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

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Date:   Fri, 23 Dec 2022 23:22:31 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        "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 23, 2022 at 09:54:34PM +0100, Andreas Grünbacher wrote:
> > But for the current version I don't really understand why the error
> > unwinding changes here.
> 
> Currently, we have this order of operations in iomap_write_begin():
> 
>   folio_unlock() // folio_put() // iomap_write_failed() // ->page_done()
> 
> and this order in iomap_write_end():
> 
>   folio_unlock() // ->page_done() // folio_put() // iomap_write_failed()
> 
> The unwinding in iomap_write_begin() works because this is the trivial
> case in which nothing happens to the page. We might just as well use
> the same order of operations there as in iomap_write_end() though, and
> when you switch to that, this is what you get.

Please document this in the commit message.