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[2607:7c80:54:e::133]) by mx.google.com with ESMTPS id 1si16117258plo.217.2019.04.08.07.13.23 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 08 Apr 2019 07:13:24 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of willy@infradead.org designates 2607:7c80:54:e::133 as permitted sender) client-ip=2607:7c80:54:e::133; Authentication-Results: mx.google.com; dkim=pass header.i=@infradead.org header.s=bombadil.20170209 header.b=LxXKg4kW; spf=pass (google.com: best guess record for domain of willy@infradead.org designates 2607:7c80:54:e::133 as permitted sender) smtp.mailfrom=willy@infradead.org 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=+reI6IWjQov4mB0Up+1lm8sZxqlFGW/kzlH+giQh14c=; b=LxXKg4kWXS+faNbwg9ehOyy7I ykui4EQtxXgWSPztHpHwAaSTZmBp8YI+nN5EgmMkzTz45HBo/N8Y0lbq1ugpo9vnAPKJeADDq0Hq/ 7uGezLBTg8bBVTjyZVPP2nePjTScdPOx9qapOgq+slTxvf1gFc/aHpKUUh/Z9OPYVZKWVFeKZxzTD cqqfKJmy+t0CQRofvcimWJ7tpLnxQQPpiiHf9ikR6k1fQx7/waVKLRgB9x1L1Nek0qtOwYq1mTqeg BOKtO2kTqCEEjJ0fLVxOGjFtHEATp7bjbthy0i4SnELwcCC8NN4V185M+CBfpND7xDRDxbd4QnkuG aDvpTksyQ==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hDV1N-0003P9-Lc; Mon, 08 Apr 2019 14:13:13 +0000 Date: Mon, 8 Apr 2019 07:13:13 -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: <20190408141313.GU22763@bombadil.infradead.org> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190408023746.16916-1-sjhuang@iluvatar.ai> User-Agent: Mutt/1.9.2 (2017-12-15) 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 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?