On Wed, Mar 23, 2011 at 11:20:24AM +1100, Dave Chinner wrote:
> > overlap = xfs_alloc_busy_try_reuse(pag, busyp,
> > fbno, fbno + flen);
> > - if (overlap) {
> > + if (overlap == -1 || (overlap && userdata)) {
> > spin_unlock(&pag->pagb_lock);
> > xfs_log_force(tp->t_mountp, XFS_LOG_SYNC);
> > goto restart;
>
> Ok, so the only time we'll do a log force now is on an complete
> overlap or a partial userdata overlap?
We do it if we would have to split the busy extent, or reallocate
metadata to userdata.
> > + if (!args->userdata ||
> > + (busyp->flags & XFS_ALLOC_BUSY_USERDATA)) {
> > + int overlap;
> > +
> > + overlap = xfs_alloc_busy_try_reuse(args->pag, busyp,
> > + fbno, fbno + flen);
> > + if (unlikely(overlap == -1)) {
> > + spin_unlock(&args->pag->pagb_lock);
> > + xfs_log_force(args->mp, XFS_LOG_SYNC);
> > + goto restart;
> > + }
>
> Hmmmm - I'm not so sure we can reuse overlapped data extents for
> data allocations without a log force at all as there is no guarantee
> that the data will not be overwritten before the original free
> transaction is on disk.
>
> That is, recovery may not replay the original data extent free
> transaction or the new allocation transaction, but there is nothing
> stopping us from having written the new data into the extent before
> the crash occurred, especially as delayed allocation places the
> allocation very close the data IO issue. e.g.:
>
> thread X thread Y
> free data extent ABC
> allocate data extent BCD
> partial overlap, no log force
> issue data IO
> .....
>
> <crash>
>
> That leads to corruption of the data in the original file because
> neither transaction is written to disk, but new data is....
You're right. It's fine from the transaction point of view, but
it could leave wrong data in a file. I'll disallow this case, too.
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