On 9 December 2016 at 14:52, Robert Richter <robert.rich...@cavium.com> wrote: > On 09.12.16 21:15:12, Yisheng Xie wrote: >> For invalid pages, their zone and node information is not initialized, and it >> do have risk to trigger the BUG_ON, so I have a silly question, >> why not just change the BUG_ON: > > We need to get the page handling correct. Modifying the BUG_ON() just > hides that something is wrong. >
Actually, I think this is a reasonable question. We are trying very hard to work around the BUG_ON(), which arguably does something wrong by calling page_to_nid() on a struct page without checking if it is a valid page. Looking at commit 344c790e3821 ("mm: make setup_zone_migrate_reserve() aware of overlapping nodes"), the BUG_ON() has a specific purpose related to adjacent zones. What will go wrong if we ignore this check? > >> ----------- >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6de9440..af199b8 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1860,12 +1860,13 @@ int move_freepages(struct zone *zone, >> * Remove at a later date when no bug reports exist related to >> * grouping pages by mobility >> */ >> - VM_BUG_ON(page_zone(start_page) != page_zone(end_page)); >> + VM_BUG_ON(early_pfn_valid(start_page) && early_pfn_valid(end_page) && >> + page_zone(start_page) != page_zone(end_page)); >> #endif >> >> for (page = start_page; page <= end_page;) { >> /* Make sure we are not inadvertently changing nodes */ >> - VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page); >> + VM_BUG_ON_PAGE(early_pfn_valid(page) && (page_to_nid(page) >> != zone_to_nid(zone)), page); >> >> if (!pfn_valid_within(page_to_pfn(page))) { >> page++; >