Re: [PATCH v6 04/19] mm: Rearrange readahead loop

Dave Chinner
2020-02-18T05:08:35Z

On Mon, Feb 17, 2020 at 10:45:45AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> Move the declaration of 'page' to inside the loop and move the 'kick
> off a fresh batch' code to the end of the function for easier use in
> subsequent patches.

Stale? the "kick off" code is moved to the tail of the loop, not the
end of the function.

> @@ -183,14 +183,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
> page = xa_load(&mapping->i_pages, page_offset);
> if (page && !xa_is_value(page)) {
> /*
> - * Page already present? Kick off the current batch of
> - * contiguous pages before continuing with the next
> - * batch.
> + * Page already present? Kick off the current batch
> + * of contiguous pages before continuing with the
> + * next batch. This page may be the one we would
> + * have intended to mark as Readahead, but we don't
> + * have a stable reference to this page, and it's
> + * not worth getting one just for that.
> */
> - if (readahead_count(&rac))
> - read_pages(&rac, &page_pool, gfp_mask);
> - rac._nr_pages = 0;
> - continue;
> + goto read;
> }
>
> page = __page_cache_alloc(gfp_mask);
> @@ -201,6 +201,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> if (page_idx == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> rac._nr_pages++;
> + continue;
> +read:
> + if (readahead_count(&rac))
> + read_pages(&rac, &page_pool, gfp_mask);
> + rac._nr_pages = 0;
> }

Also, why? This adds a goto from branched code that continues, then
adds a continue so the unbranched code doesn't execute the code the
goto jumps to. In absence of any explanation, this isn't an
improvement and doesn't make any sense...

--
Dave Chinner
david@fromorbit.com