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@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. > 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@xxxxxxxxxxxxxx> > >Signed-off-by: Jan Kara <jack@xxxxxxx> > >--- > > 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... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
