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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 5E792C10F13 for ; Tue, 9 Apr 2019 02:49:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 24E81213F2 for ; Tue, 9 Apr 2019 02:49:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="IQx6HQ79" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726539AbfDICti (ORCPT ); Mon, 8 Apr 2019 22:49:38 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:38944 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726388AbfDICth (ORCPT ); Mon, 8 Apr 2019 22:49:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=+sTnbP4kwIRGvZ3/0p8aO28bolyQS4PZg/H+Zbxqxf0=; b=IQx6HQ79QqGX7eO8jyasd4v0j m0c3D2/Hhq55NUArw4A9igZl44KnhAW/Sv4D9pcn9ybJiD1U9U71AOe4AmNjs6nHdT0V54uLfvQK9 ZECByfGXchDZxYKEHi1vGG2paLGfUqgreqgIBGpnn5ucQnO01Ft48qBLY1uEMm0VrT3HM/MXxl/Xq jwluodV18EV3WZ/b9691OyDEwNOaRBAhZtPtRtEU8kTf0w+Mh/cZmGMTMeSu63RA5m6r6MCKwwv4M apckaLCSx3COmCMyv98QI5NS1daHb5QAPSE2I5Bho5fP3vvWu3gX+CrXYF3BcK1Cp7lAr5fNtfs+8 i11Fn3SWg==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hDgpG-0001c2-5h; Tue, 09 Apr 2019 02:49:30 +0000 Date: Mon, 8 Apr 2019 19:49:29 -0700 From: Matthew Wilcox To: Huang Shijie Cc: akpm@linux-foundation.org, william.kucharski@oracle.com, ira.weiny@intel.com, palmer@sifive.com, axboe@kernel.dk, keescook@chromium.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] mm/gup.c: fix the wrong comments Message-ID: <20190409024929.GW22763@bombadil.infradead.org> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> <20190408141313.GU22763@bombadil.infradead.org> <20190409010832.GA28081@hsj-Precision-5520> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190409010832.GA28081@hsj-Precision-5520> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 09, 2019 at 09:08:33AM +0800, Huang Shijie wrote: > On Mon, Apr 08, 2019 at 07:13:13AM -0700, Matthew Wilcox wrote: > > On Mon, Apr 08, 2019 at 10:37:45AM +0800, Huang Shijie wrote: > > > The root cause is that sg_alloc_table_from_pages() requires the > > > page order to keep the same as it used in the user space, but > > > get_user_pages_fast() will mess it up. > > > > I don't understand how get_user_pages_fast() can return the pages in a > > different order in the array from the order they appear in userspace. > > Can you explain? > Please see the code in gup.c: > > int get_user_pages_fast(unsigned long start, int nr_pages, > unsigned int gup_flags, struct page **pages) > { > ....... > if (gup_fast_permitted(start, nr_pages)) { > local_irq_disable(); > gup_pgd_range(addr, end, gup_flags, pages, &nr); // The @pages array maybe filled at the first time. Right ... but if it's not filled entirely, it will be filled part-way, and then we stop. > local_irq_enable(); > ret = nr; > } > ....... > if (nr < nr_pages) { > /* Try to get the remaining pages with get_user_pages */ > start += nr << PAGE_SHIFT; > pages += nr; // The @pages is moved forward. Yes, to the point where gup_pgd_range() stopped. > if (gup_flags & FOLL_LONGTERM) { > down_read(¤t->mm->mmap_sem); > ret = __gup_longterm_locked(current, current->mm, // The @pages maybe filled at the second time Right. > /* > * retain FAULT_FOLL_ALLOW_RETRY optimization if > * possible > */ > ret = get_user_pages_unlocked(start, nr_pages - nr, // The @pages maybe filled at the second time. > pages, gup_flags); Yes. But they'll be in the same order. > BTW, I do not know why we mess up the page order. It maybe used in some special case. I'm not discounting the possibility that you've found a bug. But documenting that a bug exists is not the solution; the solution is fixing the bug.