Re: [PATCH 2/6] xfs: break up xfs_buf_find() into individual pieces

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jun 27, 2022 at 04:08:37PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> xfs_buf_find() is made up of three main parts: lookup, insert and
> locking. The interactions with xfs_buf_get_map() require it to be
> called twice - once for a pure lookup, and again on lookup failure
> so the insert path can be run. We want to simplify this down a lot,
> so split it into a fast path lookup, a slow path insert and a "lock
> the found buffer" helper. This will then let use integrate these
> operations more effectively into xfs_buf_get_map() in future
> patches.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_buf.c | 159 +++++++++++++++++++++++++++++++----------------
>  1 file changed, 105 insertions(+), 54 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 143e1c70df5d..95d4b428aec0 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -503,77 +503,60 @@ xfs_buf_hash_destroy(
>  	rhashtable_destroy(&pag->pag_buf_hash);
>  }
>  
> -/*
> - * Look up a buffer in the buffer cache and return it referenced and locked
> - * in @found_bp.
> - *
> - * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
> - * cache.
> - *
> - * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
> - * -EAGAIN if we fail to lock it.
> - *
> - * Return values are:
> - *	-EFSCORRUPTED if have been supplied with an invalid address
> - *	-EAGAIN on trylock failure
> - *	-ENOENT if we fail to find a match and @new_bp was NULL
> - *	0, with @found_bp:
> - *		- @new_bp if we inserted it into the cache
> - *		- the buffer we found and locked.
> - */
>  static int
> -xfs_buf_find(
> +xfs_buf_find_verify(

Isn't this more of a xfs_buf_map verifier?  Why not call it
xfs_buf_map_verify()?

