Re: [PATCH 04/10] xfs: implement freezing by emptying the AIL
On 04/17/12 03:26, Dave Chinner wrote:
Yeah, it's pretty clear what is happening here. We don't have freeze protection against EOF zeroing operations. At least xfs_setattr_size() and xfs_change_file_space() fail to check for freeze, and that is initially what I though was causing this problem. However, adding freeze checks into the relevant paths didn't make the hangs go away, so there's more to it than that. Basically, we've been getting races between checking for freeze, the dirtying of the pages and the flusher thread syncing out the dirty data. i.e.: Thread 1 Thread 2 freeze flusher thread write inode A check for freeze grab s_umount SB_FREEZE_WRITE writeback_inodes_sb() iterate dirty inodes inode A not in flush sync_inodes_sb() iterate dirty inodes inode A not in flush dirty pages mark inode A dirty write inode A done. SB_FREEZE_TRANS drop s_umount freeze done sync grab s_umount iterate dirty inodes Flush dirty inode A Before we added the transactional inode size updates, this race simply went unnoticed because nothing caused the flusher thread to block. All the problems I see are due to overwrites of allocated space - if there was real allocation then the delalloc conversion would have always hung. Now we see that when we need to extend the file size when writing, we ahve to allocate a transaction and hence the flusher thread now hangs. While I can "fix" the xfs_setattr_size() and xfs_change_file_space() triggers, they don't close the above race condition, so this problem is essentially unfixable in XFS. The only reason we have not tripped over it before is that the flusher thread didn't hang waiting for a transaction reservation when the race was hit. So why didn't this happen before Christoph's patch set? That's something I can't explain. Oh, wait, yes I can - 068 hangs even without this patch of Christoph's. Actually, looking at my xfstests logs, I can trace the start of the failures back to mid march, and that coincided with an update to the xfstests installed on my test boxes. Which coincides with when my machines first saw this change: commit 281627df3eb55e1b729b9bb06fff5ff112929646 Author: Christoph Hellwig<hch@xxxxxxxxxxxxx> Date: Tue Mar 13 08:41:05 2012 +0000 xfs: log file size updates at I/O completion time That confirms my analysis above - the problem is being exposed by new code in the writeback path that does transaction allocation where it didn't used to. Clearly the problem is not really the new code in Christoph's patches - it's an existing freeze problem that has previously resulted in data writes occuring after a freeze has completed (of which we have had rare complaints about). That sounds pretty dire, except for one thing: Jan Kara's patch set that fixes all these freeze problems: https://lkml.org/lkml/2012/4/16/356 And now that I've run some testing with Jan's patch series, along with Christoph's and mine (75-odd patches;), a couple of my test VMs have been running test 068 in a tight loop for about half an hour without a hang, so I'd consider this problem fixed by Jan's freeze fixes given I could reliably hang it in 2-3 minutes before adding Jan's patch set to my stack. So the fix for this problem is getting Jan's patch set into the kernel at the same time we get the inode size logging changes into the kernel. What do people think about that for a plan? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
Just a heads up, Jan's freeze patch did clear up the test 086 hang on my test box as well, but the 106 (quota test) hang on one of the mounts is still there.
--Mark. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs