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

Luis Chamberlain
2025-04-23T20:30:29Z

On Wed, Apr 23, 2025 at 07:09:28PM +0200, Jan Kara wrote:
> 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@e1-ext4-2k /var/lib/xfstests # fsck /dev/loop5 -y 2>&1 > log
> > > > e2fsck 1.47.2 (1-Jan-2025)
> > > > root@e1-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@suse.com>
> SUSE Labs, CR

> From 4c53fb9f4b9b3eb4a579f69b7adcb6524d55629c Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@suse.cz>
> Date: Wed, 23 Apr 2025 18:10:54 +0200
> Subject: [PATCH] ext4: Fix calculation of credits for extent tree modification
>
> Luis and David are reporting that after running generic/750 test for 90+
> hours on 2k ext4 filesystem, they are able to trigger a warning in
> jbd2_journal_dirty_metadata() complaining that there are not enough
> credits in the running transaction started in ext4_do_writepages().
>
> Indeed the code in ext4_do_writepages() is racy and the extent tree can
> change between the time we compute credits necessary for extent tree
> computation and the time we actually modify the extent tree. Thus it may
> happen that the number of credits actually needed is higher. Modify
> ext4_ext_index_trans_blocks() to count with the worst case of maximum
> tree depth.
>
> Link: https://lore.kernel.org/all/20250415013641.f2ppw6wov4kn4wq2@offworld
> Reported-by: Davidlohr Bueso <dave@stgolabs.net>
> Reported-by: Luis Chamberlain <mcgrof@kernel.org>
> Signed-off-by: Jan Kara <jack@suse.cz>

I kicked off tests! Let's see after ~ 90 hours!

Luis