Re: [PATCH v6 04/16] mm: Tweak readahead loop slightly

John Hubbard
2020-02-18T22:57:49Z

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

OK...presumably for the benefit of a following patch, since it is not
actually consumed in this patch.

> readahead_control, and move the 'kick off a fresh batch' code to
> the end of the function for easier use in the next patch.

That last bit was actually done in the previous patch, rather than this
one, right?

>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> mm/readahead.c | 31 +++++++++++++++++++------------
> 1 file changed, 19 insertions(+), 12 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 15329309231f..74791b96013f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -154,7 +154,6 @@ void __do_page_cache_readahead(struct address_space *mapping,
> unsigned long lookahead_size)
> {
> struct inode *inode = mapping->host;
> - struct page *page;
> unsigned long end_index; /* The last page we want to read */
> LIST_HEAD(page_pool);
> int page_idx;
> @@ -163,6 +162,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> struct readahead_control rac = {
> .mapping = mapping,
> .file = filp,
> + ._start = offset,
> ._nr_pages = 0,
> };
>
> @@ -175,32 +175,39 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * Preallocate as many pages as we will need.
> */
> for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
> - pgoff_t page_offset = offset + page_idx;

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 the same, for example).

What do you think?

thanks,
--
John Hubbard
NVIDIA

> + struct page *page;
>
> - 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 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);
> 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;
> }
>
> /*
>