linux-ext4 - Re: PROBLEM: ext4 resize2fs on-line resizing panic

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Message-ID: <20240624173823.qacrdbxfxwn42kmr@quack3>
Date: Mon, 24 Jun 2024 19:38:23 +0200
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Alexander Coffin <alex.coffin@...icrobots.com>,
	Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: ext4 resize2fs on-line resizing panic

On Mon 24-06-24 18:53:50, Jan Kara wrote:
> On Mon 24-06-24 11:26:58, Theodore Ts'o wrote:
> > On Sun, Jun 23, 2024 at 06:57:13PM -0700, Alexander Coffin wrote:
> > > [1.] One line summary of the problem:
> > > Using resize2fs on-line resizing on a specific ext4 partition is
> > > causing an Oops.
> > > 
> > > 
> > > [6.] Output of Oops.. message (if applicable) with symbolic information
> > >      resolved (see Documentation/admin-guide/bug-hunting.rst)
> > > 
> > > ```
> > > [  445.552287] ------------[ cut here ]------------
> > > [  445.552300] kernel BUG at fs/jbd2/journal.c:846!
> > 
> > Thanks for the bug report.  The BUG_ON is from the following assert in
> > jbd2_journal_next_log_block:
> > 
> > 	J_ASSERT(journal->j_free > 1);
> > 
> > and it indicates that we ran out of space in the journal.  There are
> > mechanisms to make sure that this should never happen, and if the
> > journal is too small and the transaction couldn't be broken up, then
> > the operation (whether it is a resize or a file truncate or some other
> > operation) should have errored out, and not triggered a BUG.
> 
> Yeah, I was debugging this today and I'll shortly send a fix for JBD2 so
> that we don't trigger this BUG. But the online resize will fail anyway
> after my fixes (just gracefully) because the add_flex_bg() code tries to
> start a transaction with more credits than the journal allows.

To be more precise, the problem is that with this size of the journal,
maximum transaction size is 250 metadata blocks (+6 blocks reserved for
descriptors). Online resizing tries to start a transaction with 252 credits
in ext4_flex_group_add(). 246 credits come from es->s_reserved_gdt_blocks
so I don't see an easy way how to avoid that because to each of these
reserve gdt blocks we need to add reserved gdt blocks from the new groups.
So I see two possibilities:

1) Just make mke2fs / tune2fs refuse so many reserved gdt blocks with a
tiny journal.

2) Allow larger transaction size - currently we require that 4 max sized
transactions fit into the journal, we could reduce it to 3 without
introducing deadlocks. But larger transactions could have other unexpected
performance side effects so I'm not sure the risk is worth it for a corner
case like this.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR