Re: [RFC PATCH v2 13/16] mm: add balance_dirty_pages_ratelimited_flags() function

[Jan Kara <jack@suse.cz>]

On Mon 16-05-22 09:47:15, Stefan Roesch wrote:
> This adds the function balance_dirty_pages_ratelimited_flags(). It adds
> the parameter is_async to balance_dirty_pages_ratelimited(). In case
> this is an async write, it will call _balance_diirty_pages() to
> determine if write throttling needs to be enabled. If write throttling
> is enabled, it retuns -EAGAIN, so the write request can be punted to
> the io-uring worker.
> 
> The new function is changed to return the sleep time, so callers can
> observe if the write has been punted.
> 
> For non-async writes the current behavior is maintained.
> 
> Signed-off-by: Stefan Roesch <shr@fb.com>
> ---
>  include/linux/writeback.h |  1 +
>  mm/page-writeback.c       | 48 ++++++++++++++++++++++++++-------------
>  2 files changed, 33 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index fec248ab1fec..d589804bb3be 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -373,6 +373,7 @@ unsigned long wb_calc_thresh(struct bdi_writeback *wb, unsigned long thresh);
>  
>  void wb_update_bandwidth(struct bdi_writeback *wb);
>  void balance_dirty_pages_ratelimited(struct address_space *mapping);
> +int  balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async);
>  bool wb_over_bg_thresh(struct bdi_writeback *wb);
>  
>  typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index cbb74c0666c6..78f1326f3f20 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1877,28 +1877,17 @@ static DEFINE_PER_CPU(int, bdp_ratelimits);
>   */
>  DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
>  
> -/**
> - * balance_dirty_pages_ratelimited - balance dirty memory state
> - * @mapping: address_space which was dirtied
> - *
> - * Processes which are dirtying memory should call in here once for each page
> - * which was newly dirtied.  The function will periodically check the system's
> - * dirty state and will initiate writeback if needed.
> - *
> - * Once we're over the dirty memory limit we decrease the ratelimiting
> - * by a lot, to prevent individual processes from overshooting the limit
> - * by (ratelimit_pages) each.
> - */
> -void balance_dirty_pages_ratelimited(struct address_space *mapping)
> +int balance_dirty_pages_ratelimited_flags(struct address_space *mapping, bool is_async)

Perhaps I'd call the other function balance_dirty_pages_ratelimited_async()
and then keep balance_dirty_pages_ratelimited_flags() as an internal
function. It is then more obvious at the external call sites what the call
is about (unlike the true/false argument).

								Honza

>  {
>  	struct inode *inode = mapping->host;
>  	struct backing_dev_info *bdi = inode_to_bdi(inode);
>  	struct bdi_writeback *wb = NULL;
>  	int ratelimit;
> +	int ret = 0;
>  	int *p;
>  
>  	if (!(bdi->capabilities & BDI_CAP_WRITEBACK))
> -		return;
> +		return ret;
>  
>  	if (inode_cgwb_enabled(inode))
>  		wb = wb_get_create_current(bdi, GFP_KERNEL);
> @@ -1937,10 +1926,37 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
>  	}
>  	preempt_enable();
>  
> -	if (unlikely(current->nr_dirtied >= ratelimit))
> -		balance_dirty_pages(wb, current->nr_dirtied);
> +	if (unlikely(current->nr_dirtied >= ratelimit)) {
> +		if (is_async) {
> +			struct bdp_ctx ctx = { BDP_CTX_INIT(ctx, wb) };
> +
> +			ret = _balance_dirty_pages(wb, current->nr_dirtied, &ctx);
> +			if (ret)
> +				ret = -EAGAIN;
> +		} else {
> +			balance_dirty_pages(wb, current->nr_dirtied);
> +		}
> +	}
>  
>  	wb_put(wb);
> +	return ret;
> +}
> +
> +/**
> + * balance_dirty_pages_ratelimited - balance dirty memory state
> + * @mapping: address_space which was dirtied
> + *
> + * Processes which are dirtying memory should call in here once for each page
> + * which was newly dirtied.  The function will periodically check the system's
> + * dirty state and will initiate writeback if needed.
> + *
> + * Once we're over the dirty memory limit we decrease the ratelimiting
> + * by a lot, to prevent individual processes from overshooting the limit
> + * by (ratelimit_pages) each.
> + */
> +void balance_dirty_pages_ratelimited(struct address_space *mapping)
> +{
> +	balance_dirty_pages_ratelimited_flags(mapping, false);
>  }
>  EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
>  
> -- 
> 2.30.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR