linux-kernel - Re: [PATCH 04/12] writeback: Simplify the loops in write_cache

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Message-ID: <ZJpijNd4G+JdVyhl@infradead.org>
Date:   Mon, 26 Jun 2023 21:16:12 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 04/12] writeback: Simplify the loops in
 write_cache_pages()

On Mon, Jun 26, 2023 at 06:35:13PM +0100, Matthew Wilcox (Oracle) wrote:
> Collapse the two nested loops into one.  This is needed as a step
> towards turning this into an iterator.
> ---
>  mm/page-writeback.c | 94 ++++++++++++++++++++++-----------------------
>  1 file changed, 47 insertions(+), 47 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 54f2972dab45..68f28eeb15ed 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2461,6 +2461,7 @@ int write_cache_pages(struct address_space *mapping,
>  		      void *data)
>  {
>  	int error;
> +	int i = 0;
>  
>  	if (wbc->range_cyclic) {
>  		wbc->index = mapping->writeback_index; /* prev offset */
> @@ -2478,65 +2479,64 @@ int write_cache_pages(struct address_space *mapping,
>  	folio_batch_init(&wbc->fbatch);
>  	wbc->err = 0;
>  
> +	for (;;) {
> +		struct folio *folio;
>  
> +		if (i == wbc->fbatch.nr) {
> +			writeback_get_batch(mapping, wbc);
> +			i = 0;
> +		}
>  		if (wbc->fbatch.nr == 0)
>  			break;
> +		folio = wbc->fbatch.folios[i++];

Did you consider moving what is currently the "i" local variable
into strut writeback_control as well?  Then writeback_get_batch
could return the current folio, and we could hae a much nicer loop
here by moving all of the above into writeback_get_batch:

	while ((folio = writeback_get_batch(mapping, wbc))) {

(and yes, writeback_get_batch probably needs a better name with that)