Re: [PATCH 04/15] xfs: dynamically allocate btree scrub context structure

[Dave Chinner <david@fromorbit.com>]

On Tue, Oct 12, 2021 at 04:32:55PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Reorganize struct xchk_btree so that we can dynamically size the context
> structure to fit the type of btree cursor that we have.  This will
> enable us to use memory more efficiently once we start adding very tall
> btree types.  Right-size the lastkey array so that we stop wasting the
> first array element.

"right size"?

I'm assuming this is the "nlevels - 1" bit?

> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/scrub/btree.c |   23 ++++++++++++-----------
>  fs/xfs/scrub/btree.h |   11 ++++++++++-
>  2 files changed, 22 insertions(+), 12 deletions(-)
> 
> 
> diff --git a/fs/xfs/scrub/btree.c b/fs/xfs/scrub/btree.c
> index d5e1ca521fc4..6d4eba85ef77 100644
> --- a/fs/xfs/scrub/btree.c
> +++ b/fs/xfs/scrub/btree.c
> @@ -189,9 +189,9 @@ xchk_btree_key(
>  
>  	/* If this isn't the first key, are they in order? */
>  	if (cur->bc_ptrs[level] > 1 &&
> -	    !cur->bc_ops->keys_inorder(cur, &bs->lastkey[level], key))
> +	    !cur->bc_ops->keys_inorder(cur, &bs->lastkey[level - 1], key))
>  		xchk_btree_set_corrupt(bs->sc, cur, level);
> -	memcpy(&bs->lastkey[level], key, cur->bc_ops->key_len);
> +	memcpy(&bs->lastkey[level - 1], key, cur->bc_ops->key_len);
>  
>  	if (level + 1 >= cur->bc_nlevels)
>  		return;
> @@ -631,17 +631,24 @@ xchk_btree(
>  	union xfs_btree_ptr		*pp;
>  	union xfs_btree_rec		*recp;
>  	struct xfs_btree_block		*block;
> -	int				level;
>  	struct xfs_buf			*bp;
>  	struct check_owner		*co;
>  	struct check_owner		*n;
> +	size_t				cur_sz;
> +	int				level;
>  	int				error = 0;
>  
>  	/*
>  	 * Allocate the btree scrub context from the heap, because this
> -	 * structure can get rather large.
> +	 * structure can get rather large.  Don't let a caller feed us a
> +	 * totally absurd size.
>  	 */
> -	bs = kmem_zalloc(sizeof(struct xchk_btree), KM_NOFS | KM_MAYFAIL);
> +	cur_sz = xchk_btree_sizeof(cur->bc_nlevels);
> +	if (cur_sz > PAGE_SIZE) {
> +		xchk_btree_set_corrupt(sc, cur, 0);
> +		return 0;
> +	}
> +	bs = kmem_zalloc(cur_sz, KM_NOFS | KM_MAYFAIL);
>  	if (!bs)
>  		return -ENOMEM;
>  	bs->cur = cur;
> @@ -653,12 +660,6 @@ xchk_btree(
>  	/* Initialize scrub state */
>  	INIT_LIST_HEAD(&bs->to_check);
>  
> -	/* Don't try to check a tree with a height we can't handle. */
> -	if (cur->bc_nlevels > XFS_BTREE_MAXLEVELS) {
> -		xchk_btree_set_corrupt(sc, cur, 0);
> -		goto out;
> -	}
> -
>  	/*
>  	 * Load the root of the btree.  The helper function absorbs
>  	 * error codes for us.
> diff --git a/fs/xfs/scrub/btree.h b/fs/xfs/scrub/btree.h
> index 7671108f9f85..62c3091ef20f 100644
> --- a/fs/xfs/scrub/btree.h
> +++ b/fs/xfs/scrub/btree.h
> @@ -39,9 +39,18 @@ struct xchk_btree {
>  
>  	/* internal scrub state */
>  	union xfs_btree_rec		lastrec;
> -	union xfs_btree_key		lastkey[XFS_BTREE_MAXLEVELS];
>  	struct list_head		to_check;
> +
> +	/* this element must come last! */
> +	union xfs_btree_key		lastkey[];
>  };
> +
> +static inline size_t
> +xchk_btree_sizeof(unsigned int nlevels)
> +{
> +	return struct_size((struct xchk_btree *)NULL, lastkey, nlevels - 1);
> +}

I'd like a comment here indicating that the max number of keys is
"nlevels - 1" because the last level of the tree is records and
that's held in a separate lastrec field...

That way there's a reminder of why there's a "- 1" here without
having work it out from first principles every time we look at this
code...

Otherwise it seems reasonable.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com