Re: [PATCH 05/21] xfs: introduce XFS_BLI_PREALLOC

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

On Thu, Sep 24, 2026 at 11:59:37AM +0200, Christoph Hellwig wrote:
> Add a flag so that the shadow CIL buffer for a buffer log item is always
> sizes to the maximum to prevent reallocations.  This will be used for the

  sized

> RT checksum item, where we know that we are going to fill it up very soon,
> and (almost) sequentially, so there is no point in doing a constant
> realloc cycle when more data is added to it.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/libxfs/xfs_trans_resv.c |  2 +-
>  fs/xfs/libxfs/xfs_trans_resv.h |  1 +
>  fs/xfs/xfs_buf_item.c          | 10 ++++++++++
>  fs/xfs/xfs_buf_item.h          |  4 +++-
>  4 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_trans_resv.c b/fs/xfs/libxfs/xfs_trans_resv.c
> index 3151e97ca8ff..5382ece51812 100644
> --- a/fs/xfs/libxfs/xfs_trans_resv.c
> +++ b/fs/xfs/libxfs/xfs_trans_resv.c
> @@ -53,7 +53,7 @@ xfs_buf_log_overhead(void)
>   * will be changed in a transaction.  size is used to tell how many
>   * bytes should be reserved per item.
>   */
> -STATIC uint
> +uint
>  xfs_calc_buf_res(
>  	uint		nbufs,
>  	uint		size)
> diff --git a/fs/xfs/libxfs/xfs_trans_resv.h b/fs/xfs/libxfs/xfs_trans_resv.h
> index 336279e0fc61..1804e821f382 100644
> --- a/fs/xfs/libxfs/xfs_trans_resv.h
> +++ b/fs/xfs/libxfs/xfs_trans_resv.h
> @@ -96,6 +96,7 @@ struct xfs_trans_resv {
>  #define	XFS_ITRUNCATE_LOG_COUNT_REFLINK	8
>  #define	XFS_WRITE_LOG_COUNT_REFLINK	8
>  
> +uint xfs_calc_buf_res(uint nbufs, uint size);
>  void xfs_trans_resv_calc(struct xfs_mount *mp, struct xfs_trans_resv *resp);
>  uint xfs_allocfree_block_count(struct xfs_mount *mp, uint num_ops);
>  
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 1a4ef34af8d5..644c3fb18310 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -252,6 +252,16 @@ xfs_buf_item_size(
>  		offset += BBTOB(bp->b_maps[i].bm_len);
>  	}
>  
> +	/*
> +	 * For buffers with the prealloc flag, always size the allocation size
> +	 * to the maximum as per the log reservation.  This avoids constant
> +	 * realloc cycles for buffers that are filled sequentially in rapid
> +	 * pace.  Note that the nvecs calaculation is kept from the regular

	                              calculation

> +	 * look as the buffer item formatting expects it.

"...is kept from the regular look as the buffer item formatting expects
it" ?

I don't understand that.  Is the nvecs calculation kept as the buffer
item formatting code expects it, even though we're allocating more
shadow buffer space?

--D

> +	 */
> +	if (bip->bli_flags & XFS_BLI_PREALLOC)
> +		*nbytes = xfs_calc_buf_res(bip->bli_format_count, bp->b_length);
> +
>  	/*
>  	 * Round up the buffer size required to minimise the number of memory
>  	 * allocations that need to be done as this item grows when relogged by
> diff --git a/fs/xfs/xfs_buf_item.h b/fs/xfs/xfs_buf_item.h
> index 3159325dd17b..ddc8ecc4683e 100644
> --- a/fs/xfs/xfs_buf_item.h
> +++ b/fs/xfs/xfs_buf_item.h
> @@ -20,6 +20,7 @@ struct xfs_mount;
>  #define XFS_BLI_STALE_INODE	(1u << 5)
>  #define	XFS_BLI_INODE_BUF	(1u << 6)
>  #define	XFS_BLI_ORDERED		(1u << 7)
> +#define	XFS_BLI_PREALLOC	(1u << 8)
>  
>  #define XFS_BLI_FLAGS \
>  	{ XFS_BLI_HOLD,		"HOLD" }, \
> @@ -29,7 +30,8 @@ struct xfs_mount;
>  	{ XFS_BLI_INODE_ALLOC_BUF, "INODE_ALLOC" }, \
>  	{ XFS_BLI_STALE_INODE,	"STALE_INODE" }, \
>  	{ XFS_BLI_INODE_BUF,	"INODE_BUF" }, \
> -	{ XFS_BLI_ORDERED,	"ORDERED" }
> +	{ XFS_BLI_ORDERED,	"ORDERED" }, \
> +	{ XFS_BLI_PREALLOC,	"PREALLOC" }
>  
>  /*
>   * This is the in core log item structure used to track information
> -- 
> 2.53.0
> 
>