From: Hugh Dickins hughd@google.com Subject: mm, futex: fix shared futex pgoff on shmem huge page

If more than one futex is placed on a shmem huge page, it can happen that waking the second wakes the first instead, and leaves the second waiting: the key's shared.pgoff is wrong.

When 3.11 commit 13d60f4b6ab5 ("futex: Take hugepages into account when generating futex_key"), the only shared huge pages came from hugetlbfs, and the code added to deal with its exceptional page->index was put into hugetlb source. Then that was missed when 4.8 added shmem huge pages.

page_to_pgoff() is what others use for this nowadays: except that, as currently written, it gives the right answer on hugetlbfs head, but nonsense on hugetlbfs tails. Fix that by calling hugetlbfs-specific hugetlb_basepage_index() on PageHuge tails as well as on head.

Yes, it's unconventional to declare hugetlb_basepage_index() there in pagemap.h, rather than in hugetlb.h; but I do not expect anything but page_to_pgoff() ever to need it.

[akpm@linux-foundation.org: give hugetlb_basepage_index() prototype the correct scope] Link: https://lkml.kernel.org/r/b17d946b-d09-326e-b42a-52884c36df32@google.com Fixes: 800d8c63b2e9 ("shmem: add huge pages support") Reported-by: Neel Natu neelnatu@google.com Signed-off-by: Hugh Dickins hughd@google.com Reviewed-by: Matthew Wilcox (Oracle) willy@infradead.org Acked-by: Thomas Gleixner tglx@linutronix.de Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: Zhang Yi wetpzy@gmail.com Cc: Mel Gorman mgorman@techsingularity.net Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Ingo Molnar mingo@redhat.com Cc: Peter Zijlstra peterz@infradead.org Cc: Darren Hart dvhart@infradead.org Cc: Davidlohr Bueso dave@stgolabs.net Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/hugetlb.h | 16 ---------------- include/linux/pagemap.h | 13 +++++++------ kernel/futex.c | 3 +-- mm/hugetlb.c | 5 +---- 4 files changed, 9 insertions(+), 28 deletions(-)

--- a/include/linux/hugetlb.h~mm-futex-fix-shared-futex-pgoff-on-shmem-huge-page +++ a/include/linux/hugetlb.h @@ -741,17 +741,6 @@ static inline int hstate_index(struct hs return h - hstates; }

-pgoff_t __basepage_index(struct page *page); - -/* Return page->index in PAGE_SIZE units */ -static inline pgoff_t basepage_index(struct page *page) -{ - if (!PageCompound(page)) - return page->index; - - return __basepage_index(page); -} - extern int dissolve_free_huge_page(struct page *page); extern int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn); @@ -988,11 +977,6 @@ static inline int hstate_index(struct hs return 0; }

-static inline pgoff_t basepage_index(struct page *page) -{ - return page->index; -} - static inline int dissolve_free_huge_page(struct page *page) { return 0; --- a/include/linux/pagemap.h~mm-futex-fix-shared-futex-pgoff-on-shmem-huge-page +++ a/include/linux/pagemap.h @@ -516,7 +516,7 @@ static inline struct page *read_mapping_ }

/* - * Get index of the page with in radix-tree + * Get index of the page within radix-tree (but not for hugetlb pages). * (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE) */ static inline pgoff_t page_to_index(struct page *page) @@ -535,15 +535,16 @@ static inline pgoff_t page_to_index(stru return pgoff; }

+extern pgoff_t hugetlb_basepage_index(struct page *page); + /* - * Get the offset in PAGE_SIZE. - * (TODO: hugepage should have ->index in PAGE_SIZE) + * Get the offset in PAGE_SIZE (even for hugetlb pages). + * (TODO: hugetlb pages should have ->index in PAGE_SIZE) */ static inline pgoff_t page_to_pgoff(struct page *page) { - if (unlikely(PageHeadHuge(page))) - return page->index << compound_order(page); - + if (unlikely(PageHuge(page))) + return hugetlb_basepage_index(page); return page_to_index(page); }

--- a/kernel/futex.c~mm-futex-fix-shared-futex-pgoff-on-shmem-huge-page +++ a/kernel/futex.c @@ -35,7 +35,6 @@ #include <linux/jhash.h> #include <linux/pagemap.h> #include <linux/syscalls.h> -#include <linux/hugetlb.h> #include <linux/freezer.h> #include <linux/memblock.h> #include <linux/fault-inject.h> @@ -650,7 +649,7 @@ again:

key->both.offset |= FUT_OFF_INODE; /* inode-based key */ key->shared.i_seq = get_inode_sequence_number(inode); - key->shared.pgoff = basepage_index(tail); + key->shared.pgoff = page_to_pgoff(tail); rcu_read_unlock(); }

--- a/mm/hugetlb.c~mm-futex-fix-shared-futex-pgoff-on-shmem-huge-page +++ a/mm/hugetlb.c @@ -1588,15 +1588,12 @@ struct address_space *hugetlb_page_mappi return NULL; }

-pgoff_t __basepage_index(struct page *page) +pgoff_t hugetlb_basepage_index(struct page *page) { struct page *page_head = compound_head(page); pgoff_t index = page_index(page_head); unsigned long compound_idx;

- if (!PageHuge(page_head)) - return page_index(page); - if (compound_order(page_head) >= MAX_ORDER) compound_idx = page_to_pfn(page) - page_to_pfn(page_head); else _