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[103.91.158.24]) by mx.google.com with ESMTP id w189si10601007pgd.381.2019.04.08.18.08.36 for ; Mon, 08 Apr 2019 18:08:37 -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=Fj4O1eqL; 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: 0a650161-78bff700000078a3-8f-5cabf093b18d 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 06.44.30883.390FBAC5; Tue, 9 Apr 2019 09:08:35 +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=1554772115; h=from:subject:to:date:message-id; bh=7PoxFksW/7Y5mrW/Yb3Kse3GMSMGjxnGjzU1zXRdg8I=; b=Fj4O1eqLq9eKptCrCrpUPwnVDMk/Bcb81pxPlkLUsXqS951w230uGx869AGQh+OKFwcd7xE4amB QE8j8AJ/1tRPncTNw/YMemB4TG8C3Fvuqe/Ub3CJOBuYz0h5FHNOIEgIrnwOEoTa0baTlQrNSEwWR l5d7WBuX91Fqq4b1Flo= 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 09:08:35 +0800 Date: Tue, 9 Apr 2019 09:08:33 +0800 From: Huang Shijie To: Matthew Wilcox CC: , , , , , , , Subject: Re: [PATCH 1/2] mm/gup.c: fix the wrong comments Message-ID: <20190409010832.GA28081@hsj-Precision-5520> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> <20190408141313.GU22763@bombadil.infradead.org> MIME-Version: 1.0 In-Reply-To: <20190408141313.GU22763@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+NgFprCIsWRmVeSWpSXmKPExsXClcqYpjv5w+oYg7ufDSzmrF/DZrH6bj+b xf6nz1ksznTnWlzeNYfN4t6a/6wWmycsABKLu5gsfv+Yw+bA6TG74SKLx+YVWh6L97xk8rh8 ttRj06dJ7B4nZvxm8fj49BaLx6Xm6+wenzfJBXBGcdmkpOZklqUW6dslcGWsejaBueCtQMWZ e5eZGhiX8nQxcnJICJhITP28jLWLkYtDSOAEo8Tl+50sIAlmAR2JBbs/sXUxcgDZ0hLL/3GA 1LAIvGWSuP3vBTtEwzdGib377rKDNLAIqEgse/wXzGYT0JCYe+IuM0izCJD9ZosRSD2zwEVG iQ0zjoMtEBawlFjXfQysnlfAXGLHk7lsILaQQJbEsZ7HzBBxQYmTM5+A1XMK2EjsXriJCcQW FVCWOLDtOBPIfCEBBYkXK7UgnlGSWLJ3FhOEXSjx/eVdlgmMwrOQvDML4Z1ZSBYsYGRexchf nJuul5lTWpZYklikl5i5iRESVYk7GG90vtQ7xCjAwajEw6vguDpGiDWxrLgy9xCjBAezkgjv zqmrYoR4UxIrq1KL8uOLSnNSiw8xSnOwKInzlk00iRESSE8sSc1OTS1ILYLJMnFwSjUwJc7U m8r46sXv7J3/Y4Oa6lQ8HRnNzsgGPzm9uWRz8O3Jc80mcEw7flz22iQzSw826yzFnUeaZ+Zr FCj9DoxnZFlxmLf16HU7v9Sjgnc/BsTO6V7EejBijpeN2vXIslt+Ke9/1GmbTb06qXyBl0b/ szsGmc/Xdcelr5oatfPwhafuv/Rue6zm/bP7QHfFz42eftlPileFLax5oLR7Rg93lu2WpwfW rVTyCuermLxP6Jm2w2EvsbjLPUFd1/RubGWs9rmxsu/z1OuNArusb/GFl7B88rj5Km/y+9My /5/O5FvVv/jyialzTZev/9RWwbnm3P0qWZZ9N0RmfXGuf/b+0zWuk3bhE7yCjbkmfcxhmKPE UpyRaKjFXFScCADnserWJwMAAA== 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 07:13:13AM -0700, Matthew Wilcox wrote: > On Mon, Apr 08, 2019 at 10:37:45AM +0800, Huang Shijie wrote: > > When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own > > get_user_pages_fast(). > > > > In the following scenario, we will may meet the bug in the DMA case: > > ..................... > > get_user_pages_fast(start,,, pages); > > ...... > > sg_alloc_table_from_pages(, pages, ...); > > ..................... > > > > 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. 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. 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 start, nr_pages - nr, pages, NULL, gup_flags); up_read(¤t->mm->mmap_sem); } else { /* * 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); } } ..................... BTW, I do not know why we mess up the page order. It maybe used in some special case. Thanks Huang Shijie