On Tue, Dec 06, 2011 at 06:23:23AM -0500, Christoph Hellwig wrote:

On Tue, Dec 06, 2011 at 07:06:40PM +0800, Miao Xie wrote:

quoted

quoted

I can't see why you need the writeout when the trylocks fails.  Umount
needs to take care of writing out all pending file data anyway, so doing
it from the cleaner thread in addition doesn't sound like it would help.

umount invokes sync_fs() and write out all the dirty file data. For the
other file systems, its OK because the file system does not introduce dirty pages
by itself. But btrfs is different. Its automatic defragment will make lots of dirty
pages after sync_fs() and reserve lots of meta-data space for those pages.
And then the cleaner thread may find there is no enough space to reserve, it must
sync the dirty file data and release the reserved space which is for the dirty
file data.

I think the safest way to fix is is to write out all dirty data again
once the cleaner thread has been safely stopped.

Said another way we want to stop the autodefrag code before the unmount
is ready to continue.  We also want to stop balancing, scrub etc.

-chris