From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dominique Martinet Subject: Re: [V9fs-developer] [PATCH] net/9p: avoid request size exceed to the virtqueue number in the zero copy Date: Fri, 3 Aug 2018 10:27:25 +0200 Message-ID: <20180803082725.GA8655@nautica> References: <5B63FB4D.1050703@huawei.com> <20180803073240.GA26848@nautica> <5B640FCC.3000704@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: Eric Van Hensbergen , Ron Minnich , Latchesar Ionkov , Linux Kernel Mailing List , v9fs-developer@lists.sourceforge.net, netdev@vger.kernel.org To: jiangyiwen Return-path: Content-Disposition: inline In-Reply-To: <5B640FCC.3000704@huawei.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org jiangyiwen wrote on Fri, Aug 03, 2018: > On 2018/8/3 15:32, Dominique Martinet wrote: > > jiangyiwen wrote on Fri, Aug 03, 2018: > >> Unfortunately, when the address(input and response headers) are not > >> at page boundary, it will need two extra entry in the zero copy, or > >> else it will cause sg array out of bounds. > >> > >> To avoid the problem, we should subtract two pages for maxsize. > > > > Good catch, that must have been painful to figure. > > > > Given we know how big the headers are (something like 11 or 23 bytes > > depending on the op/direction, it's capped by P9_IOHDRSZ at 24), > > couldn't we just cheat and not use the start of the buffer if we detect > > it's overlapping? > > Actually, generally the P9_IOHDRSZ will not cause the problem, because > 24 bytes is too small, but P9_ZC_HDR_SZ(4096 bytes) often cause two pages. > > So I have a question about why we need to use P9_ZC_HDR_SZ, actually we > may use P9_IOHDRSZ instead. The reason is historical (for non-dotl versions of 9P), but the reply if error could be longer than P9_IOHDRSZ in this case - see the code in p9_check_zc_errors that copies the end of the string back from the zc buffer to the 4k buffer I don't see much other reason, though... But I don't understand how that is a problem - we don't actually put the full P9_ZC_HDR_SZ in chan->sg for headers, but these: out = pack_sg_list(chan->sg, 0, VIRTQUEUE_NUM, req->tc.sdata, req->tc.size); in = pack_sg_list(chan->sg, out, VIRTQUEUE_NUM, req->rc.sdata, in_hdr_len); req->tc.size is the size of the header up till that point and in_hdr_len is the size expected from the header. That's why I suggested that if these are on a page boundary, we could just memcpy that data further within the P9_ZC_HDR_SZ-sized buffer and use that for the header (then move it back to the start of the buffer for the reply header) -- Dominique