From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755977Ab2D3JaL (ORCPT ); Mon, 30 Apr 2012 05:30:11 -0400 Received: from cantor2.suse.de ([195.135.220.15]:57267 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755076Ab2D3JaJ (ORCPT ); Mon, 30 Apr 2012 05:30:09 -0400 From: Jan Kara To: Michael Kerrisk Cc: LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, Jan Kara , mgorman@suse.de, Jeff Moyer Subject: [PATCH] Describe race of direct read and fork for unaligned buffers Date: Mon, 30 Apr 2012 11:30:07 +0200 Message-Id: <1335778207-6511-1-git-send-email-jack@suse.cz> X-Mailer: git-send-email 1.7.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is a long standing problem (or a surprising feature) in our implementation of get_user_pages() (used by direct IO). Since several attempts to fix it failed (e.g. http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg06542.html, or http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html refused in http://comments.gmane.org/gmane.linux.kernel.mm/31569) and it's not completely clear whether we really want to fix it given the costs, let's at least document it. CC: mgorman@suse.de CC: Jeff Moyer Signed-off-by: Jan Kara --- --- a/man2/open.2 2012-04-27 00:07:51.736883092 +0200 +++ b/man2/open.2 2012-04-27 00:29:59.489892980 +0200 @@ -769,7 +769,12 @@ and the file offset must all be multiples of the logical block size of the file system. Under Linux 2.6, alignment to 512-byte boundaries -suffices. +suffices. However, if the user buffer is not page aligned and direct read +runs in parallel with a +.BR fork (2) +of the reader process, it may happen that the read data is split between +pages owned by the original process and its child. Thus effectively read +data is corrupted. .LP The .B O_DIRECT