig

... but not if I don't enable UBSAN.

No idea why UBSAN makes a difference here, though.  I'm using gcc 9.2.0.

- Eric

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