Date:	Mon, 20 Nov 2006 23:18:27 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...pend2.net
Cc:	David Chinner <dgc@....com>, Andrew Morton <akpm@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH -mm 0/2] Use freezeable workqueues to avoid suspend-related XFS corruptions

Hi,

On Monday, 20 November 2006 23:03, Nigel Cunningham wrote:
> Hi.
> 
> On Mon, 2006-11-20 at 21:40 +0100, Rafael J. Wysocki wrote:
> > On Monday, 20 November 2006 12:02, Nigel Cunningham wrote:
> > > If I put a BUG_ON() in submit_bio for non suspend2 I/O, it catches this
> > > trace:
> > > 
> > > submit_bio
> > > xfs_buf_iorequest
> > > xlog_bdstrat_cb
> > > xlog_state_release_iclog
> > > xlog_state_sync_all
> > > xfs_log_force
> > > xfs_syncsub
> > > xfs_sync
> > > vfs_sync
> > > vfs_sync_worker
> > > xfssyncd
> > > keventd_create_kthread
> > 
> > Well, this trace apparently comes from xfssyncd wich explicitly calls
> > try_to_freeze().  When does this happen?
> 
> I'm sorry. Since sending the email I looked at this more, deciding in
> the end that I had a false positive. I was resetting the flag that
> enabled the check after thawing kernel threads instead of before.
> 
> I've looked back in logs to this time last year, when we were doing the
> modifications that lead to using bdev freezing. This is one instance of
> the BUG_ON() triggering then, from a different path. I don't believe
> it's bogus :)
> 
> Nov 12 11:43:06 home-nb kernel: CPU:    0
> Nov 12 11:43:06 home-nb kernel: EIP:    0060:[submit_bio+244/256]    Not tainted VLI
> Nov 12 11:43:06 home-nb kernel: EIP:    0060:[<c025ff54>]    Not tainted VLI
> Nov 12 11:43:06 home-nb kernel: EFLAGS: 00010246   (2.6.14d) 
> Nov 12 11:43:06 home-nb kernel: EIP is at submit_bio+0xf4/0x100
> Nov 12 11:43:06 home-nb kernel: eax: 00000003   ebx: 00001000   ecx: 00000202   edx: 00000000
> Nov 12 11:43:06 home-nb kernel: esi: c63700c0   edi: 00000001   ebp: df86dec8   esp: df86de80
> Nov 12 11:43:07 home-nb kernel: ds: 007b   es: 007b   ss: 0068
> Nov 12 11:43:07 home-nb kernel: Process xfsbufd (pid: 780, threadinfo=df86c000 task=c17cca50)
> Nov 12 11:43:07 home-nb kernel: Stack: 0000001c 00000008 00000000 00000000 df86deb4 c016b942 dff45440 00000010 
> Nov 12 11:43:08 home-nb kernel:        00001000 00001000 00000001 df86dec8 c016bee3 dfcb74f8 c63700c0 00001000 
> Nov 12 11:43:08 home-nb kernel:        00000000 00000000 df86df14 e0b4dd52 00000001 c63700c0 00001000 00000000 
> Nov 12 11:43:08 home-nb kernel: Call Trace:
> Nov 12 11:43:08 home-nb kernel:  [show_stack+127/160] show_stack+0x7f/0xa0
> Nov 12 11:43:08 home-nb kernel:  [<c010357f>] show_stack+0x7f/0xa0
> Nov 12 11:43:08 home-nb kernel:  [show_registers+344/464] show_registers+0x158/0x1d0
> Nov 12 11:43:09 home-nb kernel:  [<c0103718>] show_registers+0x158/0x1d0
> Nov 12 11:43:09 home-nb kernel:  [die+204/368] die+0xcc/0x170
> Nov 12 11:43:09 home-nb kernel:  [<c01038fc>] die+0xcc/0x170
> Nov 12 11:43:09 home-nb kernel:  [do_trap+137/208] do_trap+0x89/0xd0
> Nov 12 11:43:09 home-nb kernel:  [<c03138c9>] do_trap+0x89/0xd0
> Nov 12 11:43:09 home-nb kernel:  [do_invalid_op+184/208] do_invalid_op+0xb8/0xd0
> Nov 12 11:43:10 home-nb kernel:  [<c0103c48>] do_invalid_op+0xb8/0xd0
> Nov 12 11:43:10 home-nb kernel:  [error_code+79/84] error_code+0x4f/0x54
> Nov 12 11:43:10 home-nb kernel:  [<c010323b>] error_code+0x4f/0x54
> Nov 12 11:43:10 home-nb kernel:  [pg0+544066898/1069077504] _pagebuf_ioapply+0x1a2/0x2f0 [xfs]
> Nov 12 11:43:10 home-nb kernel:  [<e0b4dd52>] _pagebuf_ioapply+0x1a2/0x2f0 [xfs]
> Nov 12 11:43:10 home-nb kernel:  [pg0+544067295/1069077504] pagebuf_iorequest+0x3f/0x180 [xfs]
> Nov 12 11:43:10 home-nb kernel:  [<e0b4dedf>] pagebuf_iorequest+0x3f/0x180 [xfs]
> Nov 12 11:43:11 home-nb kernel:  [pg0+544091130/1069077504] xfs_bdstrat_cb+0x3a/0x50 [xfs]
> Nov 12 11:43:11 home-nb kernel:  [<e0b53bfa>] xfs_bdstrat_cb+0x3a/0x50 [xfs]
> Nov 12 11:43:11 home-nb kernel:  [pg0+544069743/1069077504] xfsbufd+0x12f/0x1c0 [xfs]
> Nov 12 11:43:11 home-nb kernel:  [<e0b4e86f>] xfsbufd+0x12f/0x1c0 [xfs]
> Nov 12 11:43:11 home-nb kernel:  [kthread+182/256] kthread+0xb6/0x100
> Nov 12 11:43:11 home-nb kernel:  [<c012c5c6>] kthread+0xb6/0x100
> Nov 12 11:43:11 home-nb kernel:  [kernel_thread_helper+5/12] kernel_thread_helper+0x5/0xc
> Nov 12 11:43:12 home-nb kernel:  [<c01013a9>] kernel_thread_helper+0x5/0xc
> Nov 12 11:43:12 home-nb kernel: Code: 24 0c 8b 81 9c 00 00 00 89 5c 24 04 c7 04 24 70 c7 34 c0 89 44 24 08 e8 1b 99 eb ff e9 63 ff ff ff f6 46 11 01 0f 85 66 ff ff ff <0f> 0b e9 5f ff ff ff 90 8d 74 26 00 55 89 e5 57 56 53 83 ec 2c 
> 
> Looking at the start of the xfsbufd function, this now supports
> freezing, so it shouldn't be an issue anymore, right?

