From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 20C86C3A59D for ; Mon, 24 Oct 2022 04:33:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229889AbiJXEdW (ORCPT ); Mon, 24 Oct 2022 00:33:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229872AbiJXEdQ (ORCPT ); Mon, 24 Oct 2022 00:33:16 -0400 Received: from mga07.intel.com (mga07.intel.com [134.134.136.100]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 306BA754BC for ; Sun, 23 Oct 2022 21:33:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1666585995; x=1698121995; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=ObwjONVUV1m6qgO+y0lFIH+jDLUNTJNwafsT6+bksmU=; b=lcPUjuqYqGSIRxorcfoUyv7MFrikZc9F6mbTLR85g+56kOSlhWTAsfmu B07hQElMoTSZh4tMByzlAfOaTQWFoQOR/Hmz+BtfqKGXzPSOWmv1VEAD5 9xQp3A8sqW+5z+uUJp7OiTYyZ25ZPwqrgsvqe7CQnvpxj8BO0lVeNAStS /a+icmkPM0Z+qT8N87nG64HnIpe8aSkJAQuisk+Pc9OaJO7t+SlPp45wU P0VbmC0P8ZH0RgBVP5XvpVDuzjiyzi/WVqaQKmaXoT4sHwQza5Wap6xv+ iVSkuKarhd5lxmRtSCveQQIJ5crqikibPNQJlT+C7bgb4Ow+5XyFKT6Fc A==; X-IronPort-AV: E=McAfee;i="6500,9779,10509"; a="371555184" X-IronPort-AV: E=Sophos;i="5.95,207,1661842800"; d="scan'208";a="371555184" Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Oct 2022 21:33:14 -0700 X-IronPort-AV: E=McAfee;i="6500,9779,10509"; a="694424439" X-IronPort-AV: E=Sophos;i="5.95,207,1661842800"; d="scan'208";a="694424439" Received: from iweiny-mobl.amr.corp.intel.com (HELO localhost) ([10.252.130.95]) by fmsmga008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Oct 2022 21:33:12 -0700 From: ira.weiny@intel.com To: Andrew Morton Cc: Ira Weiny , Randy Dunlap , Peter Xu , Andrea Arcangeli , Matthew Wilcox , kernel test robot , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH for rc] mm/shmem: Ensure proper fallback if page faults Date: Sun, 23 Oct 2022 21:33:05 -0700 Message-Id: <20221024043305.1491403-1-ira.weiny@intel.com> X-Mailer: git-send-email 2.37.2 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ira Weiny The kernel test robot flagged a recursive lock as a result of a conversion from kmap_atomic() to kmap_local_folio()[Link] The cause was due to the code depending on the kmap_atomic() side effect of disabling page faults. In that case the code expects the fault to fail and take the fallback case. git archaeology implied that the recursion may not be an actual bug.[1] However, the mmap_lock needed in the fault may be the one held.[2] Add an explicit pagefault_disable() and a big comment to explain this for future souls looking at this code. [1] https://lore.kernel.org/all/Y1MymJ%2FINb45AdaY@iweiny-desk3/ [2] https://lore.kernel.org/all/Y1M2p9OtBGnKwGUE@x1n/ Fixes: 7a7256d5f512 ("shmem: convert shmem_mfill_atomic_pte() to use a folio") Cc: Andrew Morton Cc: Randy Dunlap Cc: Peter Xu Cc: Andrea Arcangeli Reported-by: Matthew Wilcox (Oracle) Reported-by: kernel test robot Link: https://lore.kernel.org/r/202210211215.9dc6efb5-yujie.liu@intel.com Signed-off-by: Ira Weiny --- Thanks to Matt and Andrew for initial diagnosis. Thanks to Randy for pointing out C code needs ';' :-D Thanks to Andrew for suggesting an elaborate comment Thanks to Peter for pointing out that the mm's may be the same. --- mm/shmem.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/shmem.c b/mm/shmem.c index 8280a5cb48df..c1bca31cd485 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2424,9 +2424,16 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm, if (!zeropage) { /* COPY */ page_kaddr = kmap_local_folio(folio, 0); + /* + * The mmap_lock is held here. Disable page faults to + * prevent deadlock should copy_from_user() fault. The + * copy will be retried outside the mmap_lock. + */ + pagefault_disable(); ret = copy_from_user(page_kaddr, (const void __user *)src_addr, PAGE_SIZE); + pagefault_enable(); kunmap_local(page_kaddr); /* fallback to copy_from_user outside mmap_lock */ -- 2.37.2