Re: [PATCH 3/3] xfs: sanitise sb_bad_features2 handling

[Brian Foster <bfoster@redhat.com>]

On Thu, Jan 08, 2015 at 08:51:05AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> We currently have to ensure that every time we update sb_features2
> that we update sb_bad_features2. Now that we log and format the
> superblock in it's entirety we actually don't have to care because
> we can simply update the sb_bad_features2 when we format it into the
> buffer. This removes the need for anything but the mount and
> superblock formatting code to care about sb_bad_features2, and
> hence removes the possibility that we forget to update bad_features2
> when necessary in the future.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/libxfs/xfs_format.h | 14 +++++++-------
>  fs/xfs/libxfs/xfs_sb.c     |  8 +++++++-
>  fs/xfs/xfs_mount.c         | 23 +++++++++++------------
>  3 files changed, 25 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index 4762732..8eb7189 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -151,10 +151,13 @@ typedef struct xfs_sb {
>  	__uint32_t	sb_features2;	/* additional feature bits */
>  
>  	/*
> -	 * bad features2 field as a result of failing to pad the sb
> -	 * structure to 64 bits. Some machines will be using this field
> -	 * for features2 bits. Easiest just to mark it bad and not use
> -	 * it for anything else.
> +	 * bad features2 field as a result of failing to pad the sb structure to
> +	 * 64 bits. Some machines will be using this field for features2 bits.
> +	 * Easiest just to mark it bad and not use it for anything else.
> +	 *
> +	 * This is not kept up to date in memory; it is always overwritten by
> +	 * the value in sb_features2 when formatting the incore superblock to
> +	 * the disk buffer.
>  	 */
>  	__uint32_t	sb_bad_features2;
>  
> @@ -453,13 +456,11 @@ static inline void xfs_sb_version_addattr2(struct xfs_sb *sbp)
>  {
>  	sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
>  	sbp->sb_features2 |= XFS_SB_VERSION2_ATTR2BIT;
> -	sbp->sb_bad_features2 |= XFS_SB_VERSION2_ATTR2BIT;
>  }
>  
>  static inline void xfs_sb_version_removeattr2(struct xfs_sb *sbp)
>  {
>  	sbp->sb_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
> -	sbp->sb_bad_features2 &= ~XFS_SB_VERSION2_ATTR2BIT;
>  	if (!sbp->sb_features2)
>  		sbp->sb_versionnum &= ~XFS_SB_VERSION_MOREBITSBIT;
>  }
> @@ -475,7 +476,6 @@ static inline void xfs_sb_version_addprojid32bit(struct xfs_sb *sbp)
>  {
>  	sbp->sb_versionnum |= XFS_SB_VERSION_MOREBITSBIT;
>  	sbp->sb_features2 |= XFS_SB_VERSION2_PROJID32BIT;
> -	sbp->sb_bad_features2 |= XFS_SB_VERSION2_PROJID32BIT;
>  }
>  
>  /*
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index 6ee3af6..b440839 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -488,7 +488,6 @@ xfs_sb_to_disk(
>  	to->sb_fdblocks = cpu_to_be64(from->sb_fdblocks);
>  	to->sb_frextents = cpu_to_be64(from->sb_frextents);
>  
> -
>  	to->sb_flags = from->sb_flags;
>  	to->sb_shared_vn = from->sb_shared_vn;
>  	to->sb_inoalignmt = cpu_to_be32(from->sb_inoalignmt);
> @@ -498,6 +497,13 @@ xfs_sb_to_disk(
>  	to->sb_logsectlog = from->sb_logsectlog;
>  	to->sb_logsectsize = cpu_to_be16(from->sb_logsectsize);
>  	to->sb_logsunit = cpu_to_be32(from->sb_logsunit);
> +
> +	/*
> +	 * We need to ensure that bad_features2 always matches features2.
> +	 * Hence we enforce that here rather than having to remember to do it
> +	 * everywhere else that updates features2.
> +	 */
> +	from->sb_bad_features2 = from->sb_features2;
>  	to->sb_features2 = cpu_to_be32(from->sb_features2);
>  	to->sb_bad_features2 = cpu_to_be32(from->sb_bad_features2);
>  
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 5ef9aa2..4fa80e6 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -640,25 +640,24 @@ xfs_mountfs(
>  	xfs_sb_mount_common(mp, sbp);
>  
>  	/*
> -	 * Check for a mismatched features2 values.  Older kernels
> -	 * read & wrote into the wrong sb offset for sb_features2
> -	 * on some platforms due to xfs_sb_t not being 64bit size aligned
> -	 * when sb_features2 was added, which made older superblock
> -	 * reading/writing routines swap it as a 64-bit value.
> +	 * Check for a mismatched features2 values.  Older kernels read & wrote
> +	 * into the wrong sb offset for sb_features2 on some platforms due to
> +	 * xfs_sb_t not being 64bit size aligned when sb_features2 was added,
> +	 * which made older superblock reading/writing routines swap it as a
> +	 * 64-bit value.
>  	 *
>  	 * For backwards compatibility, we make both slots equal.
>  	 *
> -	 * If we detect a mismatched field, we OR the set bits into the
> -	 * existing features2 field in case it has already been modified; we
> -	 * don't want to lose any features.  We then update the bad location
> -	 * with the ORed value so that older kernels will see any features2
> -	 * flags, and mark the two fields as needing updates once the
> -	 * transaction subsystem is online.
> +	 * If we detect a mismatched field, we OR the set bits into the existing
> +	 * features2 field in case it has already been modified; we don't want
> +	 * to lose any features.  We then update the bad location with the ORed
> +	 * value so that older kernels will see any features2 flags. The
> +	 * superblock writeback code ensures the new sb_features2 is copied to
> +	 * sb_bad_features2 before it is logged or written to disk.
>  	 */
>  	if (xfs_sb_has_mismatched_features2(sbp)) {
>  		xfs_warn(mp, "correcting sb_features alignment problem");
>  		sbp->sb_features2 |= sbp->sb_bad_features2;
> -		sbp->sb_bad_features2 = sbp->sb_features2;
>  		mp->m_update_sb = true;
>  
>  		/*
> -- 
> 2.0.0
> 
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