[PATCH] mm/hugetlb: clear compound_mapcount when freeing gigantic pages

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

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.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
---
 mm/hugetlb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e197cd7..b64f8b7 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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)) {
 		clear_compound_head(p);
 		set_page_refcounted(p);
-- 
2.6.6

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