The patch titled
Subject: mm/hugetlb: clear compound_mapcount when freeing gigantic pages
has been added to the -mm tree. Its filename is
mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages.patch
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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>
Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
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);
_
Patches currently in -mm which might be from gerald.schaefer@xxxxxxxxxx are
mm-hugetlb-clear-compound_mapcount-when-freeing-gigantic-pages.patch
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