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

[Brian Foster <bfoster@redhat.com>]

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.

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com

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