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

[Eric Biggers]

On Mon, Mar 23, 2020 at 01:22:46PM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> By reducing nr_to_read, we can eliminate this check from inside the loop.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>

- Eric