linux-kernel - Re: [PATCH 13/13] writeback: Remove a use of write_cache_pages() from do

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Date: Mon, 5 Feb 2024 12:43:37 +0100
From: Jan Kara <jack@...e.cz>
To: Christoph Hellwig <hch@....de>
Cc: linux-mm@...ck.org, Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.com>, David Howells <dhowells@...hat.com>,
	Brian Foster <bfoster@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/13] writeback: Remove a use of write_cache_pages()
 from do_writepages()

On Sat 03-02-24 08:11:47, Christoph Hellwig wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> Use the new writeback_iter() directly instead of indirecting
> through a callback.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> [hch: ported to the while based iter style]
> Signed-off-by: Christoph Hellwig <hch@....de>

Looks good. Feel free to add:

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

I've just noticed one preexisting problem which was made more visible by
your reshuffling:

> +static int writeback_use_writepage(struct address_space *mapping,
> +		struct writeback_control *wbc)
>  {
> -	struct address_space *mapping = data;
> -	int ret = mapping->a_ops->writepage(&folio->page, wbc);
> -	mapping_set_error(mapping, ret);
> -	return ret;
> +	struct folio *folio = NULL;
> +	struct blk_plug plug;
> +	int err;
> +
> +	blk_start_plug(&plug);
> +	while ((folio = writeback_iter(mapping, wbc, folio, &err))) {
> +		err = mapping->a_ops->writepage(&folio->page, wbc);
> +		mapping_set_error(mapping, err);

So if ->writepage() returns AOP_WRITEPAGE_ACTIVATE, we shouldn't call
mapping_set_error() because that's just going to confuse the error
tracking.

> +		if (err == AOP_WRITEPAGE_ACTIVATE) {
> +			folio_unlock(folio);
> +			err = 0;
> +		}
> +	}
> +	blk_finish_plug(&plug);
> +
> +	return err;
>  }

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR