From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753877AbbC0Job (ORCPT ); Fri, 27 Mar 2015 05:44:31 -0400 Received: from mail.SerNet.de ([193.175.80.2]:44683 "EHLO mail.SerNet.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752724AbbC0Jo2 (ORCPT ); Fri, 27 Mar 2015 05:44:28 -0400 Date: Fri, 27 Mar 2015 10:44:09 +0100 From: Volker Lendecke To: Andrew Morton Cc: Christoph Hellwig , Milosz Tanski , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-aio@kvack.org, Mel Gorman , Tejun Heo , Jeff Moyer , "Theodore Ts'o" , Al Viro , linux-api@vger.kernel.org, Michael Kerrisk , linux-arch@vger.kernel.org, Dave Chinner Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only) Reply-To: Volker.Lendecke@SerNet.DE References: <20150326202824.65d03787.akpm@linux-foundation.org> <20150327081822.GA28669@infradead.org> <20150327013516.8c6788be.akpm@linux-foundation.org> <20150327084833.GA7689@infradead.org> <20150327020159.eadd0ce1.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20150327020159.eadd0ce1.akpm@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Message-Id: Organization: SerNet GmbH, Goettingen, Germany Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 27, 2015 at 02:01:59AM -0700, Andrew Morton wrote: > On Fri, 27 Mar 2015 01:48:33 -0700 Christoph Hellwig wrote: > > > On Fri, Mar 27, 2015 at 01:35:16AM -0700, Andrew Morton wrote: > > > fincore() doesn't have to be ugly. Please address the design issues I > > > raised. How is pread2() useful to the class of applications which > > > cannot proceed until all data is available? > > > > It actually makes them work correctly? preadv2( ..., DONTWAIT) will > > return -EGAIN, which causes them to bounce to the threadpool where > > they call preadv(...). > > (I assume you mean RWF_NONBLOCK) > > That isn't how pread2() works. If the leading one or more pages are > uptodate, pread2() will return a partial read. Now what? Either the > application reads the same data a second time via the worker thread > (dumb, but it will usually be a rare case) or it reads the remainder of > the data in the worker thread and splices the data back together. > Which, as I said, will often result in a second load of the initial > read result into CPU cache. Sorry, but I don't have a good picture how we are supposed to use that. I'm fine with two syscalls, but I need a way to tell the kernel to either block or not. Or do you want Samba to do repeated pread calls for ever shorter blocks? Right now I don't see a way to tell pread to either give me a short result or really block. To me that's the core of preadv2. I'm perfectly find for a syscall to give me a short read instead of a global EWOULDBLOCK. I need a way to tell the kernel which behaviour I want. Volker -- SerNet GmbH, Bahnhofsallee 1b, 37081 Göttingen phone: +49-551-370000-0, fax: +49-551-370000-9 AG Göttingen, HRB 2816, GF: Dr. Johannes Loxen http://www.sernet.de, mailto:kontakt@sernet.de