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Date:   Thu, 30 Jan 2020 00:00:13 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
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, linux-xfs@...r.kernel.org,
        cluster-devel@...hat.com, ocfs2-devel@....oracle.com
Subject: Re: [PATCH 04/12] mm: Add readahead address space operation

On Wed, Jan 29, 2020 at 11:24:56AM +1100, Dave Chinner wrote:
> On Fri, Jan 24, 2020 at 05:35:45PM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > 
> > This replaces ->readpages with a saner interface:
> >  - Return the number of pages not read instead of an ignored error code.
> >  - Pages are already in the page cache when ->readahead is called.
> >  - Implementation looks up the pages in the page cache instead of
> >    having them passed in a linked list.
> ....
> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 5a6676640f20..6d65dae6dad0 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -121,7 +121,18 @@ static void read_pages(struct address_space *mapping, struct file *filp,
> >  
> >  	blk_start_plug(&plug);
> >  
> > -	if (mapping->a_ops->readpages) {
> > +	if (mapping->a_ops->readahead) {
> > +		unsigned left = mapping->a_ops->readahead(filp, mapping,
> > +				start, nr_pages);
> > +
> > +		while (left) {
> > +			struct page *page = readahead_page(mapping,
> > +					start + nr_pages - left - 1);
> 
> Off by one? start = 2, nr_pages = 2, left = 1, this looks up the
> page at index 2, which is the one we issued IO on, not the one we
> "left behind" which is at index 3.

Yup.  I originally had:

		while (left--) ...

decided that was too confusing and didn't quite complete that thought.

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