On Wed, Dec 07, 2011 at 10:31:35AM +0800, Miao Xie wrote: > On tue, 6 Dec 2011 16:36:11 -0500, Chris Mason wrote: > > 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: > >>>> 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. > > But there is no good interface to do it before umount gets s_umount lock. > I think trylock(in writeback_inodes_sb_nr_if_idle()) + dirty data flush > can help us to fix the bug perfectly. But it won't fix the umount while balancing family of deadlocks (they are really of the same nature, vfs grabs s_umount mutex and we need it to proceed). (Balance cancelling code is part of restriper patches, it's just a hook in close_ctree() that waits until we are done relocating a chunk - very similar to cleaner wait) One example would be that balancing code while dirtying pages calls balance_dirty_pages_ratelimited() for each dirtied page, as it should. And if balance_dirty_pages() then decides to initiate writeback we are stuck schedule()ing forever, because writeback can't proceed w/o read-taking s_umount mutex which is fully held by vfs - it just skips the relocation inode. Thanks, Ilya -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