--D

>  	struct xfs_buftarg	*btp,
> -	struct xfs_buf_map	*map,
> -	int			nmaps,
> -	xfs_buf_flags_t		flags,
> -	struct xfs_buf		*new_bp,
> -	struct xfs_buf		**found_bp)
> +	struct xfs_buf_map	*map)
>  {
> -	struct xfs_perag	*pag;
> -	struct xfs_buf		*bp;
> -	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
>  	xfs_daddr_t		eofs;
> -	int			i;
> -
> -	*found_bp = NULL;
> -
> -	for (i = 0; i < nmaps; i++)
> -		cmap.bm_len += map[i].bm_len;
>  
>  	/* Check for IOs smaller than the sector size / not sector aligned */
> -	ASSERT(!(BBTOB(cmap.bm_len) < btp->bt_meta_sectorsize));
> -	ASSERT(!(BBTOB(cmap.bm_bn) & (xfs_off_t)btp->bt_meta_sectormask));
> +	ASSERT(!(BBTOB(map->bm_len) < btp->bt_meta_sectorsize));
> +	ASSERT(!(BBTOB(map->bm_bn) & (xfs_off_t)btp->bt_meta_sectormask));
>  
>  	/*
>  	 * Corrupted block numbers can get through to here, unfortunately, so we
>  	 * have to check that the buffer falls within the filesystem bounds.
>  	 */
>  	eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
> -	if (cmap.bm_bn < 0 || cmap.bm_bn >= eofs) {
> +	if (map->bm_bn < 0 || map->bm_bn >= eofs) {
>  		xfs_alert(btp->bt_mount,
>  			  "%s: daddr 0x%llx out of range, EOFS 0x%llx",
> -			  __func__, cmap.bm_bn, eofs);
> +			  __func__, map->bm_bn, eofs);
>  		WARN_ON(1);
>  		return -EFSCORRUPTED;
>  	}
> +	return 0;
> +}
>  
> -	pag = xfs_perag_get(btp->bt_mount,
> -			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
> +static inline struct xfs_buf *
> +xfs_buf_find_fast(
> +	struct xfs_perag	*pag,
> +	struct xfs_buf_map	*map)
> +{
> +	struct xfs_buf          *bp;
>  
> -	spin_lock(&pag->pag_buf_lock);
> -	bp = rhashtable_lookup_fast(&pag->pag_buf_hash, &cmap,
> -				    xfs_buf_hash_params);
> -	if (bp) {
> -		atomic_inc(&bp->b_hold);
> -		goto found;
> -	}
> +	bp = rhashtable_lookup(&pag->pag_buf_hash, map, xfs_buf_hash_params);
> +	if (!bp)
> +		return NULL;
> +	atomic_inc(&bp->b_hold);
> +	return bp;
> +}
>  
> +/*
> + * Insert the new_bp into the hash table. This consumes the perag reference
> + * taken for the lookup.
> + */
> +static int
> +xfs_buf_find_insert(
> +	struct xfs_buftarg	*btp,
> +	struct xfs_perag	*pag,
> +	struct xfs_buf		*new_bp)
> +{
>  	/* No match found */
>  	if (!new_bp) {
> -		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
> -		spin_unlock(&pag->pag_buf_lock);
>  		xfs_perag_put(pag);
> +		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
>  		return -ENOENT;
>  	}
>  
> @@ -581,14 +564,15 @@ xfs_buf_find(
>  	new_bp->b_pag = pag;
>  	rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
>  			       xfs_buf_hash_params);
> -	spin_unlock(&pag->pag_buf_lock);
> -	*found_bp = new_bp;
>  	return 0;
> +}
>  
> -found:
> -	spin_unlock(&pag->pag_buf_lock);
> -	xfs_perag_put(pag);
> -
> +static int
> +xfs_buf_find_lock(
> +	struct xfs_buftarg	*btp,
> +	struct xfs_buf          *bp,
> +	xfs_buf_flags_t		flags)
> +{
>  	if (!xfs_buf_trylock(bp)) {
>  		if (flags & XBF_TRYLOCK) {
>  			xfs_buf_rele(bp);
> @@ -609,6 +593,73 @@ xfs_buf_find(
>  		bp->b_flags &= _XBF_KMEM | _XBF_PAGES;
>  		bp->b_ops = NULL;
>  	}
> +	return 0;
> +}
> +
> +/*
> + * Look up a buffer in the buffer cache and return it referenced and locked
> + * in @found_bp.
> + *
> + * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
> + * cache.
> + *
> + * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
> + * -EAGAIN if we fail to lock it.
> + *
> + * Return values are:
> + *	-EFSCORRUPTED if have been supplied with an invalid address
> + *	-EAGAIN on trylock failure
> + *	-ENOENT if we fail to find a match and @new_bp was NULL
> + *	0, with @found_bp:
> + *		- @new_bp if we inserted it into the cache
> + *		- the buffer we found and locked.
> + */
> +static int
> +xfs_buf_find(
> +	struct xfs_buftarg	*btp,
> +	struct xfs_buf_map	*map,
> +	int			nmaps,
> +	xfs_buf_flags_t		flags,
> +	struct xfs_buf		*new_bp,
> +	struct xfs_buf		**found_bp)
> +{
> +	struct xfs_perag	*pag;
> +	struct xfs_buf		*bp;
> +	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
> +	int			error;
> +	int			i;
> +
> +	*found_bp = NULL;
> +
> +	for (i = 0; i < nmaps; i++)
> +		cmap.bm_len += map[i].bm_len;
> +
> +	error = xfs_buf_find_verify(btp, &cmap);
> +	if (error)
> +		return error;
> +
> +	pag = xfs_perag_get(btp->bt_mount,
> +			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
> +
> +	spin_lock(&pag->pag_buf_lock);
> +	bp = xfs_buf_find_fast(pag, &cmap);
> +	if (bp)
> +		goto found;
> +
> +	error = xfs_buf_find_insert(btp, pag, new_bp);
> +	spin_unlock(&pag->pag_buf_lock);
> +	if (error)
> +		return error;
> +	*found_bp = new_bp;
> +	return 0;
> +
> +found:
> +	spin_unlock(&pag->pag_buf_lock);
> +	xfs_perag_put(pag);
> +
> +	error = xfs_buf_find_lock(btp, bp, flags);
> +	if (error)
> +		return error;
>  
>  	trace_xfs_buf_find(bp, flags, _RET_IP_);
>  	XFS_STATS_INC(btp->bt_mount, xb_get_locked);
> -- 
> 2.36.1
>