Re: [PATCH 06/19] writeback: Factor out writeback_finish()

[Brian Foster]

On Thu, Jan 25, 2024 at 09:57:45AM +0100, 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>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Acked-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  include/linux/writeback.h |  6 +++
>  mm/page-writeback.c       | 79 ++++++++++++++++++++-------------------
>  2 files changed, 47 insertions(+), 38 deletions(-)
> 
..
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 437745a511c634..fcd90a176d806c 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
..
> @@ -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;
>  			}
> +		

JFYI: whitespace damage on the above line.

> +			if (error && !wbc->err)
> +				wbc->err = error;
>  

Also what happened to the return of the above "first error encountered"
for the WB_SYNC_ALL case? Is that not needed for some reason (and so the
comment just below might require an update)?

Brian

>  			/*
>  			 * 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
> 
>