Date:   Sat, 13 Mar 2021 13:29:43 +0530
From:   Shashidhar Patil <shashidhar.patil@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: jbd2 task hung in jbd2_journal_commit_transaction

Hi Theodore,
   Thank you for the explaination and clarification about the real
problem. Some more queries below to understand the
problem better.

On Fri, Mar 12, 2021 at 6:21 PM Theodore Ts'o <tytso@....edu> wrote:
>
> On Fri, Mar 12, 2021 at 01:33:26PM +0530, Shashidhar Patil wrote:
> > Hi,
> >       We are seeing problem with many tasks hung in 'D' state and
> > stack output of jbd2 thread is hung at
> > jbd2_journal_commit_transaction().
> >
> > The stack trace of jdb2 task is:
> >
> > [<0>] jbd2_journal_commit_transaction+0x26e/0x1870^M
> > [<0>] kjournald2+0xc8/0x250^M
> > [<0>] kthread+0x105/0x140^M
> > [<0>] ret_from_fork+0x35/0x40^M
> > [<0>] 0xffffffffffffffff^M
> >
> > The symptoms look similar to
> > https://www.spinics.net/lists/linux-ext4/msg53502.html.
> >
> > The system has zswap configured with swap backing is an ext4 file.
> > There are oom kills recorded in the dmesg.
> >
> > As per Theodore in the above link the issue is caused by a leaked
> > atomic handle. But one of the backtraces collected (below) points to
> > a possile problem of cylic dependency during direct memory reclaim as
> > poined here
> > https://www.kernel.org/doc/html/latest/core-api/gfp_mask-from-fs-io.html
> >
> > This issue is explained in below steps
> > 1. sys_write processing allocates atomic handle for journaling and
> > handle got allocated
> > , journalling started on the handle
> > 2. as part of write processing page cache allocation successed and the
> > write operation also started
> > 3. During the write iteration jbd2_alloc() was called with (GFP_NOFS |
> > __GFP_NOFAIL) which imply __GFP_DIRECT_RECLAIM
> >
> > 4. zswap backup ram is full so it tries to reclaim memory to free up
> > some pages and initiates a swap page disk writeback. In this case the
> > swap file is on the same partition on which write() call is done.
>
> From what I can tell zswap is using writepage(), and since the swap
> file should be *completely* preallocated and initialized, we should
> never be trying to start a handle from zswap.  This should prevent the
> deadlock from happening.  If zswap is doing something which is causing
> ext4 to start a handle when it tries to writeout a swap page, then
> that would certainly be a problem.  But that really shouldn't be the
> case.

Yes. But the the first sys_write() called by the application did
allocate an journal handle as required and since
this specific request now is waiting for IO to complete the handle is
not closed. Elsewhere in jbd2 task the commit_transaction is
blocked since there is one or more open journalling handles. Is my
understanding correct ?

>
> >
> >  The backtrace which might cause the deadlock
> >
> > 4,1736499,1121675690224,-;Call Trace:
> > 4,1736500,1121675690231,-; __schedule+0x3d6/0x8b0
> > 4,1736501,1121675690237,-; schedule+0x36/0x80
> > 4,1736502,1121675690245,-; io_schedule+0x16/0x40
> > 4,1736503,1121675690253,-; wbt_wait+0x22f/0x380
> > 4,1736504,1121675690261,-; ? trace_event_raw_event_wbt_timer+0x100/0x100
> > 4,1736505,1121675690269,-; ? end_swap_bio_read+0xd0/0xd0
> > 4,1736506,1121675690275,-; blk_mq_make_request+0x103/0x5b0
>
> We can see that zswap has called _swap_writepage(), and then the I/O
> has been submitted... and we're waiting for the I/O to complete ---
> see the call io_schedule().  So we're not actually deadlocking on

I missed this important part of submitting the requests directly to
generic bio. So clearly no journalling handles are allocated.
I do see there are many such io requests in "waiting" state which
means the storage device is not responding.
something like:

4,1737846,1121675697013,-; schedule+0x36/0x80
4,1737847,1121675697015,-; io_schedule+0x16/0x40
4,1737848,1121675697016,-; blk_mq_get_tag+0x161/0x250
4,1737849,1121675697018,-; ? wait_woken+0x80/0x80
4,1737850,1121675697020,-; blk_mq_get_request+0xdc/0x3b0
4,1737851,1121675697021,-; blk_mq_make_request+0x128/0x5b0
4,1737852,1121675697023,-; generic_make_request+0x122/0x2f0
4,1737853,1121675697024,-; ? bio_alloc_bioset+0xd2/0x1e0
4,1737854,1121675697026,-; submit_bio+0x73/0x140
.....
So all those IO requests are waiting for response from the raid port,
is that right ?

But the megaraid_sas driver( the system has LSI MEGARAID port) in most
cases handles the unresponsive behavior
by resetting the device. IN this case the reset did not happen, maybe
there is some other bug in the megaraid driver.

Thanks
-Shashidhar

> anything other else than... the storage device not getting back to us
> with the I/O completion notification.  And that's not really a cyclic
> deadlock, but probably a lost I/O completion interrupt, or the storage
> device firmware panicing/crashing/show stopping, or falling off the
> SATA bus and then needing to reconnect, etc.
>
>                                                 - Ted
>
>
> > 4,1736507,1121675690283,-; ? end_swap_bio_read+0xd0/0xd0
> > 4,1736508,1121675690288,-; generic_make_request+0x122/0x2f0
> > 4,1736509,1121675690295,-; submit_bio+0x73/0x140
> > 4,1736510,1121675690302,-; ? submit_bio+0x73/0x140
> > 4,1736511,1121675690308,-; ? get_swap_bio+0xcf/0x100
> > 4,1736512,1121675690316,-; __swap_writepage+0x33f/0x3b0
> > 4,1736513,1121675690322,-; ? lru_cache_add_file+0x37/0x40
> > 4,1736514,1121675690329,-; ? lzo_decompress+0x38/0x70
> > 4,1736515,1121675690336,-; zswap_writeback_entry+0x249/0x350
> > 4,1736516,1121675690343,-; zbud_zpool_evict+0x31/0x40
> > 4,1736517,1121675690349,-; zbud_reclaim_page+0x1e9/0x250
> > 4,1736518,1121675690356,-; zbud_zpool_shrink+0x3b/0x60
> > 4,1736519,1121675690362,-; zpool_shrink+0x1c/0x20
> > 4,1736520,1121675690369,-; zswap_frontswap_store+0x274/0x530
> > 4,1736521,1121675690376,-; __frontswap_store+0x78/0x100
> > 4,1736522,1121675690382,-; swap_writepage+0x3f/0x80
> > 4,1736523,1121675690390,-; pageout.isra.53+0x1e6/0x340
> > 4,1736524,1121675690397,-; shrink_page_list+0x992/0xbe0
> > 4,1736525,1121675690403,-; shrink_inactive_list+0x2af/0x5f0
> > 4,1736526,1121675690409,-; ? _find_next_bit+0x40/0x70
> > 4,1736527,1121675690416,-; shrink_node_memcg+0x36f/0x7f0
> > 4,1736528,1121675690423,-; shrink_node+0xe1/0x310
> > 4,1736529,1121675690429,-; ? shrink_node+0xe1/0x310
> > 4,1736530,1121675690435,-; do_try_to_free_pages+0xee/0x360
> > 4,1736531,1121675690439,-; try_to_free_pages+0xf1/0x1c0
> > 4,1736532,1121675690442,-; __alloc_pages_slowpath+0x399/0xe90
> > 4,1736533,1121675690446,-; __alloc_pages_nodemask+0x289/0x2d0
> > 4,1736534,1121675690449,-; alloc_pages_current+0x6a/0xe0
> > 4,1736535,1121675690452,-; __get_free_pages+0xe/0x30
> > 4,1736536,1121675690455,-; jbd2_alloc+0x3a/0x60
> > 4,1736537,1121675690458,-; do_get_write_access+0x182/0x3e0
> > 4,1736538,1121675690461,-; jbd2_journal_get_write_access+0x51/0x80
> > 4,1736539,1121675690464,-; __ext4_journal_get_write_access+0x3b/0x80
> > 4,1736540,1121675690466,-; ext4_reserve_inode_write+0x95/0xc0
> > 4,1736541,1121675690467,-; ? ext4_dirty_inode+0x48/0x70
> > 4,1736542,1121675690469,-; ext4_mark_inode_dirty+0x53/0x1d0
> > 4,1736543,1121675690470,-; ? __ext4_journal_start_sb+0x6d/0x120
> > 4,1736544,1121675690473,-; ext4_dirty_inode+0x48/0x70
> > 4,1736545,1121675690475,-; __mark_inode_dirty+0x184/0x3b0
> > 4,1736546,1121675690479,-; generic_update_time+0x7b/0xd0
> > 4,1736547,1121675690482,-; ? current_time+0x32/0x70
> > 4,1736548,1121675690484,-; file_update_time+0xbe/0x110
> > 4,1736549,1121675690487,-; __generic_file_write_iter+0x9d/0x1f0
> > 4,1736550,1121675690490,-; ext4_file_write_iter+0xc4/0x3b0
> > 4,1736551,1121675690492,-; ? sock_sendmsg+0x3e/0x50
> > 4,1736552,1121675690495,-; new_sync_write+0xe5/0x140
> > 4,1736553,1121675690498,-; __vfs_write+0x29/0x40
> > 4,1736554,1121675690501,-; vfs_write+0xb8/0x1b0
> > 4,1736555,1121675690503,-; ? syscall_trace_enter+0x1d6/0x2f0
> > 4,1736556,1121675690505,-; SyS_write+0x5c/0xe0
> > 4,1736557,1121675690508,-; do_syscall_64+0x73/0x130
> > 4,1736558,1121675690509,-; entry_SYSCALL_64_after_hwframe+0x3d/0xa2

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