linux-ext4 - Re: [PATCH v7 21/24] iomap: Restructure iomap_readpages

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Message-ID: <20200220162404.GY24185@bombadil.infradead.org>
Date:   Thu, 20 Feb 2020 08:24:04 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v7 21/24] iomap: Restructure iomap_readpages_actor

On Thu, Feb 20, 2020 at 07:47:41AM -0800, Christoph Hellwig wrote:
> On Wed, Feb 19, 2020 at 01:01:00PM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > 
> > By putting the 'have we reached the end of the page' condition at the end
> > of the loop instead of the beginning, we can remove the 'submit the last
> > page' code from iomap_readpages().  Also check that iomap_readpage_actor()
> > didn't return 0, which would lead to an endless loop.
> 
> I'm obviously biassed a I wrote the original code, but I find the new
> very much harder to understand (not that the previous one was easy, this
> is tricky code..).

Agreed, I found the original code hard to understand.  I think this is
easier because now cur_page doesn't leak outside this loop, so it has
an obvious lifecycle.

I'm kind of optimistic for Dave Howells' iov_iter addition of an
ITER_MAPPING.  That might simplify all of this code.