On Thu 06-03-08 01:42:34, Aneesh Kumar K.V wrote: > On Mon, Feb 04, 2008 at 05:31:56PM +0100, Jan Kara wrote: > > Hi, > > > > On Mon 04-02-08 15:42:28, Aneesh Kumar K.V wrote: > > > This is with the new ext3 -> ext4 migrate code added. The recently added > > > lockdep for jbd2 helped to find this out. We want to hold the i_data_sem > > > on the ext3 inode during migration to prevent walking the ext3 inode > > > when it is being converted to ext4 format. Also we want to avoid > > > file truncation and new blocks being added while converting to ext4. > > > Also we dont want to reserve large number of credits for journal. > > > Any idea how to fix this ? > > Hmm, while briefly looking at the code - why do you introduce i_data_sem > > and not use i_alloc_sem which is already in VFS inode? That is aimed > > exactly at the serialization of truncates, writes and similar users. > > That doesn't solve problems with lock ordering but I was just wondering... > > Another problem - ext4_fallocate() has the same lock ordering problem as > > the migration code and maybe there are others... > > One (stupid) solution to your problem is to make i_data_sem be > > always locked before the transaction is started. It could possibly have > > negative performance impact because you'd have to hold the semaphore for > > a longer time and thus a writer would block readers for longer time. So one > > would have to measure how big difference that would make. > > Another possibility is to start a single transaction for migration and > > extend it as long as you can (as truncate does it). And when you can't > > extend any more, you drop the i_data_sem and start a new transaction and > > acquire the semaphore again. This has the disadvantage that after dropping > > the semaphore you have to resync your original inode with the temporary > > one your are building which probably ends up being ugly as night... Hmm, > > but maybe we could get rid of this - hold i_mutex to protect against all > > writes (that ranks outside of transaction start so you can hold it for the > > whole migration time - maybe you even hold it if you are called from the > > write path...). After dropping i_data_sem you let some readers proceed > > but writers still wait on i_mutex so the file shouldn't change under you > > (but I suggest adding some BUG_ONs to verify that the file really doesn't > > change :). > > > > This one came up again when i was doing the mmap ENOSPC patch. Now the > current code with migrate is taking i_mutex to protech against all > writes. But it seems a write to a mmap area mapping a hole can still go > through fine. And that path cannot take i_mutex. Ah, yeah. I forgot about this case. Yes, mmaped write can happen anytime and when we writeout these dirty data through writepage() we are not guarded by i_mutex :(. > So i guess the migrate locking need to be fixed. Any suggestion ? Not a simple one. Will think about it. 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
