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

[Baokun Li]

On 2025/3/8 23:21, Ojaswin Mujoo wrote:
> On Sat, Mar 08, 2025 at 06:37:41PM +0530, Ojaswin Mujoo wrote:
> > 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@xxxxxxxxxxxxx/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@xxxxxxxxxxxxx/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
> Hey Baokun,
>
> So while coding this up, I started looking at some codepaths and it got
> me wondering when can we actually change the sbi->s_es->s_journal_inum
> (or sbi->s_sbh) from the time it gets populated to the umount time?
>
> Since the sbi->s_sbh buffer head is always in memory and never reclaimed
> due the elevated reference, the only way to modify it should be if we
> modify the memory page somehow. Or is there some codepath/tooling magic
> I'm missing that can modify this value?
For the ext4 file system, s_journal_inum is fixed after mkfs and is not
expected to be modified. However, the sbi->s_sbh buffer head belongs to
the block device, so direct write operations to the file system's block
device /dev/xxx may directly modify sbi->s_sbh.

For example, you can use the following script to trigger a journal bmap
failure:

mkfs.ext4 -F /dev/sda
mount /dev/sda /tmp/test
dd if=/dev/zero of=/dev/sda bs=4096 count=1
echo 1 > /tmp/test/file
umount /tmp/test

(Adding a delay in put_super and performing the script during the delay
 can trigger the issue described in the patch.)


Cheers,
Baokun
> Regards,
> 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(-)