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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BAE2FECAAD6 for ; Fri, 26 Aug 2022 19:43:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=h9WVnwndfj9Ncs3v4MSrTwteRvvjIhvNeVW+NH7bd4g=; b=u6AzXI/Gtgq+awc+F/1pPWiNCL YI/9PZ3uhcq9/TjFR4ofo9twchY2nrsR/AcDSJPwGWR4FtUk7idKuY+EDiXQaM3AjSPLpHYWGyf2c By0TwsC/Fzk8LO2E8F0DlyRmJwY/Y38U3TtI3ztNuRUb/unW7aPv5X2uZwB19DhjWuzZKv6ThLBSJ gaLm4bQ1ceI8mtQhiht6wYvbt1uxs2YBZmHACIKhiOcIUWs0AcnHfVv9tNpNM+c/MI2NzuVc6wHhl 6XFncxtl+ImsJadq6LibAc068Wkr9/RosQXjGJ60JR8z3GDzIaqZfgAII4QrF4iu1I7HwFA2t/wzh V/Gs0UKg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1oRfEr-00Ar5u-CU; Fri, 26 Aug 2022 19:43:33 +0000 Received: from zeniv.linux.org.uk ([2a03:a000:7:0:5054:ff:fe1c:15ff]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1oRf4t-00AiPt-O5 for linux-nvme@lists.infradead.org; Fri, 26 Aug 2022 19:33:18 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=h9WVnwndfj9Ncs3v4MSrTwteRvvjIhvNeVW+NH7bd4g=; b=ZSyTODQKMGcnl9NyDkktbrnaIA 3emfSaArlZ+4sXqOWl9GZYo7J/oYgXrLKG/pckQM1r59OYXoizsEiQSz9eVt/7G/KauhNtMlTcjK9 jzuvolo2fzh8Cz56Ei3ZDwi1MdY0WyHzhXwRlzAH+jaTfTjM7211tVOSuAxqlDUgKqpxCS0zStNG9 9+3hCZk9Uv06MzEjdshcL+WUkVb+9RnAJRFM4Z/CkKT0nXhg2rqRCItrFw6w+CBeK5BQs0OMiX5fZ RzF1dD1yxqrowDcs9Oe7QL6zsrID/Gsy0luO5Vu3MIUkrYvVa4vay+W5DLj0Aqbnz27RAXZMm/Ewt YDko7gbQ==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.95 #2 (Red Hat Linux)) id 1oRf4O-008dka-LS; Fri, 26 Aug 2022 19:32:44 +0000 Date: Fri, 26 Aug 2022 20:32:44 +0100 From: Al Viro To: "Fabio M. De Francesco" Cc: Christoph Hellwig , Sagi Grimberg , Chaitanya Kulkarni , James Smart , Ira Weiny , "Venkataramanan, Anirudh" , linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org, Chaitanya Kulkarni , Keith Busch Subject: Re: [PATCH v3 1/1] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM Message-ID: References: <20220822142438.5954-1-fmdefrancesco@gmail.com> <20220822142438.5954-2-fmdefrancesco@gmail.com> <2887364.VdNmn5OnKV@opensuse> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2887364.VdNmn5OnKV@opensuse> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220826_123315_820509_769527E7 X-CRM114-Status: GOOD ( 16.52 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On Fri, Aug 26, 2022 at 08:16:59PM +0200, Fabio M. De Francesco wrote: > As you may have already read, I'm so new to kernel development that I still > know very little about many subsystems and drivers. I am not currently > able to tell the difference between BVEC and KVEC. I could probably try to > switch from one to the other (after learning from other code), however I won't > be able to explain in the commit message why users should better use BVEC in > this case. struct kvec: pairs of form struct bio_vec: triples of form Either is a way to refer to a chunk of memory; the former obviously has it already mapped (you don't get kernel addresses otherwise), the latter doesn't need to. iov_iter instances might be backed by different things, including arrays of kvec (iov_iter_kvec() constructs such) and arrays of bio_vec (iov_iter_bvec() is the constructor for those). iov_iter primitives (copy_to_iter/copy_from_iter/copy_page_to_iter/etc.) work with either variant - they look at the flavour and act accordingly. ITER_BVEC ones tend to do that kmap_local_page() + copy + kunmap_local(). ITER_KVEC obviously use memcpy() for copying and that's it. If you need e.g. to send some subranges of some pages you could kmap them, form kvec array, make msg.msg_iter a KVEC-backed iterator over those and feed it to sendmsg(). Or you could take a bio_vec array instead, make msg.msg_iter a BVEC-backed iterator over that and feed to sendmsg(). The difference is, in the latter case kmap_local() will be done on demand *inside* ->sendmsg() instance, when it gets around to copying some data from the source and calls something like csum_and_copy_from_iter() or whichever primitive it chooses to use. Why bother with mapping the damn thing in the caller and having it pinned all along whatever ->sendmsg() you end up calling? Just give it page/offset/length instead of address/length and let lib/iov_iter.c do the right thing...