Yes, it properly freezes itself so this kind of failure should not happen.

> > > So, it would appear that freezing kthreads without freezing bdevs should
> > > be a possible solution. It may however leave some I/O unsynced
> > > pre-resume and therefore result in possible dataloss if no resume
> > > occurs. I therefore wonder whether it's better to stick with bdev
> > > freezing
> > 
> > It looks like xfs is the only filesystem that really implements bdev freezing,
> > so for other filesystems it's irrelevant.  However, it may affect the dm, and
> > I'm not sure what happens if someone tries to use a swap file on a dm
> > device for the suspend _after_ we have frozen bdevs.
> > 
> > > or create some variant wherein XFS is taught to fully flush 
> > > pending writes and not create new I/O.
> > 
> > I think we should prevent filesystems from submitting any new I/O after
> > processes have been frozen, this way or another.
> 
> Yes. We should have some means of properly telling them to put things in
> a sane state (however you define sane). It seems to belong between
> freezing userspace and freezing kernelspace because we otherwise might
> have problems with processes blocking, waiting for I/O and never
> entering the refrigerator. But doing it there also creates problems with
> (*cough*) fuse. Gah!
> 
> I would suggest that other filesystems be encouraged to implement bdev
> freezing.

I think I/O can only be submitted from the process context.  Thus if we freeze
all (and I mean _all_) threads that are used by filesystems, including worker
threads, we should effectively prevent fs-related I/O from being submitted
after tasks have been frozen.

This can be done with the help of create_freezeable_workqueue() introduced in
this patch and I'd like to implement it (and there are only a few filesystems
that use work queues).

The freezing of bdevs might be a good solution if:
(1) we were sure it wouldn't interact with dm in a wrong way,
(2) _all_ of the filesystems implemented it.
For now, neither (1) nor (2) are satisfied and we need to know we're safe
_now_.

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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