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SubjectRe: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt
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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@intel.com>
>>> Link: https://lore.kernel.org/oe-kbuild-all/202303152343.D93IbJmn-lkp@intel.com/
>>> Signed-off-by: "Kirill A. Shutemov" <kirill@shutemov.name>
>>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Signed-off-by: Andrew Morton <akpm@linux-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;
?

Guenter

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