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 Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Michel Lespinasse <walken@xxxxxxxxxx> Cc: Dave Jones <davej@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [3.7+] Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> --- mm/mremap.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/mm/mremap.c b/mm/mremap.c index 0843feb66f3d..05f1180e9f21 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -194,10 +194,17 @@ 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; -- 2.0.0.rc0 -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>