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 X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B90F7C4363D for ; Tue, 22 Sep 2020 06:41:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 55D3C208A9 for ; Tue, 22 Sep 2020 06:41:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="GPzc5iW3" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729484AbgIVGlR (ORCPT ); Tue, 22 Sep 2020 02:41:17 -0400 Received: from hqnvemgate26.nvidia.com ([216.228.121.65]:13729 "EHLO hqnvemgate26.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728307AbgIVGlR (ORCPT ); Tue, 22 Sep 2020 02:41:17 -0400 Received: from hqmail.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate26.nvidia.com (using TLS: TLSv1.2, AES256-SHA) id ; Mon, 21 Sep 2020 23:41:04 -0700 Received: from [10.2.52.174] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Tue, 22 Sep 2020 06:41:16 +0000 Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork() To: Peter Xu , , CC: Linus Torvalds , Michal Hocko , Kirill Shutemov , Jann Horn , Oleg Nesterov , Kirill Tkhai , Hugh Dickins , Leon Romanovsky , Jan Kara , Christoph Hellwig , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli References: <20200921211744.24758-1-peterx@redhat.com> <20200921212031.25233-1-peterx@redhat.com> From: John Hubbard Message-ID: <5e594e71-537f-3e9f-85b6-034b7f5fedbe@nvidia.com> Date: Mon, 21 Sep 2020 23:41:16 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <20200921212031.25233-1-peterx@redhat.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL111.nvidia.com (172.20.187.18) To HQMAIL107.nvidia.com (172.20.187.13) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1600756864; bh=PGSDeSt3BFe4Hd1n3brR/Jj7+kPght+eaqYsY8xTtNs=; h=Subject:To:CC:References:From:Message-ID:Date:User-Agent: MIME-Version:In-Reply-To:Content-Type:Content-Language: Content-Transfer-Encoding:X-Originating-IP:X-ClientProxiedBy; b=GPzc5iW3MVT6FTZ2HzvongIWa7QtLZ/1lD308KA39n/lcVCtoBOVu45lbUwFbF0Bi D7u/mxFBuYinhv5sztuSHdymukaPyyiBQrP7THzLyZyqSq7G3PhtkvyejmsDGzf3Vk oki+DjD+97hHcJAqjzDIYW4UUFOZLmWmPw/6jzEWrVVe7YfPNyyyGODxDA9yiDSjax r/zJR4IEisqyTuWRvD8CC0kGVUR9PuAnOT40qTi/1Mt5zleKl/tDVE7PA9ckOfsiYB t0nr1YEWStoXuP6MCK9tbwTORiYy47IZY7JX8VHoIgM/swmWGYKzq2ZJw5qKo1n1G2 g97H6LWqW7Oew== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/21/20 2:20 PM, Peter Xu wrote: ... > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 7ff29cc3d55c..c40aac0ad87e 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1074,6 +1074,23 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > src_page = pmd_page(pmd); > VM_BUG_ON_PAGE(!PageHead(src_page), src_page); > + > + /* > + * If this page is a potentially pinned page, split and retry the fault > + * with smaller page size. Normally this should not happen because the > + * userspace should use MADV_DONTFORK upon pinned regions. This is a > + * best effort that the pinned pages won't be replaced by another > + * random page during the coming copy-on-write. > + */ > + if (unlikely(READ_ONCE(src_mm->has_pinned) && > + page_maybe_dma_pinned(src_page))) { This condition would make a good static inline function. It's used in 3 places, and the condition is quite special and worth documenting, and having a separate function helps with that, because the function name adds to the story. I'd suggest approximately: page_likely_dma_pinned() for the name. > + pte_free(dst_mm, pgtable); > + spin_unlock(src_ptl); > + spin_unlock(dst_ptl); > + __split_huge_pmd(vma, src_pmd, addr, false, NULL); > + return -EAGAIN; > + } Why wait until we are so deep into this routine to detect this and unwind? It seems like if you could do a check near the beginning of this routine, and handle it there, with less unwinding? In fact, after taking only the src_ptl, the check could be made, right? thanks, -- John Hubbard NVIDIA