The race is as follows. Suppose a multi-threaded task forks a new process (on cpu A), thus bumping up the ref count on all the pages. While the fork is occurring (and thus we have marked all the PTEs as read-only), another thread in the original process (on cpu B) tries to write to a huge page, taking an access violation from the write-protect and calling hugetlb_cow(). Now, suppose the fork() fails. It will undo the COW and decrement the ref count on the pages, so the ref count on the huge page drops back to 1. Meanwhile hugetlb_cow() also decrements the ref count by one on the original page, since the original address space doesn't need it any more, having copied a new page to replace the original page. This leaves the ref count at zero, and when we call unlock_page(), we panic. fork on CPU A fault on CPU B ============= ============== ... down_write(&parent->mmap_sem); down_write_nested(&child->mmap_sem); ... while duplicating vmas if error break; ... up_write(&child->mmap_sem); up_write(&parent->mmap_sem); ... down_read(&parent->mmap_sem); ... lock_page(page); handle COW page_mapcount(old_page) == 2 alloc and prepare new_page ... handle error page_remove_rmap(page); put_page(page); ... fold new_page into pte page_remove_rmap(page); put_page(page); ... oops ==> unlock_page(page); up_read(&parent->mmap_sem); The solution is to take an extra reference to the page while we are holding the lock on it. Reviewed-by: Hillf Danton <dhillf@xxxxxxxxx> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx> --- mm/hugetlb.c | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 1871753..2a04cfd 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -2701,6 +2701,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, * so no worry about deadlock. */ page = pte_page(entry); + get_page(page); if (page != pagecache_page) lock_page(page); @@ -2732,6 +2733,7 @@ out_page_table_lock: } if (page != pagecache_page) unlock_page(page); + put_page(page); out_mutex: mutex_unlock(&hugetlb_instantiation_mutex); -- 1.6.5.2 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>