From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755463AbaA1UDF (ORCPT ); Tue, 28 Jan 2014 15:03:05 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:46082 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754636AbaA1UDD (ORCPT ); Tue, 28 Jan 2014 15:03:03 -0500 Date: Tue, 28 Jan 2014 12:03:01 -0800 From: Andrew Morton To: Mark Rutland Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: readahead: fix do_readahead for no readpage(s) Message-Id: <20140128120301.581c90caeb1c49e346b6aa66@linux-foundation.org> In-Reply-To: <1390907659-11675-1-git-send-email-mark.rutland@arm.com> References: <1390907659-11675-1-git-send-email-mark.rutland@arm.com> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 28 Jan 2014 11:14:19 +0000 Mark Rutland wrote: > Commit 63d0f0a3c7e1 (mm/readahead.c:do_readhead(): don't check for > ->readpage) unintentionally made do_readahead return 0 for all valid > files regardless of whether readahead was supported, rather than the > expected -EINVAL. This gets forwarded on to userspace, and results in > sys_readahead appearing to succeed in cases that don't make sense (e.g. > when called on pipes or sockets). This issue is detected by the LTP > readahead01 testcase. How can this be? : static ssize_t : do_readahead(struct address_space *mapping, struct file *filp, : pgoff_t index, unsigned long nr) : { : if (!mapping || !mapping->a_ops) : return -EINVAL; : : return force_page_cache_readahead(mapping, filp, index, nr); : } and : int force_page_cache_readahead(struct address_space *mapping, struct file *filp, : pgoff_t offset, unsigned long nr_to_read) : { : if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) : return -EINVAL; Clearly, do_readahead() will return -EINVAL if neither ->readpage or ->readpages are implemented. I can see that the behaviour would change if the address_space implements only one of ->readpage and ->readpages, but that doesn't appear to match your description and the new behaviour is correct - we can now perform readahead for address_spaces which implement ->readpages and not ->readpage (which would be odd and might not work for other reasons..).