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 E2123C10F0E for ; Tue, 9 Apr 2019 11:19:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B3E092084B for ; Tue, 9 Apr 2019 11:19:14 +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="PwT0K4nl" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726632AbfDILTN (ORCPT ); Tue, 9 Apr 2019 07:19:13 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:54224 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726062AbfDILTN (ORCPT ); Tue, 9 Apr 2019 07:19:13 -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=E01rP1CzaZ83xn8o+c2fpKU03Lvuczg2ah01j9WWPH4=; b=PwT0K4nl0C08dBLQN0wy7x7xH Wa02Me+d0qh8DbWHZYZE9bp7fomcBEi3facq6HUAVgZW29g43+x9/jjvwrxASsZi1LkDPVdV1kkXA spYD2wmPJo0vIB/Lq/lWuw7DQWRI5vzxORwGjcELcZ4GRHk3STryCC5fzqC0VG3if87CfNyPyc39e 0vYX7KALh6ly40A3kGUuEVPiNkqM5wW2TCE9DWNE+umNaUPsTWtDGtdRlYlVtsv8IcFqqubvqUcbo tsMRXp3md5QlcqVwmqPGTLK/r8GUeM0c+uxIjwwoR1tljkTYjr6O9kqK7i/YUnC0jmvANRPve5QkG LVQuBTjaQ==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hDomQ-0000gi-0S; Tue, 09 Apr 2019 11:19:06 +0000 Date: Tue, 9 Apr 2019 04:19:05 -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: <20190409111905.GY22763@bombadil.infradead.org> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> <20190408141313.GU22763@bombadil.infradead.org> <20190409010832.GA28081@hsj-Precision-5520> <20190409024929.GW22763@bombadil.infradead.org> <20190409030417.GA3324@hsj-Precision-5520> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190409030417.GA3324@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 11:04:18AM +0800, Huang Shijie wrote: > On Mon, Apr 08, 2019 at 07:49:29PM -0700, Matthew Wilcox wrote: > > 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. > I do not think it is a bug :) > > If we use the get_user_pages_unlocked(), DMA is okay, such as: > .... > get_user_pages_unlocked() > sg_alloc_table_from_pages() > ..... > > I think the comment is not accurate enough. So just add more comments, and tell the driver > users how to use the GUPs. gup_fast() and gup_unlocked() should return the pages in the same order. If they do not, then it is a bug.