Re: [PATCH v2 04/16] writeback: affine inode to a writeback ctx within a bdi

[Jan Kara <jack@suse.cz>]

On Tue 14-10-25 17:38:33, Kundan Kumar wrote:
> Affine inode to a writeback context. This helps in minimizing the
> filesytem fragmentation due to inode being processed by different
> threads.
> 
> To support parallel writeback, wire up a new superblock operation
> get_inode_wb_ctx(). Filesystems can override this callback and select
> desired writeback context for a inode. FS can use the wb context based
> on its geometry and also use 64 bit inode numbers.
> 
> If a filesystem doesn't implement this callback, it defaults to
> DEFALT_WB_CTX = 0, maintaining its original behavior.
> 
> An example implementation for XFS is provided, where XFS selects the
> writeback context based on its Allocation Group number.
> 
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>

As Christoph asked in other patch. Please introduce generic writeback
changes in one patch and then provide xfs implementation of
xfs_get_inode_wb_ctx() in another patch. Thanks.

								Honza

> ---
>  fs/fs-writeback.c           |  3 ++-
>  fs/xfs/xfs_super.c          | 13 +++++++++++++
>  include/linux/backing-dev.h |  5 ++++-
>  include/linux/fs.h          |  1 +
>  4 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 0715a7617391..56c048e22f72 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -265,7 +265,8 @@ void __inode_attach_wb(struct inode *inode, struct folio *folio)
>  {
>  	struct backing_dev_info *bdi = inode_to_bdi(inode);
>  	struct bdi_writeback *wb = NULL;
> -	struct bdi_writeback_ctx *bdi_writeback_ctx = bdi->wb_ctx[0];
> +	struct bdi_writeback_ctx *bdi_writeback_ctx =
> +						fetch_bdi_writeback_ctx(inode);
>  
>  	if (inode_cgwb_enabled(inode)) {
>  		struct cgroup_subsys_state *memcg_css;
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index bb0a82635a77..b3ec9141d902 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -53,6 +53,7 @@
>  #include <linux/magic.h>
>  #include <linux/fs_context.h>
>  #include <linux/fs_parser.h>
> +#include <linux/backing-dev.h>
>  
>  static const struct super_operations xfs_super_operations;
>  
> @@ -1294,6 +1295,17 @@ xfs_fs_show_stats(
>  	return 0;
>  }
>  
> +static struct bdi_writeback_ctx *
> +xfs_get_inode_wb_ctx(
> +	struct inode		*inode)
> +{
> +	struct xfs_inode *ip = XFS_I(inode);
> +	struct backing_dev_info *bdi = inode_to_bdi(inode);
> +	xfs_agino_t agno = XFS_INO_TO_AGNO(ip->i_mount, ip->i_ino);
> +
> +	return bdi->wb_ctx[agno % bdi->nr_wb_ctx];
> +}
> +
>  static const struct super_operations xfs_super_operations = {
>  	.alloc_inode		= xfs_fs_alloc_inode,
>  	.destroy_inode		= xfs_fs_destroy_inode,
> @@ -1310,6 +1322,7 @@ static const struct super_operations xfs_super_operations = {
>  	.free_cached_objects	= xfs_fs_free_cached_objects,
>  	.shutdown		= xfs_fs_shutdown,
>  	.show_stats		= xfs_fs_show_stats,
> +	.get_inode_wb_ctx       = xfs_get_inode_wb_ctx,
>  };
>  
>  static int
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 951ab5497500..59bbb69d300c 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -148,6 +148,7 @@ static inline bool mapping_can_writeback(struct address_space *mapping)
>  	return inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK;
>  }
>  
> +#define DEFAULT_WB_CTX 0
>  #define for_each_bdi_wb_ctx(bdi, wbctx) \
>  	for (int __i = 0; __i < (bdi)->nr_wb_ctx \
>  		&& ((wbctx) = (bdi)->wb_ctx[__i]) != NULL; __i++)
> @@ -157,7 +158,9 @@ fetch_bdi_writeback_ctx(struct inode *inode)
>  {
>  	struct backing_dev_info *bdi = inode_to_bdi(inode);
>  
> -	return bdi->wb_ctx[0];
> +	if (inode->i_sb->s_op->get_inode_wb_ctx)
> +		return inode->i_sb->s_op->get_inode_wb_ctx(inode);
> +	return bdi->wb_ctx[DEFAULT_WB_CTX];
>  }
>  
>  #ifdef CONFIG_CGROUP_WRITEBACK
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 754fec84f350..5199b0d49fa5 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2379,6 +2379,7 @@ struct super_operations {
>  	 */
>  	int (*remove_bdev)(struct super_block *sb, struct block_device *bdev);
>  	void (*shutdown)(struct super_block *sb);
> +	struct bdi_writeback_ctx *(*get_inode_wb_ctx)(struct inode *inode);
>  };
>  
>  /*
> -- 
> 2.25.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR