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    Subject[PATCH 4.9 91/94] xfs: xfs_trans_alloc_empty
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Christoph Hellwig <hch@lst.de>

    This is a partial cherry-pick of commit e89c041338
    ("xfs: implement the GETFSMAP ioctl"), which also adds this helper, and
    a great example of why feature patches should be properly split into
    their parts.

    Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
    [hch: split from the larger patch for -stable]
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    ---
    fs/xfs/xfs_trans.c | 22 ++++++++++++++++++++++
    fs/xfs/xfs_trans.h | 2 ++
    2 files changed, 24 insertions(+)

    --- a/fs/xfs/xfs_trans.c
    +++ b/fs/xfs/xfs_trans.c
    @@ -263,6 +263,28 @@ xfs_trans_alloc(
    }

    /*
    + * Create an empty transaction with no reservation. This is a defensive
    + * mechanism for routines that query metadata without actually modifying
    + * them -- if the metadata being queried is somehow cross-linked (think a
    + * btree block pointer that points higher in the tree), we risk deadlock.
    + * However, blocks grabbed as part of a transaction can be re-grabbed.
    + * The verifiers will notice the corrupt block and the operation will fail
    + * back to userspace without deadlocking.
    + *
    + * Note the zero-length reservation; this transaction MUST be cancelled
    + * without any dirty data.
    + */
    +int
    +xfs_trans_alloc_empty(
    + struct xfs_mount *mp,
    + struct xfs_trans **tpp)
    +{
    + struct xfs_trans_res resv = {0};
    +
    + return xfs_trans_alloc(mp, &resv, 0, 0, XFS_TRANS_NO_WRITECOUNT, tpp);
    +}
    +
    +/*
    * Record the indicated change to the given field for application
    * to the file system's superblock when the transaction commits.
    * For now, just store the change in the transaction structure.
    --- a/fs/xfs/xfs_trans.h
    +++ b/fs/xfs/xfs_trans.h
    @@ -159,6 +159,8 @@ typedef struct xfs_trans {
    int xfs_trans_alloc(struct xfs_mount *mp, struct xfs_trans_res *resp,
    uint blocks, uint rtextents, uint flags,
    struct xfs_trans **tpp);
    +int xfs_trans_alloc_empty(struct xfs_mount *mp,
    + struct xfs_trans **tpp);
    void xfs_trans_mod_sb(xfs_trans_t *, uint, int64_t);

    struct xfs_buf *xfs_trans_get_buf_map(struct xfs_trans *tp,

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