From 619131122438f5da4c9f8467a64f4f111c8ed823 Mon Sep 17 00:00:00 2001 From: "Kirill A. Shutemov" Date: Fri, 9 May 2014 15:37:00 -0700 Subject: mm, thp: close race between mremap() and split_huge_page() commit dd18dbc2d42af75fffa60c77e0f02220bc329829 upstream. It's critical for split_huge_page() (and migration) to catch and freeze all PMDs on rmap walk. It gets tricky if there's concurrent fork() or mremap() since usually we copy/move page table entries on dup_mm() or move_page_tables() without rmap lock taken. To get it work we rely on rmap walk order to not miss any entry. We expect to see destination VMA after source one to work correctly. But after switching rmap implementation to interval tree it's not always possible to preserve expected walk order. It works fine for dup_mm() since new VMA has the same vma_start_pgoff() / vma_last_pgoff() and explicitly insert dst VMA after src one with vma_interval_tree_insert_after(). But on move_vma() destination VMA can be merged into adjacent one and as result shifted left in interval tree. Fortunately, we can detect the situation and prevent race with rmap walk by moving page table entries under rmap lock. See commit 38a76013ad80. Problem is that we miss the lock when we move transhuge PMD. Most likely this bug caused the crash[1]. [1] http://thread.gmane.org/gmane.linux.kernel.mm/96473 Fixes: 108d6642ad81 ("mm anon rmap: remove anon_vma_moveto_tail") Signed-off-by: Kirill A. Shutemov Reviewed-by: Andrea Arcangeli Cc: Rik van Riel Acked-by: Michel Lespinasse Cc: Dave Jones Cc: David Miller Acked-by: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/mremap.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'mm') diff --git a/mm/mremap.c b/mm/mremap.c index 463a25705ac6..2201d060c31b 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -175,10 +175,17 @@ unsigned long move_page_tables(struct vm_area_struct *vma, break; if (pmd_trans_huge(*old_pmd)) { int err = 0; - if (extent == HPAGE_PMD_SIZE) + if (extent == HPAGE_PMD_SIZE) { + VM_BUG_ON(vma->vm_file || !vma->anon_vma); + /* See comment in move_ptes() */ + if (need_rmap_locks) + anon_vma_lock_write(vma->anon_vma); err = move_huge_pmd(vma, new_vma, old_addr, new_addr, old_end, old_pmd, new_pmd); + if (need_rmap_locks) + anon_vma_unlock_write(vma->anon_vma); + } if (err > 0) { need_flush = true; continue; -- cgit v1.2.3