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[103.91.158.24]) by mx.google.com with ESMTP id f8si15727512pfd.105.2019.04.08.20.04.21 for ; Mon, 08 Apr 2019 20:04:22 -0700 (PDT) Received-SPF: pass (google.com: domain of sjhuang@iluvatar.ai designates 103.91.158.24 as permitted sender) client-ip=103.91.158.24; Authentication-Results: mx.google.com; dkim=pass header.i=@iluvatar.ai header.s=key_2018 header.b=Icf5Z+Q0; spf=pass (google.com: domain of sjhuang@iluvatar.ai designates 103.91.158.24 as permitted sender) smtp.mailfrom=sjhuang@iluvatar.ai; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=iluvatar.ai X-AuditID: 0a650161-78bff700000078a3-ba-5cac0bb4f71c Received: from owa.iluvatar.ai (s-10-101-1-102.iluvatar.local [10.101.1.102]) by smg.iluvatar.ai (Symantec Messaging Gateway) with SMTP id C0.64.30883.4BB0CAC5; Tue, 9 Apr 2019 11:04:20 +0800 (HKT) Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline DKIM-Signature: v=1; a=rsa-sha256; d=iluvatar.ai; s=key_2018; c=relaxed/relaxed; t=1554779060; h=from:subject:to:date:message-id; bh=XCvCiL37W5XW4bkYbgyGsUhyFfYmLTpkuWZ3Cpm5jks=; b=Icf5Z+Q01cvzcZ7vZhssppW9D6x0aQHpeVzdLw9NbZRCdZtffiSzQDx6XPkdqqmWPApbMsQNvbc ubo2xAis69pAI21zQerkgM7XrhwU78oxG8Yjllqo2c9kzshySVFda+fvaj+BhDlgpQ/oqxUW1+WNA 8ZYnn8rzbZWEhGYO+zI= Received: from hsj-Precision-5520 (10.101.199.253) by S-10-101-1-102.iluvatar.local (10.101.1.102) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1415.2; Tue, 9 Apr 2019 11:04:19 +0800 Date: Tue, 9 Apr 2019 11:04:18 +0800 From: Huang Shijie To: Matthew Wilcox CC: , , , , , , , Subject: Re: [PATCH 1/2] mm/gup.c: fix the wrong comments Message-ID: <20190409030417.GA3324@hsj-Precision-5520> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> <20190408141313.GU22763@bombadil.infradead.org> <20190409010832.GA28081@hsj-Precision-5520> <20190409024929.GW22763@bombadil.infradead.org> MIME-Version: 1.0 In-Reply-To: <20190409024929.GW22763@bombadil.infradead.org> User-Agent: Mutt/1.9.4 (2018-02-28) X-Originating-IP: [10.101.199.253] X-ClientProxiedBy: S-10-101-1-102.iluvatar.local (10.101.1.102) To S-10-101-1-102.iluvatar.local (10.101.1.102) X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFprEIsWRmVeSWpSXmKPExsXClcqYpruFe02MwdOzmhZz1q9hs1h9t5/N Yv/T5ywWZ7pzLS7vmsNmcW/Nf1aLzRMWAInFXUwWv3/MYXPg9JjdcJHFY/MKLY/Fe14yeVw+ W+qx6dMkdo8TM36zeHx8eovF41LzdXaPz5vkAjijuGxSUnMyy1KL9O0SuDIOntnEWHBApOLg 3MQGxid8XYycHBICJhK3uhYxdjFycQgJnGCUOPT0NytIgllAR2LB7k9sXYwcQLa0xPJ/HCA1 LAJvmSRe7j3FCFIjJPCNUWLhbWuQGhYBFYn9B01BwmwCGhJzT9xlBgmLANlvthiBtDILXGSU 2DDjOAtIjbCApcS67mPsIDavgJnEmv1PWSBuOMUosWbrdCaIhKDEyZlPwBo4BWwkznxsYAOx RQWUJQ5sO84EskBIQEHixUotiF+UJJbsncUEYRdKfH95l2UCo/AsJN/MQvhmFpIFCxiZVzHy F+em62XmlJYlliQW6SVmbmKERFTiDsYbnS/1DjEKcDAq8fAqOK6OEWJNLCuuzD3EKMHBrCTC u3Pqqhgh3pTEyqrUovz4otKc1OJDjNIcLErivGUTTWKEBNITS1KzU1MLUotgskwcnFINTI92 lno9Wb1np4ignf+3W1q3kvWCH7GkSDzftOnS2a3rHh934KwU6+2fqnXDIeDmL9+8J9OM/mad 4GjdcbVCxNLlSfE/WWWFlHt7b1d99tD/EsdcwJoasPf4dJ7lTxfmhj2TZPg6vzLxwONa++Mi Ei9+HGn1kd78z6T3go1g1oqlG+dueZ7gtettBHvb/E1nxf9+mtge8kFp55TjXNM+7OmpuzJV e99C/TVPwiZ1OuhfTGd9o7PnXtbX4yWtXzrebfW88fKinIzTy7Vvq5ZLRHxYLfR72q7VPduz e84cUVxjoD7jpwiH1JMPov/eGE//IHmJ89zOmezp0imXXwdUKfac9VJcVvMjI8L+xpzDVRNe K7EUZyQaajEXFScCANDZWEQlAwAA X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. Thanks Huang Shijie