From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx134.postini.com [74.125.245.134]) by kanga.kvack.org (Postfix) with SMTP id 5796E6B0044 for ; Tue, 1 May 2012 10:32:05 -0400 (EDT) Received: by ghrr18 with SMTP id r18so2637084ghr.14 for ; Tue, 01 May 2012 07:32:04 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1335778207-6511-1-git-send-email-jack@suse.cz> References: <1335778207-6511-1-git-send-email-jack@suse.cz> From: KOSAKI Motohiro Date: Tue, 1 May 2012 10:31:44 -0400 Message-ID: Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable 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, Jeff Moyer , npiggin@gmail.com On Mon, Apr 30, 2012 at 5:30 AM, Jan Kara wrote: > This is a long standing problem (or a surprising feature) in our implemen= tation > 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 compl= etely > clear whether we really want to fix it given the costs, let's at least do= cument > it. > > CC: mgorman@suse.de > CC: Jeff Moyer > Signed-off-by: Jan Kara > --- > > --- a/man2/open.2 =A0 =A0 =A0 2012-04-27 00:07:51.736883092 +0200 > +++ b/man2/open.2 =A0 =A0 =A0 2012-04-27 00:29:59.489892980 +0200 > @@ -769,7 +769,12 @@ > =A0and the file offset must all be multiples of the logical block size > =A0of the file system. > =A0Under Linux 2.6, alignment to 512-byte boundaries > -suffices. > +suffices. However, if the user buffer is not page aligned and direct rea= d > +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. > =A0.LP > =A0The > =A0.B O_DIRECT Hello, Thank you revisit this. But as far as my remember is correct, this issue is= NOT unaligned access issue. It's just get_user_pages(_fast) vs fork race issue.= i.e. DIRECT_IO w/ multi thread process should not use fork(). -- 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