Re: [PATCH v10 12/25] mm: Move end_index check out of readahead loop

Eric Biggers
2020-03-23T20:50:01Z

On Mon, Mar 23, 2020 at 01:22:46PM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> By reducing nr_to_read, we can eliminate this check from inside the loop.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> ---
> mm/readahead.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index d01531ef9f3c..998fdd23c0b1 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -167,8 +167,6 @@ 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);
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> @@ -178,22 +176,26 @@ void __do_page_cache_readahead(struct address_space *mapping,
> ._index = index,
> };
> unsigned long i;
> + pgoff_t end_index; /* The last page we want to read */
>
> if (isize == 0)
> return;
>
> - end_index = ((isize - 1) >> PAGE_SHIFT);
> + end_index = (isize - 1) >> PAGE_SHIFT;
> + if (index > end_index)
> + return;
> + /* Don't read past the page containing the last byte of the file */
> + if (nr_to_read > end_index - index)
> + nr_to_read = end_index - index + 1;
>
> /*
> * Preallocate as many pages as we will need.
> */
> for (i = 0; i < nr_to_read; i++) {
> - if (index + i > end_index)
> - break;
> + struct page *page = xa_load(&mapping->i_pages, index + i);
>
> BUG_ON(index + i != rac._index + rac._nr_pages);
>
> - page = xa_load(&mapping->i_pages, index + i);
> if (page && !xa_is_value(page)) {
> /*
> * Page already present? Kick off the current batch of
> --

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric