On 21 Mar 2024, at 10:24, Johannes Weiner wrote: > 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? Yes. This and patch 9 look good to me. Thanks. For both, Reviewed-by: Zi Yan <ziy@xxxxxxxxxx> I also tested them locally and confirm it is a gcc bug and this fix works for gcc-9.3: 1. gcc-13.2 does not give any warning for the original patch 9 2. gcc-9.3 gives the warning for the origin patch, but the warning goes away with this patch applied. -- Best Regards, Yan, Zi
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