mm, isolation: avoid checking unmovable pages across pageblock boundary

In has_unmovable_pages(), the page parameter would not always be the first
page within a pageblock (see how the page pointer is passed in from
start_isolate_page_range() after call __first_valid_page()), so that would
cause checking unmovable pages span two pageblocks.

After this patch, the checking is enforced within one pageblock no matter
the page is first one or not, and obey the semantics of this function.

This issue is found by code inspection.

Michal said "this might lead to false negatives when an unrelated block
would cause an isolation failure".
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Hildenbrand <david@redhat.com>
Link: https://lkml.kernel.org/r/20200824065811.383266-1-lixinhai.lxh@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -8234,6 +8234,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 {
 	unsigned long iter = 0;
 	unsigned long pfn = page_to_pfn(page);
+	unsigned long offset = pfn % pageblock_nr_pages;
 
 	if (is_migrate_cma_page(page)) {
 		/*
@@ -8247,7 +8248,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 		return page;
 	}
 
-	for (; iter < pageblock_nr_pages; iter++) {
+	for (; iter < pageblock_nr_pages - offset; iter++) {
 		if (!pfn_valid_within(pfn + iter))
 			continue;