On Wed, Feb 14, 2007 at 03:26:22PM -0600, Dave Kleikamp wrote: > On Thu, 2007-02-15 at 07:31 +1100, David Chinner wrote: > > On Wed, Feb 14, 2007 at 11:54:54AM -0800, Valerie Henson wrote: > > > Just some quick notes on possible ways to fix the ext2 fsync bug that > > > eXplode found. Whether or not anyone will bother to implement it is > > > another matter. > > > > > > Background: The eXplode file system checker found a bug in ext2 fsync > > > behavior. Do the following: truncate file A, create file B which > > > reallocates one of A's old indirect blocks, fsync file B. If you then > > > crash before file A's metadata is all written out, fsck will complete > > > the truncate for file A... thereby deleting file B's data. So fsync > > > file B doesn't guarantee data is on disk after a crash. Details: > > > > > > http://www.stanford.edu/~engler/explode-osdi06.pdf > > > > > > Two possible solutions I can think of: > > > > > > * Rearrange order of duplicate block checking and fixing file size in > > > fsck. Not sure how hard this is. (Ted?) > > > > > > * Keep a set of "still allocated on disk" block bitmaps that gets > > > flushed whenever a sync happens. Don't allocate these blocks. > > > Journaling file systems already have to do this. > > > > You don't need anything on disk or to fsck to fix this problem - just > > avoid it completely by keeping a list of recently truncated blocks in > > memory and don't reuse them until the old owner inode is sync'd to disk. > > I think that's pretty much what Val is suggesting. She suggests bitmaps > rather than a list though. Maybe she should have used a better term than > "flushed", as this list only needs to be cleared, rather than written to > disk. Yeah, probably was - I misparsed the still allocated on disk block bitmaps phrase differently to what may have been intended... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - 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
