Commit c8cc708a authored by Gerald Schaefer's avatar Gerald Schaefer Committed by Linus Torvalds

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: default avatarGerald Schaefer <>
Reviewed-by: default avatarMike Kravetz <>
Cc: Luiz Capitulino <>
Cc: Naoya Horiguchi <>
Cc: Hillf Danton <>
Cc: "Kirill A . Shutemov" <>
Cc: Dave Hansen <>
Cc: Paul Gortmaker <>
Cc: "Aneesh Kumar K . V" <>
Cc: Martin Schwidefsky <>
Cc: Heiko Carstens <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 8f182270
......@@ -1030,6 +1030,7 @@ static void destroy_compound_gigantic_page(struct page *page,
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)) {
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