linux-ext4 - Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Date:	Thu, 31 Mar 2011 21:03:55 +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
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

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.

Thanks,
Toshiyuki Okajima

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