Re: [PATCH v6 05/19] mm: Remove 'page_offset' from readahead loop

John Hubbard
2020-02-18T23:08:52Z

On 2/17/20 10:45 AM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> Eliminate the page_offset variable which was confusing with the
> 'offset' parameter and record the start of each consecutive run of
> pages in the readahead_control.

...presumably for the benefit of a subsequent patch, since it's not
consumed in this patch.

Thanks for breaking these up, btw, it really helps.

>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> mm/readahead.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 3eca59c43a45..74791b96013f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -162,6 +162,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> struct readahead_control rac = {
> .mapping = mapping,
> .file = filp,
> + ._start = offset,
> ._nr_pages = 0,
> };
>
> @@ -175,12 +176,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> */
> for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
> struct page *page;
> - pgoff_t page_offset = offset + page_idx;

OK, this is still something I want to mention (I wrote the same thing when reviewing
the wrong version of this patch, a moment ago).

You know...this ends up incrementing offset each time through the
loop, so yes, the behavior is the same as when using "offset + page_idx".
However, now it's a little harder to see that.

IMHO the page_offset variable is not actually a bad thing, here. I'd rather
keep it, all other things being equal (and I don't see any other benefits
here: line count is about the same, for example).

What do you think? (I don't feel strongly about this fine point.)

thanks,
--
John Hubbard
NVIDIA

>
> - if (page_offset > end_index)
> + if (offset > end_index)
> break;
>
> - page = xa_load(&mapping->i_pages, page_offset);
> + page = xa_load(&mapping->i_pages, offset);
> if (page && !xa_is_value(page)) {
> /*
> * Page already present? Kick off the current batch
> @@ -196,16 +196,18 @@ void __do_page_cache_readahead(struct address_space *mapping,
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> break;
> - page->index = page_offset;
> + page->index = offset;
> list_add(&page->lru, &page_pool);
> if (page_idx == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> rac._nr_pages++;
> + offset++;
> continue;
> read:
> if (readahead_count(&rac))
> read_pages(&rac, &page_pool, gfp_mask);
> rac._nr_pages = 0;
> + rac._start = ++offset;
> }
>
> /*
>