Re: [PATCH] xfs: remove XFS_TRANS_RESERVE in collapse range

Dave Chinner
2014-05-08T22:05:27Z

On Thu, May 08, 2014 at 05:51:16PM -0400, Brian Foster wrote:
> On Fri, May 09, 2014 at 07:17:12AM +1000, Dave Chinner wrote:
> > On Thu, May 08, 2014 at 09:36:37AM -0400, Brian Foster wrote:
> > > On Thu, May 08, 2014 at 06:49:14PM +0900, Namjae Jeon wrote:
> > > > There is no need to dip into reserve pool. Reserve pool is used for much
> > > > more important things. And xfs_trans_reserve will never return ENOSPC
> > > > because punch hole is already done. If we get ENOSPC, collapse range
> > > > will be simply failed.
> > > >
> > > > Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
> > > > Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
> > > > ---
> > > > fs/xfs/xfs_bmap_util.c | 3 +--
> > > > 1 file changed, 1 insertion(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > > > index 296160b..91a43c5 100644
> > > > --- a/fs/xfs/xfs_bmap_util.c
> > > > +++ b/fs/xfs/xfs_bmap_util.c
> > > > @@ -1519,7 +1519,6 @@ xfs_collapse_file_space(
> > > >
> > > > while (!error && !done) {
> > > > tp = xfs_trans_alloc(mp, XFS_TRANS_DIOSTRAT);
> > > > - tp->t_flags |= XFS_TRANS_RESERVE;
> > >
> > > Makes sense.
> > >
> > > > /*
> > > > * We would need to reserve permanent block for transaction.
> > > > * This will come into picture when after shifting extent into
> > > > @@ -1529,7 +1528,7 @@ xfs_collapse_file_space(
> > > > error = xfs_trans_reserve(tp, &M_RES(mp)->tr_write,
> > > > XFS_DIOSTRAT_SPACE_RES(mp, 0), 0);
> > > > if (error) {
> > > > - ASSERT(error == ENOSPC || XFS_FORCED_SHUTDOWN(mp));
> > > > + ASSERT(XFS_FORCED_SHUTDOWN(mp));
> > >
> > > The xfs_trans_reserve() call still reserves XFS_DIOSTRAT_SPACE_RES()
> > > blocks, so therefore I think ENOSPC is still a possibility. The question
> > > is probably whether or not we need to reserve blocks for this
> > > transaction.
> > >
> > > Making a pass through the code... we have the possibility of deleting a
> > > btree record in xfs_bmap_shift_extents(). This in turn could potentially
> > > free a btree block, which frees space. I _think_ this could mean we
> > > want to keep the block reservation because we update the free space
> > > trees, but I suppose that could be handled by the freelist...
> > >
> > > Perhaps Dave can confirm which direction we should go here..?
> >
> > Having collapse range fail with ENOSPC is not an issue - it is being
> > executed in a context where we can fail safely and return an error
> > to the user.
> >
> > XFS_TRANS_RESERVE is used in places where a failure is unrecoverable
> > or there is no one to report the error to. e.g. prevent data loss
> > due to ENOSPC in unwritten extent conversion during background
> > buffered write IO completion
> >
> > So here there is no need for it at all....
> >
>
> Yeah, dropping XFS_TRANS_RESERVE makes sense. The question I have is
> whether we should reserve blocks for this transaction (for a potential
> bmbt block free). If we do reserve blocks, then I assume the ENOSPC
> assert should stick around.

Yes, the assert is wrong because xfs_trans_reserve() can
return ENOMEM as well as ENOSPC. It should just be removed.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com