On Thu, Apr 06, 2023 at 03:44:23PM -0700, Andrew Morton wrote:
> On Fri, 7 Apr 2023 00:14:31 +0300 Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> > > > 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
> > */
>
> Meanwhile I'd like to fix "various boot failures (hang) on arm targets"
> in -next, so I queued up Kirill's informal fix for now.
Here's my variant of the fix up with more vervose comments.
diff --git a/mm/memblock.c b/mm/memblock.c
index 7911224b1ed3..381e36ac9e4d 100644
--- 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 c8f0a8c2d049..8e0fa209d533 100644
--- 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);
--
Kiryl Shutsemau / Kirill A. Shutemov
