Re: [PATCH 05/13] writeback: rework the loop termination condition in write_cache_pages

[Jan Kara]

On Sat 03-02-24 08:11:39, Christoph Hellwig wrote:
> Rework the way we deal with the cleanup after the writepage call.
> 
> First handle the magic AOP_WRITEPAGE_ACTIVATE separately from real error
> returns to get it out of the way of the actual error handling path.
> 
> The split the handling on intgrity vs non-integrity branches first,
> and return early using a goto for the non-ingegrity early loop condition
> to remove the need for the done and done_index local variables, and for
> assigning the error to ret when we can just return error directly.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  mm/page-writeback.c | 84 ++++++++++++++++++---------------------------
>  1 file changed, 33 insertions(+), 51 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index c7c494526bc650..88b2c4c111c01b 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2396,13 +2396,12 @@ int write_cache_pages(struct address_space *mapping,
>  		      void *data)
>  {
>  	int ret = 0;
> -	int done = 0;
>  	int error;
>  	struct folio_batch fbatch;
> +	struct folio *folio;
>  	int nr_folios;
>  	pgoff_t index;
>  	pgoff_t end;		/* Inclusive */
> -	pgoff_t done_index;
>  	xa_mark_t tag;
>  
>  	folio_batch_init(&fbatch);
> @@ -2419,8 +2418,7 @@ int write_cache_pages(struct address_space *mapping,
>  	} else {
>  		tag = PAGECACHE_TAG_DIRTY;
>  	}
> -	done_index = index;
> -	while (!done && (index <= end)) {
> +	while (index <= end) {
>  		int i;
>  
>  		nr_folios = filemap_get_folios_tag(mapping, &index, end,
> @@ -2430,11 +2428,7 @@ int write_cache_pages(struct address_space *mapping,
>  			break;
>  
>  		for (i = 0; i < nr_folios; i++) {
> -			struct folio *folio = fbatch.folios[i];
> -			unsigned long nr;
> -
> -			done_index = folio->index;
> -
> +			folio = fbatch.folios[i];
>  			folio_lock(folio);
>  
>  			/*
> @@ -2469,45 +2463,32 @@ int write_cache_pages(struct address_space *mapping,
>  
>  			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
>  			error = writepage(folio, wbc, data);
> -			nr = folio_nr_pages(folio);
> -			wbc->nr_to_write -= nr;
> -			if (unlikely(error)) {
> -				/*
> -				 * Handle errors according to the type of
> -				 * writeback. There's no need to continue for
> -				 * background writeback. Just push done_index
> -				 * past this page so media errors won't choke
> -				 * writeout for the entire file. For integrity
> -				 * writeback, we must process the entire dirty
> -				 * set regardless of errors because the fs may
> -				 * still have state to clear for each page. In
> -				 * that case we continue processing and return
> -				 * the first error.
> -				 */
> -				if (error == AOP_WRITEPAGE_ACTIVATE) {
> -					folio_unlock(folio);
> -					error = 0;
> -				} else if (wbc->sync_mode != WB_SYNC_ALL) {
> -					ret = error;
> -					done_index = folio->index + nr;
> -					done = 1;
> -					break;
> -				}
> -				if (!ret)
> -					ret = error;
> +			wbc->nr_to_write -= folio_nr_pages(folio);
> +
> +			if (error == AOP_WRITEPAGE_ACTIVATE) {
> +				folio_unlock(folio);
> +				error = 0;
>  			}
>  
>  			/*
> -			 * We stop writing back only if we are not doing
> -			 * integrity sync. In case of integrity sync we have to
> -			 * keep going until we have written all the pages
> -			 * we tagged for writeback prior to entering this loop.
> +			 * For integrity writeback we have to keep going until
> +			 * we have written all the folios we tagged for
> +			 * writeback above, even if we run past wbc->nr_to_write
> +			 * or encounter errors.
> +			 * We stash away the first error we encounter in
> +			 * wbc->saved_err so that it can be retrieved when we're
> +			 * done.  This is because the file system may still have
> +			 * state to clear for each folio.
> +			 *
> +			 * For background writeback we exit as soon as we run
> +			 * past wbc->nr_to_write or encounter the first error.
>  			 */
> -			done_index = folio->index + nr;
> -			if (wbc->nr_to_write <= 0 &&
> -			    wbc->sync_mode == WB_SYNC_NONE) {
> -				done = 1;
> -				break;
> +			if (wbc->sync_mode == WB_SYNC_ALL) {
> +				if (error && !ret)
> +					ret = error;
> +			} else {
> +				if (error || wbc->nr_to_write <= 0)
> +					goto done;
>  			}
>  		}
>  		folio_batch_release(&fbatch);
> @@ -2524,14 +2505,15 @@ int write_cache_pages(struct address_space *mapping,
>  	 * 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;
> -	}
> -
> +	if (wbc->range_cyclic)
> +		mapping->writeback_index = 0;
>  	return ret;
> +
> +done:
> +	folio_batch_release(&fbatch);
> +	if (wbc->range_cyclic)
> +		mapping->writeback_index = folio->index + folio_nr_pages(folio);
> +	return error;
>  }
>  EXPORT_SYMBOL(write_cache_pages);
>  
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR