Re: [PATCH 09/17] writeback: Simplify the loops in write_cache_pages()

[Jan Kara <jack@suse.cz>]

On Mon 18-12-23 16:35:45, Christoph Hellwig wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Collapse the two nested loops into one.  This is needed as a step
> towards turning this into an iterator.
> 
> Note that this drops the "index <= end" check in the previous outer loop
> and just relies on filemap_get_folios_tag() to return 0 entries when
> index > end.  This actually has a subtle implication when end == -1
> because then the returned index will be -1 as well and thus if there is
> page present on index -1, we could be looping indefinitely. But as the
> comment in filemap_get_folios_tag documents this as already broken anyway
> we should not worry about it here either.  The fix for that would
> probably a change to the filemap_get_folios_tag() calling convention.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> [hch: updated the commit log based on feedback from Jan Kara]
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks good. Feel free to add:

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

								Honza

> ---
>  mm/page-writeback.c | 94 ++++++++++++++++++++++-----------------------
>  1 file changed, 46 insertions(+), 48 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 2a004c0b9bdfbf..c7983ea3040be4 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2473,6 +2473,7 @@ int write_cache_pages(struct address_space *mapping,
>  {
>  	int error;
>  	pgoff_t end;		/* Inclusive */
> +	int i = 0;
>  
>  	if (wbc->range_cyclic) {
>  		wbc->index = mapping->writeback_index; /* prev offset */
> @@ -2487,63 +2488,60 @@ int write_cache_pages(struct address_space *mapping,
>  	folio_batch_init(&wbc->fbatch);
>  	wbc->err = 0;
>  
> -	while (wbc->index <= end) {
> -		int i;
> -
> -		writeback_get_batch(mapping, wbc);
> +	for (;;) {
> +		struct folio *folio;
> +		unsigned long nr;
>  
> +		if (i == wbc->fbatch.nr) {
> +			writeback_get_batch(mapping, wbc);
> +			i = 0;
> +		}
>  		if (wbc->fbatch.nr == 0)
>  			break;
>  
> -		for (i = 0; i < wbc->fbatch.nr; i++) {
> -			struct folio *folio = wbc->fbatch.folios[i];
> -			unsigned long nr;
> +		folio = wbc->fbatch.folios[i++];
>  
> -			folio_lock(folio);
> -			if (!should_writeback_folio(mapping, wbc, folio)) {
> -				folio_unlock(folio);
> -				continue;
> -			}
> +		folio_lock(folio);
> +		if (!should_writeback_folio(mapping, wbc, folio)) {
> +			folio_unlock(folio);
> +			continue;
> +		}
>  
> -			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
> +		trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
>  
> -			error = writepage(folio, wbc, data);
> -			nr = folio_nr_pages(folio);
> -			wbc->nr_to_write -= nr;
> +		error = writepage(folio, wbc, data);
> +		nr = folio_nr_pages(folio);
> +		wbc->nr_to_write -= nr;
>  
> -			/*
> -			 * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return
> -			 * value.  Eventually all instances should just unlock
> -			 * the folio themselves and return 0;
> -			 */
> -			if (error == AOP_WRITEPAGE_ACTIVATE) {
> -				folio_unlock(folio);
> -				error = 0;
> -			}
> -		
> -			if (error && !wbc->err)
> -				wbc->err = error;
> +		/*
> +		 * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return value.
> +		 * Eventually all instances should just unlock the folio
> +		 * themselves and return 0;
> +		 */
> +		if (error == AOP_WRITEPAGE_ACTIVATE) {
> +			folio_unlock(folio);
> +			error = 0;
> +		}
>  
> -			/*
> -			 * For integrity sync  we have to keep going until we
> -			 * have written all the folios we tagged for writeback
> -			 * prior to entering this loop, even if we run past
> -			 * wbc->nr_to_write or encounter errors.  This is
> -			 * because the file system may still have state to clear
> -			 * for each folio.   We'll eventually return the first
> -			 * error encountered.
> -			 *
> -			 * For background writeback just push done_index past
> -			 * this folio so that we can just restart where we left
> -			 * off and media errors won't choke writeout for the
> -			 * entire file.
> -			 */
> -			if (wbc->sync_mode == WB_SYNC_NONE &&
> -			    (wbc->err || wbc->nr_to_write <= 0)) {
> -				writeback_finish(mapping, wbc,
> -						folio->index + nr);
> -				return error;
> -			}
> +		if (error && !wbc->err)
> +			wbc->err = error;
> +
> +		/*
> +		 * For integrity sync  we have to keep going until we have
> +		 * written all the folios we tagged for writeback prior to
> +		 * entering this loop, even if we run past wbc->nr_to_write or
> +		 * encounter errors.  This is because the file system may still
> +		 * have state to clear for each folio.   We'll eventually return
> +		 * the first error encountered.
> +		 *
> +		 * For background writeback just push done_index past this folio
> +		 * so that we can just restart where we left off and media
> +		 * errors won't choke writeout for the entire file.
> +		 */
> +		if (wbc->sync_mode == WB_SYNC_NONE &&
> +		    (wbc->err || wbc->nr_to_write <= 0)) {
> +			writeback_finish(mapping, wbc, folio->index + nr);
> +			return error;
>  		}
>  	}
>  
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR