Message-Id: <43801f2e-a2e5-4035-9757-930ea7dc1757@www.fastmail.com>
Date:   Wed, 24 Feb 2021 10:58:27 -0800
From:   "Seamus Connor" <seamus@...foilwizard.net>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: reproducible corruption in journal

> *) It appears that your test is generating a large number of very
> small transactions, and you are then "crashing" the file system by
> disconnecting the file system from further updates, and running e2fsck
> to replay the journal, throwing away the block writes after the
> "disconnection", and then remounting the file system.  I'm going to
> further guess that size of the small transactions are very similar,
> and the amount of time between when the file system is mounted, and
> when the file system is forcibly disconnected, is highly predictable
> (e.g., always N seconds, plus or minus a small delta).

Yes, this matches the workload. I assume the transactions are very small 
because we are doing a large number of metadata operations, and 
because we are mounted sync?

> 
> Is that last point correct?  If so, that's a perfect storm where it's
> possible for the journal replay to get confused, and mistake previous
> blocks in the journal as ones part of the last valid file system
> mount.  It's something which probably never happens in practice in
> production, since users are generally not running a super-fixed
> workload, and then causing the system to repeatedly crash after a
> fixed interval, such that the mistake described above could happen.
> That being said, it's arguably still a bug.
> 
> Does this hypothesis consistent with what you are seeing?

Yes, this is consistent with what I am seeing. The only thing to add is that
the workload isn't particularly fixed. The data being written is generated
by a production workload (we are recording statistics about hardware).
The interval at which we are shutting down the block device is regular
but not precise (+/- 30 seconds).

> 
> If so, I can see two possible solutions to avoid this:
> 
> 1) When we initialize the journal, after replaying the journal and
> writing a new journal superblock, we issue a discard for the rest of
> the journal.  This won't help for block devices that don't support
> discard, but it should slightly reduce work for the FTL, and perhaps
> slightly improve the write endurance for flash.
Our virtual device doesn't support discard, could that be why others aren't
seeing this issue?

> 
> 2) We should stop resetting the sequence number to zero, but instead,
> keep the sequence number at the last used number.  For testing
> purposes, we should have an option where the sequence number is forced
> to (0U - 300) so that we test what happens when the 4 byte unsigned
> integer wraps.
I can give this a try with my workload. Just so I can be sure I understand, the 
hypothesis is that we are running into issues during do_one_pass(..., PASS_SCAN)
because we are getting unlucky with  "if (sequence != next_commit_ID) {..."?
The solution is to reduce the occurrence of this issue (to basically zero) by not
resetting the sequence number? Have I understood you correctly? Looking
through e2fsprogs, I think there is a commit that already does this
(32448f50df7d974ded956bbc78a419cf65ec09a3) during replay. Another thing
that I could try is zeroing out the contents of inode 8 after a journal replay and
recreating the journal after each event.

Thanks for your help!

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