From: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> Subject: mm/hugetlb: clear compound_mapcount when freeing gigantic pages While working on s390 support for gigantic hugepages I ran into the following "Bad page state" warning when freeing gigantic pages: BUG: Bad page state in process bash pfn:580001 page:000003d116000040 count:0 mapcount:0 mapping:ffffffff00000000 index:0x0 flags: 0x7fffc0000000000() page dumped because: non-NULL mapping This is because page->compound_mapcount, which is part of a union with page->mapping, is initialized with -1 in prep_compound_gigantic_page(), and not cleared again during destroy_compound_gigantic_page(). Fix this by clearing the compound_mapcount in destroy_compound_gigantic_page() before clearing compound_head. Interestingly enough, the warning will not show up on x86_64, although this should not be architecture specific. Apparently there is an endianness issue, combined with the fact that the union contains both a 64 bit ->mapping pointer and a 32 bit atomic_t ->compound_mapcount as members. The resulting bogus page->mapping on x86_64 therefore contains 00000000ffffffff instead of ffffffff00000000 on s390, which will falsely trigger the PageAnon() check in free_pages_prepare() because page->mapping & PAGE_MAPPING_ANON is true on little-endian architectures like x86_64 in this case (the page is not compound anymore, ->compound_head was already cleared before). As a result, page->mapping will be cleared before doing the checks in free_pages_check(). Not sure if the bogus "PageAnon() returning true" on x86_64 for the first tail page of a gigantic page (at this stage) has other theoretical implications, but they would also be fixed with this patch. Link: http://lkml.kernel.org/r/1466612719-5642-1-git-send-email-gerald.schaefer@xxxxxxxxxx Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> Cc: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx> Cc: "Aneesh Kumar K . V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/hugetlb.c | 1 + 1 file changed, 1 insertion(+) diff -puN mm/hugetlb.c~mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages mm/hugetlb.c --- a/mm/hugetlb.c~mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages +++ a/mm/hugetlb.c @@ -1030,6 +1030,7 @@ static void destroy_compound_gigantic_pa int nr_pages = 1 << order; struct page *p = page + 1; + atomic_set(compound_mapcount_ptr(page), 0); for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) { clear_compound_head(p); set_page_refcounted(p); _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
