Re: [PATCH] xfs: make tr_growdata a permanent transaction

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Apr 17, 2019 at 05:54:32AM -0400, Brian Foster wrote:
> On Wed, Apr 17, 2019 at 05:36:08AM -0400, Brian Foster wrote:
> > The growdata transaction is used by growfs operations to increase
> > the data size of the filesystem. Part of this sequence involves
> > extending the size of the last preexisting AG in the fs, if
> > necessary. This is implemented by freeing the newly available
> > physical range to the AG.
> > 
> > tr_growdata is not a permanent transaction, however, and block
> > allocation transactions must be permanent to handle deferred frees
> > of AGFL blocks. If the grow operation extends an existing AG that
> > requires AGFL fixing, assert failures occur due to a populated dfops
> > list on a non-permanent transaction and the AGFL free does not
> > occur. This is reproduced (rarely) by xfs/104.
> > 
> > Change tr_growdata to a permanent transaction with a default log
> > count. This increases initial transaction reservation size, but
> > growfs is an infrequent and non-performance critical operation and
> > so should have minimal impact. Also add an assert in the block
> > allocation path to make this transaction requirement explicit and
> > obvious to future callers.
> > 
> > Reported-by: Darrick J. Wong <darrick.wong@oracle.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > This is motivated by Darrick's recent xfs/104 failure report[1]. Note
> > that I made the assert a bit more explicit than originally suggested
> > because I think any transaction that performs block allocation should be
> > expected to be able to handle arbitrary AGFL fixups. This survives an
> > fstests auto run without any regressions[2] or assert failures.
> > 
> > Brian
> > 
> > [1] https://marc.info/?l=linux-xfs&m=155537961822223&w=2
> > [2] Note that I was never able to reproduce the original xfs/104
> > failure.
> > 
> >  fs/xfs/libxfs/xfs_alloc.c      | 2 ++
> >  fs/xfs/libxfs/xfs_trans_resv.c | 6 +++++-
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index bc3367b8b7bb..e4df1866f949 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -2237,6 +2237,8 @@ xfs_alloc_fix_freelist(
> >  	xfs_extlen_t		need;	/* total blocks needed in freelist */
> >  	int			error = 0;
> >  
> > +	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
> > +
> 
> Naturally, just after sending this I'm reminded I never went back to
> actually audit usage of XFS_ALLOC_FLAG_NOSHRINK. :P NOSHRINK is
> currently used in userspace (repair) and scrub. The former is in a place
> that looks to me like it already uses a permanent transaction. The
> latter is not immediately clear to me. It looks like scrub can alloc
> either tr_itruncate, which is permanent, or an empty transaction. The
> noshrink call is associated with repair, which I think means we'd have
> the permanent transaction (?), but it still might make sense to exclude
> NOSHRINK callers from failing the assert in principle. Thoughts?

Scrub should never be calling xfs_alloc_fix_freelist since it's a
readonly operation, and repair should never be running with an empty
transaction, so this should be fine kernel-side.

As for xfs_repair... yes, it does call libxfs_trans_alloc_rollable to
get a "permanent" transaction (which is actually tr_itruncate too).  So
this should be fine in xfsprogs too.

--D

> Brian
> 
> >  	if (!pag->pagf_init) {
> >  		error = xfs_alloc_read_agf(mp, tp, args->agno, flags, &agbp);
> >  		if (error)
> > diff --git a/fs/xfs/libxfs/xfs_trans_resv.c b/fs/xfs/libxfs/xfs_trans_resv.c
> > index f99a7aefe418..83f4ee2afc49 100644
> > --- a/fs/xfs/libxfs/xfs_trans_resv.c
> > +++ b/fs/xfs/libxfs/xfs_trans_resv.c
> > @@ -876,9 +876,13 @@ xfs_trans_resv_calc(
> >  	resp->tr_sb.tr_logres = xfs_calc_sb_reservation(mp);
> >  	resp->tr_sb.tr_logcount = XFS_DEFAULT_LOG_COUNT;
> >  
> > +	/* growdata requires permanent res; it can free space to the last AG */
> > +	resp->tr_growdata.tr_logres = xfs_calc_growdata_reservation(mp);
> > +	resp->tr_growdata.tr_logcount = XFS_DEFAULT_PERM_LOG_COUNT;
> > +	resp->tr_growdata.tr_logflags |= XFS_TRANS_PERM_LOG_RES;
> > +
> >  	/* The following transaction are logged in logical format */
> >  	resp->tr_ichange.tr_logres = xfs_calc_ichange_reservation(mp);
> > -	resp->tr_growdata.tr_logres = xfs_calc_growdata_reservation(mp);
> >  	resp->tr_fsyncts.tr_logres = xfs_calc_swrite_reservation(mp);
> >  	resp->tr_writeid.tr_logres = xfs_calc_writeid_reservation(mp);
> >  	resp->tr_attrsetrt.tr_logres = xfs_calc_attrsetrt_reservation(mp);
> > -- 
> > 2.17.2
> >