Re: [PATCH 01/12] xfs: don't get a pag reference in xfs_buf_get_map

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

On Wed, Jul 15, 2026 at 04:50:54PM +0200, Christoph Hellwig wrote:
> As of commit 497560b9ef42 ("xfs: switch (back) to a per-buftarg buffer
> hash"), buffer lookups don't require the perag structure.  Stop looking
> it up in xfs_buf_get_map, and instead only find it when allocating a new
> buffer.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

b_pag is only set for cached buffers for the data device, right?  And
this patch doesn't change that property, it just moves the place where
we get the perag to xfs_buf_find_insert, right?

If yes to both, then
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_buf.c | 44 ++++++++++----------------------------------
>  1 file changed, 10 insertions(+), 34 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index e1465e950acc..54e091315d56 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -469,7 +469,6 @@ xfs_buf_lookup(
>  static int
>  xfs_buf_find_insert(
>  	struct xfs_buftarg	*btp,
> -	struct xfs_perag	*pag,
>  	struct xfs_buf_map	*cmap,
>  	struct xfs_buf_map	*map,
>  	int			nmaps,
> @@ -482,10 +481,13 @@ xfs_buf_find_insert(
>  
>  	error = xfs_buf_alloc(btp, map, nmaps, flags, &new_bp);
>  	if (error)
> -		goto out_drop_pag;
> +		return error;
>  
>  	/* The new buffer keeps the perag reference until it is freed. */
> -	new_bp->b_pag = pag;
> +	if (!xfs_buftarg_is_mem(btp)) {
> +		new_bp->b_pag = xfs_perag_get(btp->bt_mount,
> +			xfs_daddr_to_agno(btp->bt_mount, cmap->bm_bn));
> +	}
>  
>  retry:
>  	rcu_read_lock();
> @@ -520,25 +522,12 @@ xfs_buf_find_insert(
>  	return 0;
>  
>  out_free_buf:
> +	if (new_bp->b_pag)
> +		xfs_perag_put(new_bp->b_pag);
>  	xfs_buf_free(new_bp);
> -out_drop_pag:
> -	if (pag)
> -		xfs_perag_put(pag);
>  	return error;
>  }
>  
> -static inline struct xfs_perag *
> -xfs_buftarg_get_pag(
> -	struct xfs_buftarg		*btp,
> -	const struct xfs_buf_map	*map)
> -{
> -	struct xfs_mount		*mp = btp->bt_mount;
> -
> -	if (xfs_buftarg_is_mem(btp))
> -		return NULL;
> -	return xfs_perag_get(mp, xfs_daddr_to_agno(mp, map->bm_bn));
> -}
> -
>  /*
>   * Assembles a buffer covering the specified range. The code is optimised for
>   * cache hits, as metadata intensive workloads will see 3 orders of magnitude
> @@ -552,7 +541,6 @@ xfs_buf_get_map(
>  	xfs_buf_flags_t		flags,
>  	struct xfs_buf		**bpp)
>  {
> -	struct xfs_perag	*pag;
>  	struct xfs_buf		*bp = NULL;
>  	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
>  	int			error;
> @@ -567,28 +555,21 @@ xfs_buf_get_map(
>  	if (error)
>  		return error;
>  
> -	pag = xfs_buftarg_get_pag(btp, &cmap);
> -
>  	error = xfs_buf_lookup(btp, &cmap, flags, &bp);
>  	if (error && error != -ENOENT)
> -		goto out_put_perag;
> +		return error;
>  
>  	/* cache hits always outnumber misses by at least 10:1 */
>  	if (unlikely(!bp)) {
>  		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
>  
>  		if (flags & XBF_INCORE)
> -			goto out_put_perag;
> -
> -		/* xfs_buf_find_insert() consumes the perag reference. */
> -		error = xfs_buf_find_insert(btp, pag, &cmap, map, nmaps,
> -				flags, &bp);
> +			return 0;
> +		error = xfs_buf_find_insert(btp, &cmap, map, nmaps, flags, &bp);
>  		if (error)
>  			return error;
>  	} else {
>  		XFS_STATS_INC(btp->bt_mount, xb_get_locked);
> -		if (pag)
> -			xfs_perag_put(pag);
>  	}
>  
>  	/*
> @@ -602,11 +583,6 @@ xfs_buf_get_map(
>  	trace_xfs_buf_get(bp, flags, _RET_IP_);
>  	*bpp = bp;
>  	return 0;
> -
> -out_put_perag:
> -	if (pag)
> -		xfs_perag_put(pag);
> -	return error;
>  }
>  
>  int
> -- 
> 2.53.0
> 
>