linux-ext4 - Re: [PATCH v2 1/8] migrate: fix skipping metadata buffer heads on migration

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Message-ID: <jwciumjkfwwjeoklsi6ubcspcjswkz5s5gtttzpjqft6dtb7sp@c4ae6y5pix5w>
Date: Wed, 23 Apr 2025 19:09:28 +0200
From: Jan Kara <jack@...e.cz>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Jan Kara <jack@...e.cz>, dave@...olabs.net, brauner@...nel.org, 
	tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, 
	riel@...riel.com, willy@...radead.org, hannes@...xchg.org, oliver.sang@...el.com, 
	david@...hat.com, axboe@...nel.dk, hare@...e.de, david@...morbit.com, 
	djwong@...nel.org, ritesh.list@...il.com, linux-fsdevel@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-mm@...ck.org, gost.dev@...sung.com, p.raghav@...sung.com, 
	da.gomez@...sung.com, syzbot+f3c6fda1297c748a7076@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/8] migrate: fix skipping metadata buffer heads on
 migration

On Wed 16-04-25 09:58:30, Luis Chamberlain wrote:
> On Tue, Apr 15, 2025 at 06:28:55PM +0200, Jan Kara wrote:
> > > So I tried:
> > > 
> > > root@...ext4-2k /var/lib/xfstests # fsck /dev/loop5 -y 2>&1 > log
> > > e2fsck 1.47.2 (1-Jan-2025)
> > > root@...ext4-2k /var/lib/xfstests # wc -l log
> > > 16411 log
> > 
> > Can you share the log please?
> 
> Sure, here you go:
> 
> https://github.com/linux-kdevops/20250416-ext4-jbd2-bh-migrate-corruption
> 
> The last trace-0004.txt is a fresh one with Davidlohr's patches
> applied. It has trace-0004-fsck.txt.

Thanks for the data! I was staring at them for some time and at this point
I'm leaning towards a conclusion that this is actually not a case of
metadata corruption but rather a bug in ext4 transaction credit computation
that is completely independent of page migration.

Based on the e2fsck log you've provided the only damage in the filesystem
is from the aborted transaction handle in the middle of extent tree growth.
So nothing points to a lost metadata write or anything like that. And the
credit reservation for page writeback is indeed somewhat racy - we reserve
number of transaction credits based on current tree depth. However by the
time we get to ext4_ext_map_blocks() another process could have modified
the extent tree so we may need to modify more blocks than we originally
expected and reserved credits for.

Can you give attached patch a try please?

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

View attachment "0001-ext4-Fix-calculation-of-credits-for-extent-tree-modi.patch" of type "text/x-patch" (1983 bytes)