Date:	Wed, 06 Apr 2011 14:09:14 +0900
From:	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
To:	Jan Kara <jack@...e.cz>
CC:	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

Hi.

(2011/04/06 7:54), Jan Kara wrote:
> On Tue 05-04-11 19:25:44, Toshiyuki Okajima wrote:
>> (2011/03/31 21:03), Toshiyuki Okajima wrote:
>>> Hi, thanks for your reviewing.
>>>
>>> (2011/03/30 23:12), Jan Kara wrote:
>>>> Hello,
>>>>
>>>> On Mon 28-03-11 17:06:28, Toshiyuki Okajima wrote:
>>>>> On Thu, 17 Feb 2011 11:45:52 +0100
>>>>> Jan Kara<jack@...e.cz>  wrote:
>>>>>> On Thu 17-02-11 12:50:51, Toshiyuki Okajima wrote:
>>>>>>> (2011/02/16 23:56), Jan Kara wrote:
>>>>>>>> On Wed 16-02-11 08:17:46, Toshiyuki Okajima wrote:
>>>>>>>>> On Tue, 15 Feb 2011 18:29:54 +0100
>>>>>>>>> Jan Kara<jack@...e.cz>  wrote:
>>>>>>>>>> On Tue 15-02-11 12:03:52, Ted Ts'o wrote:
>>>>>>>>>>> On Tue, Feb 15, 2011 at 05:06:30PM +0100, Jan Kara wrote:
>>> <SNIP>
>>>>> I have deeply continued to examined the root cause of this problem, then
>>>>> I found it.
>>>>>
>>>>> It is that we can write a memory which is mmaped to a file. Then the memory
>>>>> becomes "DIRTY" so then the flusher thread (ex. wb_do_writeback) tries to
>>>>> "writeback" the memory.
>>>>>
>>>>> Therefore, the root cause of this hangup is not only ext4 component (with
>>>>> delayed allocation feature) but also writeback mechanism for mmap. If you
>>>>> use the other filesystem, you can write something to the filesystem though
>>>>> you have freezed the filesystem.
>>>
>>>> Well, you can write something only in the caches, not to the on disk
>>>> image. So it's not a problem as such.
>>> My reproducer uses the loopback device(/dev/loopX). By using it, I have confirmed that
>>> we can write in not only the caches but also the loopback device. However,
>>> I don't still confirm that we can write to the real device(/dev/sdaX).
>>>
>>>>
>>>>> A sample problem is attached on this mail. Try to execute it then you can
>>>>> confirm that we can write some data to your filesystem while freezing the
>>>>> filesystem.
>>>>> (If you change FS variable in go.sh from ext3 to ext4 and you execute
>>>>> "fsfreeze -u mnt" manually on other prompt, you can also confirm this deadlock.)
>>>>>
>>>>> I think the best approach to fix this problem is to let users not to write
>>>>> memory which is mapped to a certain file while the filesystem is freezing.
>>>>> However, it is very difficult to control users not to write memory which has
>>>>> been already mapped to the file.
>>>> It is actually possible. In case of ext4, you could add a check (+ wait)
>>>> in ext4_page_mkwrite() whether the filesystem is frozen or in the process
>>>> of being frozen and if so, wait for it to get unfrozen. The only tough
>>>> problem here might be the locking as ext4_page_mkwrite() is called with
>>>> mmap_sem held and I'm not sure we can take s_umount with mmap_sem held.
>>>> But you'd have to fix all filesystems (and all paths possibly creating
>>>> dirty data) in this way.
>>>>
>>>
>>>>> Therefore, I think there is only actual method that we stop writeback thread
>>>>> to resolve the mmap problem. Also, by this fix, the original problem
>>>>> (ext4 delayed write vs unfreeze) can be solved.
>>>> Hmm, I had a look at the code again and think we could fix the issue
>>>> cleanly (i.e. all possible users of s_umount) as follows: The lock
>>>> ordering will be
>>>> s_umount ->  "fs frozen"
>>>> and there will be a new mutex s_freeze_mutex protecting changes of
>>>> s_frozen.
>>>>
>>>> freeze_bdev() already observes this lock ordering, it will only take
>>>> s_freeze_mutex for the changes of s_frozen values. The only other code
>>>> that is relevant for the lock ordering is thaw_super() (the freezing
>>>> process is not expected to reenter kernel for the frozen filesystem).
>>>> In thaw_super() we could take s_freeze_mutex, do all the thawing work,
>>>> set s_frozen, release s_freeze_mutex and put superblock reference.
>>>>
>>>
>>>> So something like the patch below - it seems to work for me, can you test
>>>> it please?
>>> I think your patch looks good, so, the original problem seems to be solved.
>>> OK, I will test your patch.
>>> This weekend I cannot test it. So, I will reply next week.
>> I have tested whether Mizuma-san's reproducer can cause to deadlock with your
>> patch. And then any problems didn't hit while the reproducer was running.
>>
>> I think your patch solves the original deadlock problem which is reported by
>> Mizuma-san.
>    Good. Thanks.
>
>>> Reported-by: Toshiyuki Okajima<toshi.okajima@...fujitsu.com>
>>> Signed-off-by: Jan Kara<jack@...e.cz>
>>> ---
>>> fs/super.c         |   40 ++++++++++++++++++++++++++++++++++------
>>> include/linux/fs.h |    1 +
>>> 2 files changed, 35 insertions(+), 6 deletions(-)
>>

>> However, I think a write which causes the deadlock is from mmapped dirty
>> pages. So, I guess we also need to fix in the mmap path while fsfreezing.
>    Why? If you dirty a page, writeback thread can come and try to write it -
> which blocks - but now that does not matter...
I have not understood the code around writeback thread very much...
Please explain me the concrete function name which blocks some writes?

Mizuma-san's reproducer also writes the data which maps to the file (mmap).
The original problem happens after the fsfreeze operation is done.
I understand the normal write operation (not mmap) can be blocked while
fsfreezing. So, I guess we don't always block all the write operation
while fsfreezing.

Thanks
Toshiyuki Okajima

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