Re: Fix(es) for ext2 fsync bug

[Dave Kleikamp <shaggy@linux.vnet.ibm.com>]

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.

-- 
David Kleikamp
IBM Linux Technology Center