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

[Luis Chamberlain <mcgrof@kernel.org>]

On Wed, Apr 23, 2025 at 01:30:29PM -0700, Luis Chamberlain wrote:
> 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!

Tested-by: kdevops@lists.linux.dev

I have run the test over 3 separate guests and each one has tested this
over 48 hours each. There is no ext4 fs corruption reported, all is
good, so I do believe thix fixes the issue. One of the guests was on
Linus't tree which didn't yet have Davidlorh's fixes for folio migration.
And so I believe this patch should have a stable tag fix so stable gets it.

  Luis