Ocfs2 devel - Mar 2020 - [PATCH v9 12/25] mm: Move end_index check out of readahead loop

From: "Matthew Wilcox (Oracle)" <willy at infradead.org>

By reducing nr_to_read, we can eliminate this check from inside the loop.

Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
Reviewed-by: John Hubbard <jhubbard at nvidia.com>
Reviewed-by: William Kucharski <william.kucharski at oracle.com>
---
 mm/readahead.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index d01531ef9f3c..a37b68f66233 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 +176,29 @@ 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;
+	/* Avoid wrapping to the beginning of the file */
+	if (index + nr_to_read < index)
+		nr_to_read = ULONG_MAX - index + 1;
+	/* Don't read past the page containing the last byte of the file */
+	if (index + nr_to_read >= end_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
-- 
2.25.1

On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox
wrote:
> From: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> 
> By reducing nr_to_read, we can eliminate this check from inside the loop.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org>
> Reviewed-by: John Hubbard <jhubbard at nvidia.com>
> Reviewed-by: William Kucharski <william.kucharski at oracle.com>
> ---
>  mm/readahead.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index d01531ef9f3c..a37b68f66233 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 +176,29 @@ 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;
> +	/* Avoid wrapping to the beginning of the file */
> +	if (index + nr_to_read < index)
> +		nr_to_read = ULONG_MAX - index + 1;
> +	/* Don't read past the page containing the last byte of the file */
> +	if (index + nr_to_read >= end_index)
> +		nr_to_read = end_index - index + 1;
There seem to be a couple off-by-one errors here.  Shouldn't it be:

	/* Avoid wrapping to the beginning of the file */
	if (index + nr_to_read < index)
		nr_to_read = ULONG_MAX - index;
	/* Don't read past the page containing the last byte of the file */
	if (index + nr_to_read > end_index)
		nr_to_read = end_index - index + 1;

I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1',
so that
'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0.

Then 'index + nr_to_read > end_index' rather 'index + nr_to_read
>= end_index',
since otherwise nr_to_read can be increased by 1 rather than decreased or stay
the same as expected.

- Eric