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

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

On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers
wrote:
> On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote:
> > +	/* 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;
I think it's right.  Imagine that index is ULONG_MAX.  We should read one
page (the one at ULONG_MAX).  That would be ULONG_MAX - ULONG_MAX + 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;
> 
> 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.
Ooh, I missed the overflow case here.  It should be:

+	if (index + nr_to_read - 1 > end_index)
+		nr_to_read = end_index - index + 1;

Let's say index comes in at ULONG_MAX - 2, end_index is ULONG_MAX - 1
and nr_to_read is 8.  The first condition triggers and nr_to_read is
reduced to 3.  But then the second condition wouldn't trigger because
ULONG_MAX - 2 + 3 is 0.

With the rewrite I have in this message, ULONG_MAX - 2 + 3 - 1 is ULONG_MAX,
which is > ULONG_MAX - 1.  So the condition triggers and nr_to_read becomes
(ULONG_MAX - 1) - (ULONG_MAX - 2) + 1.  Which is -1 + 2 + 1, which is 2.
Which is the right answer because we want to read two pages; the one
at ULONG_MAX - 2 and the one at ULONG_MAX - 1.

Thank you!