Re: [PATCH] xfs: require a relatively recent V5 filesystem for LARP mode

[Bill O'Donnell <bodonnel@redhat.com>]

On Thu, Aug 31, 2023 at 10:07:39PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> While reviewing the FIEXCHANGE code in XFS, I realized that the function
> that enables logged xattrs doesn't actually check that the superblock
> has a LOG_INCOMPAT feature bit field.  Add a check to refuse the
> operation if we don't have a V5 filesystem...
> 
> ...but on second though, let's require either reflink or rmap so that we
> only have to deal with LARP mode on relatively /modern/ kernel.  4.14 is
> about as far back as I feel like going.
> 
> Seeing as LARP is a debugging-only option anyway, this isn't likely to
> affect any real users.
> 
> Fixes: d9c61ccb3b09 ("xfs: move xfs_attr_use_log_assist out of xfs_log.c")
> Really-Fixes: f3f36c893f26 ("xfs: Add xfs_attr_set_deferred and xfs_attr_remove_deferred")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>

> ---
>  fs/xfs/xfs_xattr.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/xfs/xfs_xattr.c b/fs/xfs/xfs_xattr.c
> index 43e5c219aaed..a3975f325f4e 100644
> --- a/fs/xfs/xfs_xattr.c
> +++ b/fs/xfs/xfs_xattr.c
> @@ -46,6 +46,17 @@ xfs_attr_grab_log_assist(
>  	if (xfs_sb_version_haslogxattrs(&mp->m_sb))
>  		return 0;
>  
> +	/*
> +	 * Check if the filesystem featureset is new enough to set this log
> +	 * incompat feature bit.  Strictly speaking, the minimum requirement is
> +	 * a V5 filesystem for the superblock field, but we'll require rmap
> +	 * or reflink to avoid having to deal with really old kernels.
> +	 */
> +	if (!xfs_has_reflink(mp) && !xfs_has_rmapbt(mp)) {
> +		error = -EOPNOTSUPP;
> +		goto drop_incompat;
> +	}
> +
>  	/* Enable log-assisted xattrs. */
>  	error = xfs_add_incompat_log_feature(mp,
>  			XFS_SB_FEAT_INCOMPAT_LOG_XATTRS);
>