linux-kernel - Re: [PATCH] mm-treewide-redefine-max

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Message-ID: <9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net>
Date:   Wed, 5 Apr 2023 22:20:26 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt

Hi,

On Wed, Mar 15, 2023 at 06:38:00PM +0300, Kirill A. Shutemov wrote:
> fix min() warning
> 
> Link: https://lkml.kernel.org/r/20230315153800.32wib3n5rickolvh@box
> Reported-by: kernel test robot <lkp@...el.com>
>   Link: https://lore.kernel.org/oe-kbuild-all/202303152343.D93IbJmn-lkp@intel.com/
> Signed-off-by: "Kirill A. Shutemov" <kirill@...temov.name>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Zi Yan <ziy@...dia.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

This patch results in various boot failures (hang) on arm targets
in linux-next. Debug messages reveal the reason.

########### MAX_ORDER=10 start=0 __ffs(start)=-1 min()=10 min_t=-1
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If start==0, __ffs(start) returns 0xfffffff or (as int) -1, which min_t()
interprets as such, while min() apparently uses the returned unsigned long
value. Obviously a negative order isn't received well by the rest of the
code.

Guenter

> ---
>  mm/memblock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 338b8cb0793e..7911224b1ed3 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2043,7 +2043,7 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
>  	int order;
>  
>  	while (start < end) {
> -		order = min(MAX_ORDER, __ffs(start));
> +		order = min_t(int, MAX_ORDER, __ffs(start));
>  
>  		while (start + (1UL << order) > end)
>  			order--;
> -- 
> 2.39.2