From: Vlastimil Babka <vbabka@suse.cz>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Michal Hocko <mhocko@suse.com>,
Andrew Morton <akpm@linux-foundation.org>,
Hillf Danton <hillf.zj@alibaba-inc.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
Vinayak Menon <vinmenon@codeaurora.org>,
Xishi Qiu <qiuxishi@huawei.com>
Cc: Hao Lee <haolee.swjtu@gmail.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback
Date: Fri, 23 Jun 2017 15:10:45 +0200 [thread overview]
Message-ID: <d03adba2-800a-e367-fe12-87278ea2abe3@suse.cz> (raw)
In-Reply-To: <20170621185529.2265-1-linux@rasmusvillemoes.dk>
On 06/21/2017 08:55 PM, 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@rasmusvillemoes.dk>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> 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,
>
next prev parent reply other threads:[~2017-06-23 13:11 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-19 13:54 [PATCH] mm: remove a redundant condition in the for loop Hao Lee
2017-06-19 14:17 ` Vlastimil Babka
2017-06-19 19:05 ` Rasmus Villemoes
2017-06-19 20:23 ` Vlastimil Babka
2017-06-20 1:39 ` Hao Lee
2017-06-21 9:43 ` Michal Hocko
2017-06-21 18:55 ` [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback Rasmus Villemoes
2017-06-23 12:22 ` Michal Hocko
2017-06-23 13:10 ` Vlastimil Babka [this message]
2017-06-24 13:26 ` Wei Yang
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