linux-kernel - Re: [PATCH v2 0/3] Fix a BUG_ON crashing the kernel in start_this

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Message-ID: <Z8xBHVU1vChK2rMO@li-dc0c254c-257c-11b2-a85c-98b6c1322444.ibm.com>
Date: Sat, 8 Mar 2025 18:37:41 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        Yang Erkun <yangerkun@...wei.com>
Subject: Re: [PATCH v2 0/3] Fix a BUG_ON crashing the kernel in
 start_this_handle

On Sat, Mar 08, 2025 at 09:09:28AM +0800, Baokun Li wrote:
> On 2025/3/6 22:28, Ojaswin Mujoo wrote:
> > ** Changes since v1 [1] **
> > 
> >   * Picked up RVBs from Jan and Ritesh
> >   * In patch 2/3, we now use a flag in sbi instead of SB_ACITVE
> >     to determine when to journal sb vs when to commit directly.
> >   * Added a prep patch 1/3
> > 
> > [1] https://lore.kernel.org/linux-ext4/cover.1740212945.git.ojaswin@linux.ibm.com/T/#m5e659425b8c8fe2ac01e7242b77fed315ff89db4
> > 
> > @Baokun, I didn't get a chance to look into the journal_inode
> > modifications we were discussing in [2]. I'll try to spend some time and
> > send that as a separate patch. Hope that's okay
> > 
> > [2] https://lore.kernel.org/linux-ext4/cover.1740212945.git.ojaswin@linux.ibm.com/T/#mad8feb44d9b6ddadf87830b92caa7b78d902dc05
> That's fine, it's not a priority. And if this patch set makes sure we
> don't crash when things go wrong, I'm okay with leaving it as is.
> 
> It's possible that jbd2_journal_commit_transaction() could call
> ext4_handle_error() in other places as the code evolves. Fixing known
> problems and protecting against potential ones is always a good thing.

Yep thats true, I did spend some time on this since the codepath was a
bit unfamiliar to me. Seems like a straighforward enough change. I'll
add it to the next patch.

thanks,
ojaswin
> 
> 
> Cheers,
> Baokun
> > ** Original Cover **
> > 
> > When running LTP stress tests on ext4, after a multiday run we seemed to
> > have hit the following BUG_ON:
> > 
> >   [NIP  : start_this_handle+268]
> >   #3 [c000001067c27a40] start_this_handle at c008000004d40f74 [jbd2]  (unreliable)
> >   #4 [c000001067c27b60] jbd2__journal_start at c008000004d415cc [jbd2]
> >   #5 [c000001067c27be0] update_super_work at c0080000053f9758 [ext4]
> >   #6 [c000001067c27c70] process_one_work at c000000000188790
> >   #7 [c000001067c27d20] worker_thread at c00000000018973c
> >   #8 [c000001067c27dc0] kthread at c000000000196c84
> >   #9 [c000001067c27e10] ret_from_kernel_thread at c00000000000cd64
> > 
> > Which comes out to
> > 
> >    382   repeat:
> >    383           read_lock(&journal->j_state_lock);
> > * 384           BUG_ON(journal->j_flags & JBD2_UNMOUNT);
> >    385           if (is_journal_aborted(journal) ||
> >    386               (journal->j_errno != 0 && !(journal->j_flags & JBD2_ACK_ERR))) {
> >    387                   read_unlock(&journal->j_state_lock);
> > 
> > 
> > Initially this seemed like it should never happen but upon crash
> > analysis it seems like it could indeed be hit as described in patch 1/2.
> > 
> > I would like to add that through the logs we only knew that:
> > 
> > - ext4_journal_bmap -> ext4_map_blocks is failing with EFSCORRUPTED.
> > - update_super_work had hit the BUG_ON
> > 
> > I was not able to hit this bug again (without modifying the kernel to
> > inject errors) but the above backtrace seems to be one possible paths
> > where this BUG_ON can be hit. Rest of the analysis and fix is in patch
> > 2/3. Patch 3 is just a small tweak that i found helpful while debugging.
> > 
> > That being said, journalling is something I'm not very familiar with and
> > there might be gaps in my understanding so thoughts and suggestions are
> > welcome.
> > 
> > Ojaswin Mujoo (3):
> >    ext4: define ext4_journal_destroy wrapper
> >    ext4: avoid journaling sb update on error if journal is destroying
> >    ext4: Make sb update interval tunable
> > 
> >   fs/ext4/ext4.h      | 11 +++++++++++
> >   fs/ext4/ext4_jbd2.h | 22 ++++++++++++++++++++++
> >   fs/ext4/super.c     | 35 +++++++++++++++++------------------
> >   fs/ext4/sysfs.c     |  4 ++++
> >   4 files changed, 54 insertions(+), 18 deletions(-)
> > 
>