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Date:   Thu, 6 Apr 2023 18:10:15 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Guenter Roeck <linux@roeck-us.net>, Mike Rapoport <rppt@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt
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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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov