mm: avoid passing 0 to __ffs()

23baf831a32c ("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: 23baf831a32c ("mm, treewide: redefine MAX_ORDER sanely")
Signed-off-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lkml.kernel.org/r/9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memblock.c b/mm/memblock.c
index 7911224b1ed392e23d17bfd3db738b16a4d66684..3feafea06ab2128b6daf332d5963ae5e2e985a89 100644 (file)
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2043,7 +2043,16 @@ 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));
+               /*
+                * 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--;

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c8f0a8c2d04995071651576e5a1b95f4af4dc062..8e0fa209d5334a05d590c54cd74c8d8d052768ea 100644 (file)
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -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);