Re: [PATCH] xfs: don't call into blockgc scan with freeze protection

From: Dave Chinner <david@fromorbit.com>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Brian Foster <bfoster@redhat.com>, linux-xfs@vger.kernel.org
Subject: Re: [PATCH] xfs: don't call into blockgc scan with freeze protection
Date: Fri, 19 Feb 2021 15:56:58 +1100	[thread overview]
Message-ID: <20210219045658.GF4662@dread.disaster.area> (raw)
In-Reply-To: <20210219032309.GX7193@magnolia>

On Thu, Feb 18, 2021 at 07:23:09PM -0800, Darrick J. Wong wrote:
> On Thu, Feb 18, 2021 at 03:14:58PM -0500, Brian Foster wrote:
> > fstest xfs/167 produced a lockdep splat that complained about a
> > nested transaction acquiring freeze protection during an eofblocks
> > scan. Drop freeze protection around the block reclaim scan in the
> > transaction allocation code to avoid this problem.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> I think it seems reasonable, though I really wish that other patchset to
> clean up all the "modify thread state when we start/end transactions"
> had landed.
> 
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> 
> --D
> 
> > ---
> >  fs/xfs/xfs_trans.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> > index 44f72c09c203..c32c62d3b77a 100644
> > --- a/fs/xfs/xfs_trans.c
> > +++ b/fs/xfs/xfs_trans.c
> > @@ -261,6 +261,7 @@ xfs_trans_alloc(
> >  {
> >  	struct xfs_trans	*tp;
> >  	int			error;
> > +	bool			retried = false;
> >  
> >  	/*
> >  	 * Allocate the handle before we do our freeze accounting and setting up
> > @@ -288,19 +289,27 @@ xfs_trans_alloc(
> >  	INIT_LIST_HEAD(&tp->t_dfops);
> >  	tp->t_firstblock = NULLFSBLOCK;
> >  
> > +retry:
> >  	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> > -	if (error == -ENOSPC) {
> > +	if (error == -ENOSPC && !retried) {
> >  		/*
> >  		 * We weren't able to reserve enough space for the transaction.
> >  		 * Flush the other speculative space allocations to free space.
> >  		 * Do not perform a synchronous scan because callers can hold
> >  		 * other locks.
> >  		 */
> > +		retried = true;
> > +		if (!(flags & XFS_TRANS_NO_WRITECOUNT))
> > +			sb_end_intwrite(mp->m_super);
> >  		error = xfs_blockgc_free_space(mp, NULL);
> > -		if (!error)
> > -			error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> > -	}
> > -	if (error) {
> > +		if (error) {
> > +			kmem_cache_free(xfs_trans_zone, tp);
> > +			return error;
> > +		}

This seems dangerous to me. If xfs_trans_reserve() adds anything to
the transaction even if it fails, this will fail to free it. e.g.
xfs_log_reserve() call allocate a ticket and attach it to the
transaction and *then fail*. This code will now leak that ticket.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com