Re: [PATCH 06/17] writeback: Factor out writeback_finish()

[Jan Kara]

On Mon 18-12-23 16:35:42, Christoph Hellwig wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> Instead of having a 'done' variable that controls the nested loops,
> have a writeback_finish() that can be returned directly.  This involves
> keeping more things in writeback_control, but it's just moving stuff
> allocated on the stack to being allocated slightly earlier on the stack.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> [hch: heavily rebased, reordered and commented struct writeback_control]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  include/linux/writeback.h |  6 +++
>  mm/page-writeback.c       | 79 ++++++++++++++++++++-------------------
>  2 files changed, 47 insertions(+), 38 deletions(-)
> 
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 833ec38fc3e0c9..390f2dd03cf27e 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -11,6 +11,7 @@
>  #include <linux/flex_proportions.h>
>  #include <linux/backing-dev-defs.h>
>  #include <linux/blk_types.h>
> +#include <linux/pagevec.h>
>  
>  struct bio;
>  
> @@ -40,6 +41,7 @@ enum writeback_sync_modes {
>   * in a manner such that unspecified fields are set to zero.
>   */
>  struct writeback_control {
> +	/* public fields that can be set and/or consumed by the caller: */
>  	long nr_to_write;		/* Write this many pages, and decrement
>  					   this for each page written */
>  	long pages_skipped;		/* Pages which were not written */
> @@ -77,6 +79,10 @@ struct writeback_control {
>  	 */
>  	struct swap_iocb **swap_plug;
>  
> +	/* internal fields used by the ->writepages implementation: */
> +	struct folio_batch fbatch;
> +	int err;
> +
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct bdi_writeback *wb;	/* wb this writeback is issued under */
>  	struct inode *inode;		/* inode being written out */
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index c798c0d6d0abb4..564d5faf562ba7 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2360,6 +2360,29 @@ void tag_pages_for_writeback(struct address_space *mapping,
>  }
>  EXPORT_SYMBOL(tag_pages_for_writeback);
>  
> +static void writeback_finish(struct address_space *mapping,
> +		struct writeback_control *wbc, pgoff_t done_index)
> +{
> +	folio_batch_release(&wbc->fbatch);
> +
> +	/*
> +	 * For range cyclic writeback we need to remember where we stopped so
> +	 * that we can continue there next time we are called.  If  we hit the
> +	 * last page and there is more work to be done, wrap back to the start
> +	 * of the file.
> +	 *
> +	 * For non-cyclic writeback we always start looking up at the beginning
> +	 * of the file if we are called again, which can only happen due to
> +	 * -ENOMEM from the file system.
> +	 */
> +	if (wbc->range_cyclic) {
> +		if (wbc->err || wbc->nr_to_write <= 0)
> +			mapping->writeback_index = done_index;
> +		else
> +			mapping->writeback_index = 0;
> +	}
> +}
> +
>  /**
>   * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
>   * @mapping: address space structure to write
> @@ -2395,17 +2418,12 @@ int write_cache_pages(struct address_space *mapping,
>  		      struct writeback_control *wbc, writepage_t writepage,
>  		      void *data)
>  {
> -	int ret = 0;
> -	int done = 0;
>  	int error;
> -	struct folio_batch fbatch;
>  	int nr_folios;
>  	pgoff_t index;
>  	pgoff_t end;		/* Inclusive */
> -	pgoff_t done_index;
>  	xa_mark_t tag;
>  
> -	folio_batch_init(&fbatch);
>  	if (wbc->range_cyclic) {
>  		index = mapping->writeback_index; /* prev offset */
>  		end = -1;
> @@ -2419,22 +2437,23 @@ int write_cache_pages(struct address_space *mapping,
>  	} else {
>  		tag = PAGECACHE_TAG_DIRTY;
>  	}
> -	done_index = index;
> -	while (!done && (index <= end)) {
> +
> +	folio_batch_init(&wbc->fbatch);
> +	wbc->err = 0;
> +
> +	while (index <= end) {
>  		int i;
>  
>  		nr_folios = filemap_get_folios_tag(mapping, &index, end,
> -				tag, &fbatch);
> +				tag, &wbc->fbatch);
>  
>  		if (nr_folios == 0)
>  			break;
>  
>  		for (i = 0; i < nr_folios; i++) {
> -			struct folio *folio = fbatch.folios[i];
> +			struct folio *folio = wbc->fbatch.folios[i];
>  			unsigned long nr;
>  
> -			done_index = folio->index;
> -
>  			folio_lock(folio);
>  
>  			/*
> @@ -2481,6 +2500,9 @@ int write_cache_pages(struct address_space *mapping,
>  				folio_unlock(folio);
>  				error = 0;
>  			}
> +		
> +			if (error && !wbc->err)
> +				wbc->err = error;
>  
>  			/*
>  			 * For integrity sync  we have to keep going until we
> @@ -2496,38 +2518,19 @@ int write_cache_pages(struct address_space *mapping,
>  			 * off and media errors won't choke writeout for the
>  			 * entire file.
>  			 */
> -			if (error && !ret)
> -				ret = error;
> -			if (wbc->sync_mode == WB_SYNC_NONE) {
> -				if (ret || wbc->nr_to_write <= 0) {
> -					done_index = folio->index + nr;
> -					done = 1;
> -					break;
> -				}
> +			if (wbc->sync_mode == WB_SYNC_NONE &&
> +			    (wbc->err || wbc->nr_to_write <= 0)) {
> +				writeback_finish(mapping, wbc,
> +						folio->index + nr);
> +				return error;
>  			}
>  		}
> -		folio_batch_release(&fbatch);
> +		folio_batch_release(&wbc->fbatch);
>  		cond_resched();
>  	}
>  
> -	/*
> -	 * For range cyclic writeback we need to remember where we stopped so
> -	 * that we can continue there next time we are called.  If  we hit the
> -	 * last page and there is more work to be done, wrap back to the start
> -	 * of the file.
> -	 *
> -	 * For non-cyclic writeback we always start looking up at the beginning
> -	 * of the file if we are called again, which can only happen due to
> -	 * -ENOMEM from the file system.
> -	 */
> -	if (wbc->range_cyclic) {
> -		if (done)
> -			mapping->writeback_index = done_index;
> -		else
> -			mapping->writeback_index = 0;
> -	}
> -
> -	return ret;
> +	writeback_finish(mapping, wbc, 0);
> +	return 0;
>  }
>  EXPORT_SYMBOL(write_cache_pages);
>  
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR