Re: [PATCH 1/7] xfs: introduce fake roots for ag-rooted btrees

[Brian Foster <bfoster@redhat.com>]

On Wed, Mar 11, 2020 at 08:45:37PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Create an in-core fake root for AG-rooted btree types so that callers
> can generate a whole new btree using the upcoming btree bulk load
> function without making the new tree accessible from the rest of the
> filesystem.  It is up to the individual btree type to provide a function
> to create a staged cursor (presumably with the appropriate callouts to
> update the fakeroot) and then commit the staged root back into the
> filesystem.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_btree.c |  168 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/libxfs/xfs_btree.h |   30 ++++++++
>  fs/xfs/xfs_trace.h        |   28 ++++++++
>  3 files changed, 225 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index 4ef9f0b42c7f..085bc070e804 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
...
> @@ -4908,3 +4910,169 @@ xfs_btree_has_more_records(
>  	else
>  		return block->bb_u.s.bb_rightsib != cpu_to_be32(NULLAGBLOCK);
>  }
> +
...
> +/*
> + * Initialize a AG-rooted btree cursor with the given AG btree fake root.  The
> + * btree cursor's bc_ops will be overridden as needed to make the staging
> + * functionality work.  If new_ops is not NULL, these new ops will be passed
> + * out to the caller for further overriding.
> + */
> +void
> +xfs_btree_stage_afakeroot(
> +	struct xfs_btree_cur		*cur,
> +	struct xbtree_afakeroot		*afake,
> +	struct xfs_btree_ops		**new_ops)
> +{
> +	struct xfs_btree_ops		*nops;
> +
> +	ASSERT(!(cur->bc_flags & XFS_BTREE_STAGING));
> +	ASSERT(!(cur->bc_flags & XFS_BTREE_ROOT_IN_INODE));
> +	ASSERT(cur->bc_tp == NULL);
> +
> +	nops = kmem_alloc(sizeof(struct xfs_btree_ops), KM_NOFS);
> +	memcpy(nops, cur->bc_ops, sizeof(struct xfs_btree_ops));
> +	nops->alloc_block = xfs_btree_fakeroot_alloc_block;
> +	nops->free_block = xfs_btree_fakeroot_free_block;
> +	nops->init_ptr_from_cur = xfs_btree_fakeroot_init_ptr_from_cur;
> +	nops->set_root = xfs_btree_afakeroot_set_root;
> +	nops->dup_cursor = xfs_btree_fakeroot_dup_cursor;
> +
> +	cur->bc_ag.afake = afake;
> +	cur->bc_nlevels = afake->af_levels;
> +	cur->bc_ops = nops;
> +	cur->bc_flags |= XFS_BTREE_STAGING;
> +
> +	if (new_ops)
> +		*new_ops = nops;

Curious why we have new_ops if the caller unconditionally assigns
->bc_ops to the same value..? That aside:

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

> +}
> +
> +/*
> + * Transform an AG-rooted staging btree cursor back into a regular cursor by
> + * substituting a real btree root for the fake one and restoring normal btree
> + * cursor ops.  The caller must log the btree root change prior to calling
> + * this.
> + */
> +void
> +xfs_btree_commit_afakeroot(
> +	struct xfs_btree_cur		*cur,
> +	struct xfs_trans		*tp,
> +	struct xfs_buf			*agbp,
> +	const struct xfs_btree_ops	*ops)
> +{
> +	ASSERT(cur->bc_flags & XFS_BTREE_STAGING);
> +	ASSERT(cur->bc_tp == NULL);
> +
> +	trace_xfs_btree_commit_afakeroot(cur);
> +
> +	kmem_free((void *)cur->bc_ops);
> +	cur->bc_ag.agbp = agbp;
> +	cur->bc_ops = ops;
> +	cur->bc_flags &= ~XFS_BTREE_STAGING;
> +	cur->bc_tp = tp;
> +}
> diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> index 0d10bbd5223a..aa4a7bd40023 100644
> --- a/fs/xfs/libxfs/xfs_btree.h
> +++ b/fs/xfs/libxfs/xfs_btree.h
> @@ -179,7 +179,10 @@ union xfs_btree_irec {
>  
>  /* Per-AG btree information. */
>  struct xfs_btree_cur_ag {
> -	struct xfs_buf		*agbp;
> +	union {
> +		struct xfs_buf		*agbp;
> +		struct xbtree_afakeroot	*afake;	/* fake ag header root */
> +	};
>  	xfs_agnumber_t		agno;
>  	union {
>  		struct {
> @@ -235,6 +238,12 @@ typedef struct xfs_btree_cur
>  #define XFS_BTREE_LASTREC_UPDATE	(1<<2)	/* track last rec externally */
>  #define XFS_BTREE_CRC_BLOCKS		(1<<3)	/* uses extended btree blocks */
>  #define XFS_BTREE_OVERLAPPING		(1<<4)	/* overlapping intervals */
> +/*
> + * The root of this btree is a fakeroot structure so that we can stage a btree
> + * rebuild without leaving it accessible via primary metadata.  The ops struct
> + * is dynamically allocated and must be freed when the cursor is deleted.
> + */
> +#define XFS_BTREE_STAGING		(1<<5)
>  
>  
>  #define	XFS_BTREE_NOERROR	0
> @@ -515,4 +524,23 @@ xfs_btree_islastblock(
>  	return block->bb_u.s.bb_rightsib == cpu_to_be32(NULLAGBLOCK);
>  }
>  
> +/* Fake root for an AG-rooted btree. */
> +struct xbtree_afakeroot {
> +	/* AG block number of the new btree root. */
> +	xfs_agblock_t		af_root;
> +
> +	/* Height of the new btree. */
> +	unsigned int		af_levels;
> +
> +	/* Number of blocks used by the btree. */
> +	unsigned int		af_blocks;
> +};
> +
> +/* Cursor interactions with with fake roots for AG-rooted btrees. */
> +void xfs_btree_stage_afakeroot(struct xfs_btree_cur *cur,
> +		struct xbtree_afakeroot *afake,
> +		struct xfs_btree_ops **new_ops);
> +void xfs_btree_commit_afakeroot(struct xfs_btree_cur *cur, struct xfs_trans *tp,
> +		struct xfs_buf *agbp, const struct xfs_btree_ops *ops);
> +
>  #endif	/* __XFS_BTREE_H__ */
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 059c3098a4a0..d8c229492973 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -3605,6 +3605,34 @@ TRACE_EVENT(xfs_check_new_dalign,
>  		  __entry->calc_rootino)
>  )
>  
> +TRACE_EVENT(xfs_btree_commit_afakeroot,
> +	TP_PROTO(struct xfs_btree_cur *cur),
> +	TP_ARGS(cur),
> +	TP_STRUCT__entry(
> +		__field(dev_t, dev)
> +		__field(xfs_btnum_t, btnum)
> +		__field(xfs_agnumber_t, agno)
> +		__field(xfs_agblock_t, agbno)
> +		__field(unsigned int, levels)
> +		__field(unsigned int, blocks)
> +	),
> +	TP_fast_assign(
> +		__entry->dev = cur->bc_mp->m_super->s_dev;
> +		__entry->btnum = cur->bc_btnum;
> +		__entry->agno = cur->bc_ag.agno;
> +		__entry->agbno = cur->bc_ag.afake->af_root;
> +		__entry->levels = cur->bc_ag.afake->af_levels;
> +		__entry->blocks = cur->bc_ag.afake->af_blocks;
> +	),
> +	TP_printk("dev %d:%d btree %s ag %u levels %u blocks %u root %u",
> +		  MAJOR(__entry->dev), MINOR(__entry->dev),
> +		  __print_symbolic(__entry->btnum, XFS_BTNUM_STRINGS),
> +		  __entry->agno,
> +		  __entry->levels,
> +		  __entry->blocks,
> +		  __entry->agbno)
> +)
> +
>  #endif /* _TRACE_XFS_H */
>  
>  #undef TRACE_INCLUDE_PATH
>