Commit 23be7468 authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
Browse files

hugetlb: fix infinite loop in get_futex_key() when backed by huge pages

If a futex key happens to be located within a huge page mapped
MAP_PRIVATE, get_futex_key() can go into an infinite loop waiting for a
page->mapping that will never exist.


 for more details
about the problem.

This patch makes page->mapping a poisoned value that includes
PAGE_MAPPING_ANON mapped MAP_PRIVATE.  This is enough for futex to
continue but because of PAGE_MAPPING_ANON, the poisoned value is not
dereferenced or used by futex.  No other part of the VM should be
dereferencing the page->mapping of a hugetlbfs page as its page cache is
not on the LRU.

This patch fixes the problem with the test case described in the bugzilla.

[ mel cant spel]
Signed-off-by: default avatarMel Gorman <>
Acked-by: default avatarPeter Zijlstra <>
Acked-by: default avatarDarren Hart <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 9a6a1ecd
...@@ -48,6 +48,15 @@ ...@@ -48,6 +48,15 @@
#define POISON_FREE 0x6b /* for use-after-free poisoning */ #define POISON_FREE 0x6b /* for use-after-free poisoning */
#define POISON_END 0xa5 /* end-byte of poisoning */ #define POISON_END 0xa5 /* end-byte of poisoning */
/********** mm/hugetlb.c **********/
* Private mappings of hugetlb pages use this poisoned value for
* page->mapping. The core VM should not be doing anything with this mapping
* but futex requires the existence of some page->mapping value even though it
* is unused if PAGE_MAPPING_ANON is set.
/********** arch/$ARCH/mm/init.c **********/ /********** arch/$ARCH/mm/init.c **********/
...@@ -546,6 +546,7 @@ static void free_huge_page(struct page *page) ...@@ -546,6 +546,7 @@ static void free_huge_page(struct page *page)
mapping = (struct address_space *) page_private(page); mapping = (struct address_space *) page_private(page);
set_page_private(page, 0); set_page_private(page, 0);
page->mapping = NULL;
BUG_ON(page_count(page)); BUG_ON(page_count(page));
INIT_LIST_HEAD(&page->lru); INIT_LIST_HEAD(&page->lru);
...@@ -2447,8 +2448,10 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, ...@@ -2447,8 +2448,10 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
spin_lock(&inode->i_lock); spin_lock(&inode->i_lock);
inode->i_blocks += blocks_per_huge_page(h); inode->i_blocks += blocks_per_huge_page(h);
spin_unlock(&inode->i_lock); spin_unlock(&inode->i_lock);
} else } else {
lock_page(page); lock_page(page);
page->mapping = HUGETLB_POISON;
} }
/* /*
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