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

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Date:   Thu, 6 Apr 2023 11:23:30 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Mike Rapoport <rppt@...nel.org>
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

On 4/6/23 08:10, Kirill A. Shutemov wrote:
> On Thu, Apr 06, 2023 at 06:57:41AM -0700, Guenter Roeck wrote:
>> On 4/6/23 00:25, Kirill A. Shutemov wrote:
>>> On Wed, Apr 05, 2023 at 10:20:26PM -0700, Guenter Roeck wrote:
>>>> 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.
>>>
>>> Actually, __ffs() is not defined for 0.
>>>
>>> Maybe something like this?
>>>
>>> diff --git a/mm/memblock.c b/mm/memblock.c
>>> index 7911224b1ed3..63603b943bd0 100644
>>> --- a/mm/memblock.c
>>> +++ b/mm/memblock.c
>>> @@ -2043,7 +2043,11 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
>>>    	int order;
>>>    	while (start < end) {
>>> -		order = min_t(int, MAX_ORDER, __ffs(start));
>>> +		/* __ffs() behaviour is undefined for 0 */
>>> +		if (start)
>>> +			order = min_t(int, MAX_ORDER, __ffs(start));
>>> +		else
>>> +			order = MAX_ORDER;
>>
>> Shouldn't that be
>> 		else
>> 			order = 0;
>> ?
> 
> +Mike.
> 
> No. start == 0 is MAX_ORDER-aligned. We want to free the pages in the
> largest chunks alignment allows.
> 

Ah, ok. Makes sense.

Thanks,
Guenter