Re: [PATCH v3 4/5] writeback, memcg: Implement cgroup_writeback_by_id()

[Jan Kara <jack@suse.cz>]

On Wed 21-08-19 14:02:10, Tejun Heo wrote:
> Implement cgroup_writeback_by_id() which initiates cgroup writeback
> from bdi and memcg IDs.  This will be used by memcg foreign inode
> flushing.
> 
> v2: Use wb_get_lookup() instead of wb_get_create() to avoid creating
>     spurious wbs.
> 
> v3: Interpret 0 @nr as 1.25 * nr_dirty to implement best-effort
>     flushing while avoding possible livelocks.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/fs-writeback.c         |   83 ++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/writeback.h |    2 +
>  2 files changed, 85 insertions(+)
> 
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -892,6 +892,89 @@ restart:
>  }
>  
>  /**
> + * cgroup_writeback_by_id - initiate cgroup writeback from bdi and memcg IDs
> + * @bdi_id: target bdi id
> + * @memcg_id: target memcg css id
> + * @nr_pages: number of pages to write, 0 for best-effort dirty flushing
> + * @reason: reason why some writeback work initiated
> + * @done: target wb_completion
> + *
> + * Initiate flush of the bdi_writeback identified by @bdi_id and @memcg_id
> + * with the specified parameters.
> + */
> +int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr,
> +			   enum wb_reason reason, struct wb_completion *done)
> +{
> +	struct backing_dev_info *bdi;
> +	struct cgroup_subsys_state *memcg_css;
> +	struct bdi_writeback *wb;
> +	struct wb_writeback_work *work;
> +	int ret;
> +
> +	/* lookup bdi and memcg */
> +	bdi = bdi_get_by_id(bdi_id);
> +	if (!bdi)
> +		return -ENOENT;
> +
> +	rcu_read_lock();
> +	memcg_css = css_from_id(memcg_id, &memory_cgrp_subsys);
> +	if (memcg_css && !css_tryget(memcg_css))
> +		memcg_css = NULL;
> +	rcu_read_unlock();
> +	if (!memcg_css) {
> +		ret = -ENOENT;
> +		goto out_bdi_put;
> +	}
> +
> +	/*
> +	 * And find the associated wb.  If the wb isn't there already
> +	 * there's nothing to flush, don't create one.
> +	 */
> +	wb = wb_get_lookup(bdi, memcg_css);
> +	if (!wb) {
> +		ret = -ENOENT;
> +		goto out_css_put;
> +	}
> +
> +	/*
> +	 * If @nr is zero, the caller is attempting to write out most of
> +	 * the currently dirty pages.  Let's take the current dirty page
> +	 * count and inflate it by 25% which should be large enough to
> +	 * flush out most dirty pages while avoiding getting livelocked by
> +	 * concurrent dirtiers.
> +	 */
> +	if (!nr) {
> +		unsigned long filepages, headroom, dirty, writeback;
> +
> +		mem_cgroup_wb_stats(wb, &filepages, &headroom, &dirty,
> +				      &writeback);
> +		nr = dirty * 10 / 8;
> +	}
> +
> +	/* issue the writeback work */
> +	work = kzalloc(sizeof(*work), GFP_NOWAIT | __GFP_NOWARN);
> +	if (work) {
> +		work->nr_pages = nr;
> +		work->sync_mode = WB_SYNC_NONE;
> +		work->range_cyclic = 1;
> +		work->reason = reason;
> +		work->done = done;
> +		work->auto_free = 1;
> +		wb_queue_work(wb, work);
> +		ret = 0;
> +	} else {
> +		ret = -ENOMEM;
> +	}
> +
> +	wb_put(wb);
> +out_css_put:
> +	css_put(memcg_css);
> +out_bdi_put:
> +	bdi_put(bdi);
> +	return ret;
> +}
> +
> +/**
>   * cgroup_writeback_umount - flush inode wb switches for umount
>   *
>   * This function is called when a super_block is about to be destroyed and
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -217,6 +217,8 @@ void wbc_attach_and_unlock_inode(struct
>  void wbc_detach_inode(struct writeback_control *wbc);
>  void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
>  			      size_t bytes);
> +int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr_pages,
> +			   enum wb_reason reason, struct wb_completion *done);
>  void cgroup_writeback_umount(void);
>  
>  /**
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR