From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx137.postini.com [74.125.245.137]) by kanga.kvack.org (Postfix) with SMTP id C162C6B0044 for ; Mon, 30 Apr 2012 09:41:53 -0400 (EDT) From: Jeff Moyer Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers References: <1335778207-6511-1-git-send-email-jack@suse.cz> Date: Mon, 30 Apr 2012 09:41:50 -0400 In-Reply-To: <1335778207-6511-1-git-send-email-jack@suse.cz> (Jan Kara's message of "Mon, 30 Apr 2012 11:30:07 +0200") Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Michael Kerrisk , LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de Jan Kara writes: > 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 I think this sufficiently distills the problem. Thanks, Jan. Acked-by: Jeff Moyer -- 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