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    SubjectRe: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt
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    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@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;
    >> ?
    >
    > +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

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