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@xxxxxxxxxxxxx> > > > > 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.