diff mbox series

[v7,11/24] mm: Move end_index check out of readahead loop

Message ID 20200219210103.32400-12-willy@infradead.org
State New
Headers show
Series Change readahead API | expand

Commit Message

Matthew Wilcox Feb. 19, 2020, 9 p.m. UTC
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>
---
 mm/readahead.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

Comments

John Hubbard Feb. 21, 2020, 3:50 a.m. UTC | #1
On 2/19/20 1:00 PM, 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>
> ---
>  mm/readahead.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 07cdfbf00f4b..ace611f4bf05 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -166,8 +166,6 @@ 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);
> @@ -179,22 +177,27 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  		._nr_pages = 0,
>  	};
>  	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;
> +	if (index + nr_to_read < index)
> +		nr_to_read = ULONG_MAX - index + 1;
> +	if (index + nr_to_read >= end_index)
> +		nr_to_read = end_index - index + 1;


This tiny patch made me pause, because I wasn't sure at first of the exact
intent of the lines above. Once I worked it out, it seemed like it might
be helpful (or overkill??) to add a few hints for the reader, especially since
there are no hints in the function's (minimal) documentation header. What
do you think of this?

	/*
	 * If we can't read *any* pages without going past the inodes's isize
	 * limit, give up entirely:
	 */
	if (index > end_index)
		return;

	/* Cap nr_to_read, in order to avoid overflowing the ULONG type: */
	if (index + nr_to_read < index)
		nr_to_read = ULONG_MAX - index + 1;

	/* Cap nr_to_read, to avoid reading past the inode's isize limit: */
	if (index + nr_to_read >= end_index)
		nr_to_read = end_index - index + 1;


Either way, it looks corrected written to me, so:

    Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
Matthew Wilcox Feb. 21, 2020, 3:35 p.m. UTC | #2
On Thu, Feb 20, 2020 at 07:50:39PM -0800, John Hubbard wrote:
> This tiny patch made me pause, because I wasn't sure at first of the exact
> intent of the lines above. Once I worked it out, it seemed like it might
> be helpful (or overkill??) to add a few hints for the reader, especially since
> there are no hints in the function's (minimal) documentation header. What
> do you think of this?
> 
> 	/*
> 	 * If we can't read *any* pages without going past the inodes's isize
> 	 * limit, give up entirely:
> 	 */
> 	if (index > end_index)
> 		return;
> 
> 	/* Cap nr_to_read, in order to avoid overflowing the ULONG type: */
> 	if (index + nr_to_read < index)
> 		nr_to_read = ULONG_MAX - index + 1;
> 
> 	/* Cap nr_to_read, to avoid reading past the inode's isize limit: */
> 	if (index + nr_to_read >= end_index)
> 		nr_to_read = end_index - index + 1;

A little verbose for my taste ... How about this?

        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;
John Hubbard Feb. 21, 2020, 7:41 p.m. UTC | #3
On 2/21/20 7:35 AM, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 07:50:39PM -0800, John Hubbard wrote:
>> This tiny patch made me pause, because I wasn't sure at first of the exact
>> intent of the lines above. Once I worked it out, it seemed like it might
>> be helpful (or overkill??) to add a few hints for the reader, especially since
>> there are no hints in the function's (minimal) documentation header. What
>> do you think of this?
>>
>> 	/*
>> 	 * If we can't read *any* pages without going past the inodes's isize
>> 	 * limit, give up entirely:
>> 	 */
>> 	if (index > end_index)
>> 		return;
>>
>> 	/* Cap nr_to_read, in order to avoid overflowing the ULONG type: */
>> 	if (index + nr_to_read < index)
>> 		nr_to_read = ULONG_MAX - index + 1;
>>
>> 	/* Cap nr_to_read, to avoid reading past the inode's isize limit: */
>> 	if (index + nr_to_read >= end_index)
>> 		nr_to_read = end_index - index + 1;
> 
> A little verbose for my taste ... How about this?


Mine too, actually. :)  I think your version below looks good.


thanks,
diff mbox series

Patch

diff --git a/mm/readahead.c b/mm/readahead.c
index 07cdfbf00f4b..ace611f4bf05 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -166,8 +166,6 @@  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);
@@ -179,22 +177,27 @@  void __do_page_cache_readahead(struct address_space *mapping,
 		._nr_pages = 0,
 	};
 	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;
+	if (index + nr_to_read < index)
+		nr_to_read = ULONG_MAX - index + 1;
+	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