Re: [PATCH 2/3] cgroup, writeback: implement submit_bh_blkcg_css()

[Jan Kara]

On Mon 09-10-17 14:29:10, Tejun Heo wrote:
> Add wbc->blkcg_css so that the blkcg_css association can be specified
> independently and implement submit_bh_blkcg_css() using it.  This will
> be used to override cgroup membership on specific buffer_heads.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>

Hum, I dislike growing wbc just to pass blkcg_css through one function in
fs/buffer.c (in otherwise empty wbc). Not that space would be a big concern
for wbc but it just gets messier... Cannot we just refactor the code a
bit like:

1) Create function

struct bio *create_bio_bh(int op, int op_flags, struct buffer_head *bh,
			  enum rw_hint write_hint);

which would create bio from bh.

2) Make submit_bh(), submit_bh_wbc(), submit_bh_blkcg_css() use this and
the latter two would further tweak the bio as needed before submitting.

Thoughts?

								Honza


> ---
>  fs/buffer.c                 | 12 ++++++++++++
>  fs/fs-writeback.c           |  1 +
>  include/linux/buffer_head.h | 11 +++++++++++
>  include/linux/writeback.h   |  6 ++++--
>  4 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 170df85..fac4f9a 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3143,6 +3143,18 @@ int submit_bh(int op, int op_flags, struct buffer_head *bh)
>  }
>  EXPORT_SYMBOL(submit_bh);
>  
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +int submit_bh_blkcg_css(int op, int op_flags,  struct buffer_head *bh,
> +			struct cgroup_subsys_state *blkcg_css)
> +{
> +	struct writeback_control wbc = { .blkcg_css = blkcg_css };
> +
> +	/* @wbc is used just to override the bio's blkcg_css */
> +	return submit_bh_wbc(op, op_flags, bh, 0, &wbc);
> +}
> +EXPORT_SYMBOL(submit_bh_blkcg_css);
> +#endif
> +
>  /**
>   * ll_rw_block: low-level access to block devices (DEPRECATED)
>   * @op: whether to %READ or %WRITE
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 245c430..cd882e6 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -538,6 +538,7 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
>  	}
>  
>  	wbc->wb = inode_to_wb(inode);
> +	wbc->blkcg_css = wbc->wb->blkcg_css;
>  	wbc->inode = inode;
>  
>  	wbc->wb_id = wbc->wb->memcg_css->id;
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index c8dae55..abb4dd4 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -13,6 +13,7 @@
>  #include <linux/pagemap.h>
>  #include <linux/wait.h>
>  #include <linux/atomic.h>
> +#include <linux/cgroup.h>
>  
>  #ifdef CONFIG_BLOCK
>  
> @@ -205,6 +206,16 @@ int bh_submit_read(struct buffer_head *bh);
>  loff_t page_cache_seek_hole_data(struct inode *inode, loff_t offset,
>  				 loff_t length, int whence);
>  
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +int submit_bh_blkcg(int op, int op_flags, struct buffer_head *bh,
> +		    struct cgroup_subsys_state *blkcg_css);
> +#else
> +static inline int submit_bh_blkcg(int op, int op_flags, struct buffer_head *bh,
> +				  struct cgroup_subsys_state *blkcg_css)
> +{
> +	return submit_bh(op, op_flags, bh);
> +}
> +#endif
>  extern int buffer_heads_over_limit;
>  
>  /*
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index d581579..81e5946 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -91,6 +91,8 @@ struct writeback_control {
>  	unsigned for_sync:1;		/* sync(2) WB_SYNC_ALL writeback */
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct bdi_writeback *wb;	/* wb this writeback is issued under */
> +	struct cgroup_subsys_state *blkcg_css; /* usually wb->blkcg_css but
> +						  may be overridden */
>  	struct inode *inode;		/* inode being written out */
>  
>  	/* foreign inode detection, see wbc_detach_inode() */
> @@ -277,8 +279,8 @@ static inline void wbc_init_bio(struct writeback_control *wbc, struct bio *bio)
>  	 * behind a slow cgroup.  Ultimately, we want pageout() to kick off
>  	 * regular writeback instead of writing things out itself.
>  	 */
> -	if (wbc->wb)
> -		bio_associate_blkcg(bio, wbc->wb->blkcg_css);
> +	if (wbc->blkcg_css)
> +		bio_associate_blkcg(bio, wbc->blkcg_css);
>  }
>  
>  #else	/* CONFIG_CGROUP_WRITEBACK */
> -- 
> 2.9.5
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR