linux-ext4 - Re: [RFC v3 2/7] iomap: Add iomap_folio

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Date:   Fri, 23 Dec 2022 07:02:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com
Subject: Re: [RFC v3 2/7] iomap: Add iomap_folio_done helper

On Fri, Dec 16, 2022 at 04:06:21PM +0100, Andreas Gruenbacher wrote:
> +static void iomap_folio_done(struct iomap_iter *iter, loff_t pos, size_t ret,
> +		struct folio *folio)
> +{
> +	const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
> +
> +	if (folio)
> +		folio_unlock(folio);
> +	if (page_ops && page_ops->page_done)
> +		page_ops->page_done(iter->inode, pos, ret, &folio->page);
> +	if (folio)
> +		folio_put(folio);
> +}

How is the folio derefence going to work if folio is NULL?

That being said, I really wonder if the current API is the right way to
go.  Can't we just have a ->get_folio method with the same signature as
__filemap_get_folio, and then do the __filemap_get_folio from the file
system and avoid the page/folio == NULL clean path entirely?  Then on
the done side move the unlock and put into the done method as well.

>  	if (!folio) {
>  		status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
> -		goto out_no_page;
> +		iomap_folio_done(iter, pos, 0, NULL);
> +		return status;
>  	}
>  
>  	/*
> @@ -656,13 +670,9 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
>  	return 0;
>  
>  out_unlock:
> -	folio_unlock(folio);
> -	folio_put(folio);
> +	iomap_folio_done(iter, pos, 0, folio);
>  	iomap_write_failed(iter->inode, pos, len);
>  
> -out_no_page:
> -	if (page_ops && page_ops->page_done)
> -		page_ops->page_done(iter->inode, pos, 0, NULL);
>  	return status;

But for the current version I don't really understand why the error
unwinding changes here.