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Date:   Wed, 17 Mar 2021 20:30:56 +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 details about the journalling layer and
insight into the block device layer.
 I think Good luck might have clicked. The swap file in our case is
attached to a loop block device before enabling swap using swapon.
Since loop driver processes its IO requests by calling
vfs_iter_write() the write requests re-enter the ext4
filesystem/journalling code.
Is that right ? There seems to be a possibility of cylic dependency.

Thanks
-Shashidhar
On Sun, Mar 14, 2021 at 9:08 AM Theodore Ts'o <tytso@....edu> wrote:
>
> On Sat, Mar 13, 2021 at 01:29:43PM +0530, Shashidhar Patil wrote:
> > > 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 ?
>
> Yes, that's correct.  When we start a transaction commit, either
> because the 5 second commit interval has been reached, or there isn't
> enough room in the journal for a particular handle to start (when we
> start a file system mutation, we estimate the worst case number of
> blocks that might need to be modified, and hence require space in the
> journal), we first (a) stop any new handles from being started, and
> then (b) wait for all currently running handles to complete.
>
> If one handle takes a lot longer to complete than all the others,
> while we are waiting for that last handle to finish, the commit can
> not make forward progress, and no other file system operation which
> requires modifying metadata can proceed.  As a result, we try to keep
> the time between starting a handle and stopping a handle as short as
> possible.  For example, if possible, we will try to read a block that
> might be needed by a mutation operation *before* we start the handle.
> That's not always possible, but we try to do that whenever possible,
> and there are various tracepoints and other debugging facilities so we
> can see which types of file system mutations require holding handles
> longest, so we can try to optimize them.
>
> > 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.
>
> Yes, it's not necessarily a problem with the storage device or the
> host bus adapter; it could also be some kind of bug in the device
> driver --- or even the block layer, although that's much, much less
> likely (mostly because a lot of people would be complaining if that
> were the case).
>
> If you have access to a SCSI/SATA bus snooper which can be inserted in
> between the storage device (HDD/SSD) and the LSI Megaraid, that might
> be helpful in terms of trying to figure out what is going on.  Failing
> that, you'll probably find some way to add/use debugging
> hooks/tracepoints in the driver.
>
> Good luck,
>
>                                         - Ted

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