Re: [PATCH 5.10 CANDIDATE 7/8] xfs: consider shutdown in bmapbt cursor delete assert

From: Dave Chinner <david@fromorbit.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: "Darrick J . Wong" <djwong@kernel.org>,
	Brian Foster <bfoster@redhat.com>, Christoph Hellwig <hch@lst.de>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Adam Manzanares <a.manzanares@samsung.com>,
	Tyler Hicks <code@tyhicks.com>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	fstests <fstests@vger.kernel.org>
Subject: Re: [PATCH 5.10 CANDIDATE 7/8] xfs: consider shutdown in bmapbt cursor delete assert
Date: Thu, 2 Jun 2022 15:15:04 +1000	[thread overview]
Message-ID: <20220602051504.GL227878@dread.disaster.area> (raw)
In-Reply-To: <CAOQ4uxg8Y_7mkf+ieWBAuspCDG+H5Ci2P7xMudxF49nV5M0czg@mail.gmail.com>

On Thu, Jun 02, 2022 at 07:24:26AM +0300, Amir Goldstein wrote:
> On Thu, Jun 2, 2022 at 3:38 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Wed, Jun 01, 2022 at 01:45:46PM +0300, Amir Goldstein wrote:
> > > From: Brian Foster <bfoster@redhat.com>
> > >
> > > commit 1cd738b13ae9b29e03d6149f0246c61f76e81fcf upstream.
> > >
> > > The assert in xfs_btree_del_cursor() checks that the bmapbt block
> > > allocation field has been handled correctly before the cursor is
> > > freed. This field is used for accurate calculation of indirect block
> > > reservation requirements (for delayed allocations), for example.
> > > generic/019 reproduces a scenario where this assert fails because
> > > the filesystem has shutdown while in the middle of a bmbt record
> > > insertion. This occurs after a bmbt block has been allocated via the
> > > cursor but before the higher level bmap function (i.e.
> > > xfs_bmap_add_extent_hole_real()) completes and resets the field.
> > >
> > > Update the assert to accommodate the transient state if the
> > > filesystem has shutdown. While here, clean up the indentation and
> > > comments in the function.
> > >
> > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > ---
> > >  fs/xfs/libxfs/xfs_btree.c | 33 ++++++++++++---------------------
> > >  1 file changed, 12 insertions(+), 21 deletions(-)
> >
> > https://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git/commit/?h=for-next&id=56486f307100e8fc66efa2ebd8a71941fa10bf6f
> >
> 
> Warm from the over :)
> 
> I will need more time to verify that this new fix is not breaking LTS
> but I don't think that it should be blocking taking the old 5.12 fix now.
> Right?

Rule #1: don't introduce new bugs into stable kernels.

This commit has a known (and fixed) bug in it. If you are going to
back port it to a stable kernel, then you need to also pull in the
fix for that commit, too.

But the bigger question is this: why propose backports of commits
that only change debug code?

ASSERT()s are not compiled into production kernels - they are only
compiled into developer builds when CONFIG_XFS_DEBUG=y is set. It is
test code, not production code, hence nobody will be using this in
production kernels.

I don't see the value in backporting debug fixes unless there
is some other dependency that requires them. But if you are going to
back port them, Rule #1 applies.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com