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Date:   Thu, 6 Apr 2023 10:25:29 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Guenter Roeck <linux@roeck-us.net>
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
Message-ID: <20230406072529.vupqyrzqnhyozeyh@box.shutemov.name>
References: <9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net>
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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;
 
 		while (start + (1UL << order) > end)
 			order--;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c8f0a8c2d049..3179c30f7958 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -605,7 +605,13 @@ static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
 	 * this and the first chunk to online will be pageblock_nr_pages.
 	 */
 	for (pfn = start_pfn; pfn < end_pfn;) {
-		int order = min_t(int, MAX_ORDER, __ffs(pfn));
+		int order;
+
+		/* __ffs() behaviour is undefined for 0 */
+		if (pfn)
+			order = min_t(int, MAX_ORDER, __ffs(pfn));
+		else
+			order = MAX_ORDER;
 
 		(*online_page_callback)(pfn_to_page(pfn), order);
 		pfn += (1UL << order);
-- 
  Kiryl Shutsemau / Kirill A. Shutemov