linux-kernel - Re: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue

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Date:   Fri, 23 Jun 2017 14:22:02 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Xishi Qiu <qiuxishi@...wei.com>,
        Hao Lee <haolee.swjtu@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in
 __rmqueue_fallback

On Wed 21-06-17 20:55:28, Rasmus Villemoes wrote:
> Since current_order starts as MAX_ORDER-1 and is then only
> decremented, the second half of the loop condition seems
> superfluous. However, if order is 0, we may decrement current_order
> past 0, making it UINT_MAX. This is obviously too subtle ([1], [2]).
> 
> Since we need to add some comment anyway, change the two variables to
> signed, making the counting-down for loop look more familiar, and
> apparently also making gcc generate slightly smaller code.
> 
> [1] https://lkml.org/lkml/2016/6/20/493
> [2] https://lkml.org/lkml/2017/6/19/345
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

I would hope for a more consistent usage of the type but his alone
should prevent future attempts to "clean up" the code.

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> Michal, something like this, perhaps?
> 
> mm/page_alloc.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2302f250d6b1..e656f4da9772 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2204,19 +2204,23 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
>   * list of requested migratetype, possibly along with other pages from the same
>   * block, depending on fragmentation avoidance heuristics. Returns true if
>   * fallback was found so that __rmqueue_smallest() can grab it.
> + *
> + * The use of signed ints for order and current_order is a deliberate
> + * deviation from the rest of this file, to make the for loop
> + * condition simpler.
>   */
>  static inline bool
> -__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
> +__rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  {
>  	struct free_area *area;
> -	unsigned int current_order;
> +	int current_order;
>  	struct page *page;
>  	int fallback_mt;
>  	bool can_steal;
>  
>  	/* Find the largest possible block of pages in the other list */
>  	for (current_order = MAX_ORDER-1;
> -				current_order >= order && current_order <= MAX_ORDER-1;
> +				current_order >= order;
>  				--current_order) {
>  		area = &(zone->free_area[current_order]);
>  		fallback_mt = find_suitable_fallback(area, current_order,
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs