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

[Brian Foster <bfoster@redhat.com>]

On Wed, Apr 17, 2019 at 07:37:59AM -0700, Darrick J. Wong 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);
> 
> /me wonders if we ought to have a comment here about why we're asserting
> on this? i.e.
> 

Yep.

> /*
>  * This function can shrink the AGFL, which uses a deferred op to avoid
>  * exceeding transaction reservation (or whatever the original reason
>  * was).  Deferred ops require a transaction with a permanent
>  * reservation, so check that here.
>  */
> ASSERT(...);
> 
> Send me back a finished comment and I'll add it on its way in; the rest
> of the patch looks ok to me.
> 

Though to me a one-liner seems sufficient since this is just an assert.
E.g.,

	/* deferred ops (AGFL block frees) require permanent transactions */
	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);

... seems enough to document the intent. Hm?

Brian

> --D
> 
> > +
> >  	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
> >