From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx196.postini.com [74.125.245.196]) by kanga.kvack.org (Postfix) with SMTP id 046F26B0044 for ; Mon, 30 Apr 2012 05:30:09 -0400 (EDT) From: Jan Kara 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> Sender: owner-linux-mm@kvack.org List-ID: To: Michael Kerrisk Cc: LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, Jan Kara , mgorman@suse.de, Jeff Moyer 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org