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@xxxxxxx> 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@xxxxxxx> 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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