On Thu, Apr 06, 2023 at 06:10:15PM +0300, 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@xxxxxxxxx> > > > > > Link: https://lore.kernel.org/oe-kbuild-all/202303152343.D93IbJmn-lkp@xxxxxxxxx/ > > > > > Signed-off-by: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> > > > > > Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > > > > > Cc: Zi Yan <ziy@xxxxxxxxxx> > > > > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > > > > > > > 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. Right. Before the changes to MAX_ORDER it was order = min(MAX_ORDER - 1UL, __ffs(start)); which would evaluate to 10. I'd just prefer the comment to include the explanation about why we choose MAX_ORDER for start == 0. Say /* * __ffs() behaviour is undefined for 0 and we want to free the * pages in the largest chunks alignment allows, so set order to * MAX_ORDER when start == 0 */ > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Sincerely yours, Mike.
