Re: [PATCH v3 3/6] xfs: add per-inode AG prediction map and dirty-AG bitmap

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

On Fri, Jan 16, 2026 at 03:38:15PM +0530, Kundan Kumar wrote:
> Add per-inode structures to track predicted AGs of dirty folios using
> an xarray and bitmap. This enables efficient identification of AGs
> involved in writeback.
> 
> Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> ---
>  fs/xfs/xfs_icache.c | 27 +++++++++++++++++++++++++++
>  fs/xfs/xfs_inode.h  |  5 +++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index e44040206851..f97aa6d66271 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -80,6 +80,25 @@ static inline xa_mark_t ici_tag_to_mark(unsigned int tag)
>  	return XFS_PERAG_BLOCKGC_MARK;
>  }
>  
> +static int xfs_inode_init_ag_bitmap(struct xfs_inode *ip)
> +{
> +	unsigned int bits = ip->i_mount->m_sb.sb_agcount;
> +	unsigned int nlongs;
> +
> +	xa_init_flags(&ip->i_ag_pmap, XA_FLAGS_LOCK_IRQ);

This increases the size of struct xfs_inode by 40 bytes...

> +	ip->i_ag_dirty_bitmap = NULL;
> +	ip->i_ag_dirty_bits = bits;
> +
> +	if (!bits)
> +		return 0;
> +
> +	nlongs = BITS_TO_LONGS(bits);
> +	ip->i_ag_dirty_bitmap = kcalloc(nlongs, sizeof(unsigned long),
> +					GFP_NOFS);

...and there could be hundreds or thousands of AGs for each filesystem.
That's a lot of kernel memory to handle this prediction stuff, and I"m
not even sure what ag_dirty_bitmap does yet.

> +
> +	return ip->i_ag_dirty_bitmap ? 0 : -ENOMEM;
> +}
> +
>  /*
>   * Allocate and initialise an xfs_inode.
>   */
> @@ -131,6 +150,8 @@ xfs_inode_alloc(
>  	ip->i_next_unlinked = NULLAGINO;
>  	ip->i_prev_unlinked = 0;
>  
> +	xfs_inode_init_ag_bitmap(ip);

Unchecked return value???

> +
>  	return ip;
>  }
>  
> @@ -194,6 +215,12 @@ xfs_inode_free(
>  	ip->i_ino = 0;
>  	spin_unlock(&ip->i_flags_lock);
>  
> +	/* free xarray contents (values are immediate packed ints) */
> +	xa_destroy(&ip->i_ag_pmap);
> +	kfree(ip->i_ag_dirty_bitmap);
> +	ip->i_ag_dirty_bitmap = NULL;
> +	ip->i_ag_dirty_bits = 0;
> +
>  	__xfs_inode_free(ip);
>  }
>  
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index bd6d33557194..dee449168605 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -99,6 +99,11 @@ typedef struct xfs_inode {
>  	spinlock_t		i_ioend_lock;
>  	struct work_struct	i_ioend_work;
>  	struct list_head	i_ioend_list;
> +
> +	/* AG prediction map: pgoff_t -> packed u32 */

What about blocksize < pagesize filesystems?  Which packed agno do you
associate with the pgoff_t?

Also, do you have an xarray entry for each pgoff_t in a large folio?

--D

> +	struct xarray           i_ag_pmap;
> +	unsigned long           *i_ag_dirty_bitmap;
> +	unsigned int            i_ag_dirty_bits;
>  } xfs_inode_t;
>  
>  static inline bool xfs_inode_on_unlinked_list(const struct xfs_inode *ip)
> -- 
> 2.25.1
> 
>