From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752259AbdLGBST (ORCPT ); Wed, 6 Dec 2017 20:18:19 -0500 Received: from mga09.intel.com ([134.134.136.24]:49306 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751718AbdLGBSS (ORCPT ); Wed, 6 Dec 2017 20:18:18 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.45,370,1508828400"; d="scan'208";a="184791203" From: "Huang, Ying" To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Huang Ying , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Alexander Viro , Zi Yan , Naoya Horiguchi , Andrea Arcangeli Subject: [PATCH -mm -V2] mm, userfaultfd, THP: Avoid waiting when PMD under THP migration Date: Thu, 7 Dec 2017 09:17:52 +0800 Message-Id: <20171207011752.3292-1-ying.huang@intel.com> X-Mailer: git-send-email 2.15.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Huang Ying If THP migration is enabled, for a VMA handled by userfaultfd, consider the following situation, do_page_fault() __do_huge_pmd_anonymous_page() handle_userfault() userfault_msg() /* a huge page is allocated and mapped at fault address */ /* the huge page is under migration, leaves migration entry in page table */ userfaultfd_must_wait() /* return true because !pmd_present() */ /* may wait in loop until fatal signal */ That is, it may be possible for userfaultfd_must_wait() encounters a PMD entry which is !pmd_none() && !pmd_present(). In the current implementation, we will wait for such PMD entries, which may cause unnecessary waiting, and potential soft lockup. This is fixed via avoiding to wait when !pmd_none() && !pmd_present(), only wait when pmd_none(). This may be not a problem in practice, because userfaultfd_must_wait() is always called with mm->mmap_sem read-locked. mremap() will write-lock mm->mmap_sem. And UFFDIO_COPY doesn't support to copy THP mapping. But the change introduced still makes the code more correct, and makes the PMD and PTE code more consistent. Signed-off-by: "Huang, Ying" Reviewed-by: Andrea Arcangeli Cc: Mike Kravetz Cc: Mike Rapoport Cc: "Kirill A. Shutemov" Cc: Alexander Viro Cc: Zi Yan Cc: Naoya Horiguchi --- fs/userfaultfd.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index ac9a4e65ca49..cbc715fc82e1 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -294,10 +294,13 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx, * pmd_trans_unstable) of the pmd. */ _pmd = READ_ONCE(*pmd); - if (!pmd_present(_pmd)) + if (pmd_none(_pmd)) goto out; ret = false; + if (!pmd_present(_pmd)) + goto out; + if (pmd_trans_huge(_pmd)) goto out; -- 2.15.0