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

Dave Chinner
2020-01-29T00:25:04Z

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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com