Re: [PATCH 04/12] mm: Add readahead address space operation

Matthew Wilcox
2020-01-30T08:00:18Z

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@infradead.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.