Commit 59f876fb authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Andrew Morton
Browse files

mm: avoid passing 0 to __ffs() 

23baf831 ("mm, treewide: redefine MAX_ORDER sanely") results in
various boot failures (hang) on arm targets 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.

[akpm@linux-foundation.org: fix comment, per Mike]
  Link: https://lkml.kernel.org/r/ZDBa7HWZK69dKKzH@kernel.org
Link: https://lkml.kernel.org/r/20230406072529.vupqyrzqnhyozeyh@box.shutemov.name


Fixes: 23baf831 ("mm, treewide: redefine MAX_ORDER sanely")
Signed-off-by: default avatar"Kirill A. Shutemov" <kirill@shutemov.name>
Reported-by: default avatarGuenter Roeck <linux@roeck-us.net>
  Link: https://lkml.kernel.org/r/9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net


Reviewed-by: default avatarMike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f8f238ff

mm/memblock.c

+10 −1
Original line number Diff line number Diff line
@@ -2043,7 +2043,16 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end) 
	int order;



	while (start < end) {

		/*

		 * Free the pages in the largest chunks alignment allows.

		 *

		 * __ffs() behaviour is undefined for 0. start == 0 is

		 * MAX_ORDER-aligned, set order to MAX_ORDER for the case.

		 */

		if (start)

			order = min_t(int, MAX_ORDER, __ffs(start));

		else

			order = MAX_ORDER;



		while (start + (1UL << order) > end)

			order--;

mm/memory_hotplug.c

+12 −1
Original line number Diff line number Diff line
@@ -605,7 +605,18 @@ 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;



		/*

		 * Free to online pages in the largest chunks alignment allows.

		 *

		 * __ffs() behaviour is undefined for 0. start == 0 is

		 * MAX_ORDER-aligned, Set order to MAX_ORDER for the case.

		 */

		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);