Return-Path: linux-nfs-owner@vger.kernel.org Received: from aserp1040.oracle.com ([141.146.126.69]:32297 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751020AbaIXVNM convert rfc822-to-8bit (ORCPT ); Wed, 24 Sep 2014 17:13:12 -0400 Content-Type: text/plain; charset=windows-1252 Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Subject: Re: [PATCH V3] svcrdma: advertise the correct max payload From: Chuck Lever In-Reply-To: <5422B14C.3040502@dev.mellanox.co.il> Date: Wed, 24 Sep 2014 14:12:58 -0700 Cc: Steve Wise , fields@fieldses.org, linux-rdma@vger.kernel.org, Linux NFS Mailing List Message-Id: <55D3FB7C-3DDD-42B0-96F7-8134096F745C@oracle.com> References: <20140923221122.24227.91718.stgit@build.ogc.int> <5422B14C.3040502@dev.mellanox.co.il> To: Sagi Grimberg Sender: linux-nfs-owner@vger.kernel.org List-ID: On Sep 24, 2014, at 4:55 AM, Sagi Grimberg wrote: > On 9/24/2014 1:11 AM, Steve Wise wrote: >> Svcrdma currently advertises 1MB, which is too large. The correct value >> is the minimum of RPCSVC_MAXPAYLOAD and the max scatter-gather allowed >> in an NFSRDMA IO chunk * the host page size. This bug is usually benign >> because the Linux X64 NFSRDMA client correctly limits the payload size to >> the correct value (64*4096 = 256KB). But if the Linux client is PPC64 >> with a 64KB page size, then the client will indeed use a payload size >> that will overflow the server. >> > > Maybe I'm a bit late with this, but can you explain why SG table is > limited to 64 (fastreg MR and page_list probably bound it). Where is it > coming from? Naturally it needs to be bound by some figure, but why 64? It?s arbitrary, AFAIK. It has been used to allocate some data structures on the stack, for example, so it can?t be too large. It?s something we need to look at. -- Chuck Lever chuck[dot]lever[at]oracle[dot]com