On Thu, Mar 21, 2024 at 09:13:57PM +0800, kernel test robot wrote: > Hi Johannes, > > kernel test robot noticed the following build warnings: > > [auto build test WARNING on akpm-mm/mm-everything] > > url: https://github.com/intel-lab-lkp/linux/commits/Johannes-Weiner/mm-page_alloc-remove-pcppage-migratetype-caching/20240321-020814 > base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything > patch link: https://lore.kernel.org/r/20240320180429.678181-10-hannes%40cmpxchg.org > patch subject: [PATCH 09/10] mm: page_isolation: prepare for hygienic freelists > config: i386-randconfig-003-20240321 (https://download.01.org/0day-ci/archive/20240321/202403212118.ye7lcKjD-lkp@xxxxxxxxx/config) > compiler: gcc-9 (Debian 9.3.0-22) 9.3.0 > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240321/202403212118.ye7lcKjD-lkp@xxxxxxxxx/reproduce) > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > the same patch/commit), kindly add following tags > | Reported-by: kernel test robot <lkp@xxxxxxxxx> > | Closes: https://lore.kernel.org/oe-kbuild-all/202403212118.ye7lcKjD-lkp@xxxxxxxxx/ > > All warnings (new ones prefixed by >>): > > mm/page_alloc.c: In function 'move_freepages_block_isolate': > >> mm/page_alloc.c:688:17: warning: array subscript 11 is above array bounds of 'struct free_area[11]' [-Warray-bounds] > 688 | zone->free_area[order].nr_free--; > | ~~~~~~~~~~~~~~~^~~~~~~ > >> mm/page_alloc.c:688:17: warning: array subscript 11 is above array bounds of 'struct free_area[11]' [-Warray-bounds] I think this is a bug in the old gcc. We have this in move_freepages_block_isolate(): /* We're the starting block of a larger buddy */ if (PageBuddy(page) && buddy_order(page) > pageblock_order) { int mt = get_pfnblock_migratetype(page, pfn); int order = buddy_order(page); if (!is_migrate_isolate(mt)) __mod_zone_freepage_state(zone, -(1UL << order), mt); del_page_from_free_list(page, zone, order); And this config doesn't have hugetlb enabled, so: /* If huge pages are not used, group by MAX_ORDER_NR_PAGES */ #define pageblock_order MAX_PAGE_ORDER If buddies were indeed >MAX_PAGE_ORDER, this would be an out-of-bounds access when delete updates the freelist count. Of course, buddies per definition cannot be larger than MAX_PAGE_ORDER. But the older gcc doesn't seem to realize this branch in this configuration is dead. Maybe we can help it out and make the impossible scenario a bit more explicit? Does this fixlet silence the warning? diff --git a/mm/page_alloc.c b/mm/page_alloc.c index efb2581ac142..4cdc356e73f6 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1698,6 +1698,10 @@ bool move_freepages_block_isolate(struct zone *zone, struct page *page, NULL, NULL)) return false; + /* No splits needed if buddies can't span multiple blocks */ + if (pageblock_order == MAX_PAGE_ORDER) + goto move; + /* We're a tail block in a larger buddy */ pfn = find_large_buddy(start_pfn); if (pfn != start_pfn) { @@ -1725,7 +1729,7 @@ bool move_freepages_block_isolate(struct zone *zone, struct page *page, split_large_buddy(zone, page, pfn, order); return true; } - +move: mt = get_pfnblock_migratetype(page, start_pfn); nr_moved = move_freepages(zone, start_pfn, end_pfn, migratetype); if (!is_migrate_isolate(mt)) Zi Yan, does this look sane to you as well?
