Fix a BUG_ON crashing the kernel in start_this_handle

** Changes in v4 [4] **

* some minor refactoring and typo fix

[4] https://lore.kernel.org/linux-ext4/[email protected]/T/#m8b5191fef8b201246ab5b34f7dc11b79fe6afe99

** Changes in v3 [3] **

* Picked RVBs from Jan and Baokun
* In patch 2/3 we now do the following:
set flag -> force commit -> flush wq
* use sbi->s_mount_flags instead of an
sbi field for journal destroy flag

[3] https://lore.kernel.org/linux-ext4/[email protected]/T/#m3a6c2e9205381d86460267e7be37ae0db688b800

** Changes in v2 [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/[email protected]/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/[email protected]/T/#mad8feb44d9b6ddadf87830b92caa7b78d902dc05

** 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      | 12 +++++++++++-
 fs/ext4/ext4_jbd2.h | 29 ++++++++++++++++++++++++++++
 fs/ext4/super.c     | 47 +++++++++++++++++++++------------------------
 fs/ext4/sysfs.c     |  4 ++++
 4 files changed, 66 insertions(+), 26 deletions(-)

2.48.1