From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757245Ab0ERPxT (ORCPT ); Tue, 18 May 2010 11:53:19 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.123]:63444 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751491Ab0ERPxS (ORCPT ); Tue, 18 May 2010 11:53:18 -0400 X-Authority-Analysis: v=1.1 cv=XlIRUSWzFvHiorqRgyN8fup7OiOLcGcwhtEWqPJTlSs= c=1 sm=0 a=8EtQVlZUVNwA:10 a=hO-oPbc3tlwA:10 a=7U3hwN5JcxgA:10 a=Q9fys5e9bTEA:10 a=gMqfjgEr1zLu/65IO0LwxA==:17 a=uB51Yq4vNIEX0SBci-4A:9 a=KYDUYj5AgdIIjgFjRt8A:7 a=SojiUwRDDj98dCSJ_KHmE5em4TcA:4 a=PUjeQqilurYA:10 a=xNuefaKZfxuTjvek:21 a=rNHTCq4KgKSdzzvO:21 a=gMqfjgEr1zLu/65IO0LwxA==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.89.75 Subject: Re: Unexpected splice "always copy" behavior observed From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Mathieu Desnoyers Cc: Peter Zijlstra , Frederic Weisbecker , Pierre Tardy , Ingo Molnar , Arnaldo Carvalho de Melo , Tom Zanussi , Paul Mackerras , linux-kernel@vger.kernel.org, arjan@infradead.org, ziga.mahkovec@gmail.com, davem , linux-mm@kvack.org, Andrew Morton , KOSAKI Motohiro , Christoph Lameter , Tejun Heo , Jens Axboe , Linus Torvalds , Nick Piggin In-Reply-To: <20100518153440.GB7748@Krystal> References: <20100518153440.GB7748@Krystal> Content-Type: text/plain; charset="ISO-8859-15" Organization: Kihon Technologies Inc. Date: Tue, 18 May 2010 11:53:13 -0400 Message-ID: <1274197993.26328.755.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hehe, I just notice this this morning too, while investigating. On Tue, 2010-05-18 at 11:34 -0400, Mathieu Desnoyers wrote: > Hi, > > I'm currently digging into the splice code to figure out why it's always in copy > mode even though I specified the SPLICE_F_MOVE flag and released the page > references from the LTTng ring buffer. I'm splicing to a pipe and then from the > pipe to an ext3 filesystem (2.6.33.4 kernel). I've got the feeling I'm missing > something and I don't like that. > > My simple test case is to add a printk around the splice copy: > > fs/splice.c: pipe_to_file() > if (buf->page != page) { > /* > * Careful, ->map() uses KM_USER0! > */ > char *src = buf->ops->map(pipe, buf, 1); > char *dst = kmap_atomic(page, KM_USER1); > > printk(KERN_WARNING "SPLICE COPY!!!\n"); > memcpy(dst + offset, src + buf->offset, this_len); > flush_dcache_page(page); > kunmap_atomic(dst, KM_USER1); > buf->ops->unmap(pipe, buf, src); I used trace_printk() since it is not as invasive. > } > > I'll start with a disclaimer that I only recently improved my splice > understanding, so AFAIU: Same here ;-) > > * pipe_to_file() allocates a struct page *page on its stack. > > * It is passed, uninitialized, to > > ret = pagecache_write_begin(file, mapping, sd->pos, this_len, > AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata); > > that looks already odd to me, as I would expect pipe_to_file to populate > this page pointer with buf->page initially if the proper conditions are met. > > * Looking at the ext2 and ext3 write_begin code, neither are using the pagep > parameter: > > ext2: > > static int > ext2_write_begin(struct file *file, struct address_space *mapping, > loff_t pos, unsigned len, unsigned flags, > struct page **pagep, void **fsdata) > { > *pagep = NULL; > return __ext2_write_begin(file, mapping, pos, len, flags, pagep,fsdata); > } > > > ext3: > > static int ext3_write_begin(struct file *file, struct address_space *mapping, > loff_t pos, unsigned len, unsigned flags, > struct page **pagep, void **fsdata) > { > struct page *page; > .... > > retry: > page = grab_cache_page_write_begin(mapping, index, flags); > if (!page) > return -ENOMEM; > *pagep = page; > > * So, considering the test to check if the page content must be copied: > > if (buf->page != page) { > > how is it ever possible that buf->page == page ? I'm currently looking at the network code to see if it is better. -- Steve